What is the most abundant element in the sun

[<< prev] 1 2 3 4 5 6 7 8 [next >>]

Nowadays scientists use special instruments called spectrographs to spread out the visible portion of the Sun’s radiation into its spectral components and separate colors. Such a display of the intensity of radiation as a function of wavelength is called a spectrum. Spectroscopy is the study and interoperation of spectra, especially with a view to determine the chemical composition of and physical conditions in the source of radiation.

When the spectrum of sunlight is examined carefully, with very fine wavelength resolution, numerous fine dark gaps are seen crossing the rainbow-like display (Fig. 1.9). When coarser resolution is used, the separate colors of sunlight are somewhat blurred together and the dark places are no longer found superimposed on its spectrum.

The dark gaps of missing colors are now called absorption lines. When a cool, tenuous gas is placed in front of a hot, dense one, atoms or ions in the cool gas absorb radiation at specific wavelengths, thereby producing the dark absorption lines. They are called lines because they look like a line in the spectrum.

Each chemical element, and only that element, produces a unique set, or pattern, of wavelengths at which the dark lines fall. It is as if each element has its own characteristic barcode that can be used to identify each element, as a fingerprint might identify a criminal.

Since a greater number of atoms will absorb more light, the relative darkness of the absorption lines establishes the relative abundance of the elements in the Sun. That is, darker absorption lines generally indicate greater absorption and therefore larger amounts of the element. Studies of the absorption lines in the Sun’s spectrum showed, in the 1920s, that hydrogen is the most abundant element in the visible solar gases. Since the Sun was most likely chemically homogenous, a high hydrogen abundance was implied for the entire star, and this was confirmed by subsequent calculations of its luminosity.

Helium, the second-most abundant element in the Sun, is so rare on Earth that it was first discovered on the Sun.

Altogether, 92.1 percent of the number of atoms in the Sun are hydrogen atoms, 7.8 percent by number are helium atoms, and all other heavier elements make up only 0.1 percent.

Atoms consist of largely of empty space, just as the room you may be sitting in appears to be mostly empty. A tiny, heavy, positively charged nucleus lies at the heart of an atom, surrounded by a cloud of relatively minute, negatively charged electrons that occupy most of an atom’s space and govern its chemical behavior.

Hydrogen is the simplest atom, consisting of a single electron circling around a single proton. The nucleus of helium contains two neutrons and two protons, and so has two electrons in orbit.

Stars are Born, Live and Die

Massive stars have an explosive death. After the core has become hot enough to produce iron, the star has reached the end of the line. It has become bankrupt, completely spending all its nuclear resources, and there is nothing left to do but collapse under the sheer weight of it all. In a matter of milliseconds the central core is crushed into a ball of neutrons about 10,000 meters across, no bigger than a city. The outer layers also plunge in toward the center, but they rebound like a tightly coiled spring. The pent-up energy generated in the collapsing center blasts the outer layers out in a violent explosion called a supernova, littering space with its cinders. These ashes will join the debris from countless other explosions, providing the raw material for the next generation of stars.

In a galaxy the size of the Milky Way, a supernova explodes on average once every hundred years or so. The energy released in a supernova is immense. For a few weeks it can be brighter than the combined brightness of all the other stars in a galaxy. Then, as the debris expands outwards, it cools and becomes fainter. Astronomers use the name supernova remnant for this expanding shell of gas (Fig. 1.11, Fig. 1.12). This material moves out into interstellar space, seeding it with heavy elements that were forged inside the former star.

Where did the Chemical Elements Come from?

The majority of atoms that we see today were formed at the dawn of time before the stars even existed, in the immediate aftermath of the big-bang explosion that produced the expanding Universe. All of the most abundant element, hydrogen, that is now in the Universe was created back then, about 10 or 20 billion years ago, and so was most of the helium, the second-most abundant element. The hydrogen and helium were synthesized in the earliest stages of the infant Universe, within just a wink of the cosmic eye. As the Universe expanded, it cooled and thinned out, prohibiting primordial nucleosynthesis after the first few hundred seconds of the big bang.

The first stars could not have had rocky planets like the Earth, because there was initially nothing but hydrogen and helium. The only possible planets would have been icy balls of frozen gas. Without carbon, life as we know it could not evolve on these planets.

Stars that contained only hydrogen and helium are called first-generation stars. Middle-aged stars like the Sun are second-generation stars, meaning that some of their material came from previous stars. Sun-like stars contain heavy elements that were formed inside massive first-generation stars or at the time of their explosions (Fig. 1.14).

During the billions of years before the Sun was born, massive stars reworked the chemical elements, fusing lighter elements into heavier ones within their nuclear furnaces. Carbon, oxygen, nitrogen, silicon, iron, and most of the other heavy elements were made this way. The enriched stellar material was then cast out into interstellar space by the short-lived massive stars, gently blowing in their stellar winds or explosively ejected within supernova remnants.

The Sun and its retinue of planets condensed from this material about 4.6 billion years ago. They are partly composed of heavy elements that were synthesized long, long ago and far, far away, in the nuclear crucibles of stars that lived and died before the Sun was born. The Earth and everything on it have been spawned from this recycled material, the cosmic leftovers and waste products of stars that disappeared long ago.

Perhaps the most fascinating aspect of stellar alchemy is its implications for life on Earth. Most of the chemical elements in our bodies, from the calcium in our teeth to the iron that makes our blood red, were created billions of years ago in the hot interiors of long-vanished stars. We are true children of the stars, for we are all made of star stuff. If the Universe were not very, very old, there would not have been time enough to forge the necessary elements of life in the ancient stellar cauldrons. The same nuclear powerhouse that synthesized heavy elements from light ones, and made these stars shine, is now at work inside the Sun.

(page 3 of 8)

[<< prev] 1 2 3 4 5 6 7 8 [next >>]

Page 2

The Earth intercepts only a modest fraction of the energy being pumped out in all directions from the Sun. When we measure the total amount of sunlight that illuminates and warms our globe, and extrapolate back to the Sun, we find that it is emitting a power of 385.4 million, million, million, million, or 3.854 x 1026, watts. An enormous amount of energy is being expended. In just one second the energy output of the Sun equals the entire energy consumption of the United States for a million years.

The astonishing thing is the Sun’s durability; the Sun has managed to last billions of years despite radiating such awesome amounts of energy. In looking back at Earth’s history, we find that the Sun has been shining steadily and relentlessly for aeons, with a brilliance that could not be substantially less than it is now. The radioactive clocks in rock fossils indicate, for example, that the Sun was hot enough to sustain primitive creatures on Earth 3.5 billion years ago.

By the early 20th century, when your grandparents might have lived, no one had any clue as to why the Sun, or any other star, could shine so brightly for billions of years. That understanding had to await the discovery of the ingredients of the atom, observations that the Sun is mainly composed of hydrogen, and the realization that sub-atomic energy can be released within the extraordinary conditions inside stars.

(page 1 of 5)

Page 3

1 2 3 4 5 6 7 8 [next >>]

The Sun is our powerhouse, sustaining life on Earth. It energizes our planet and fuels the engine of life. The Sun warms our world, keeping the temperature at a level that allows liquid water to exist and keeps the Earth teeming with life. Without the Sun’s light and heat, all life would quickly vanish from the surface of our planet.

The Sun is not only warmth, it is light to. The miracle of plants is their ability to use sunlight to make them grow (Fig. 1.1), and in doing so they create another miracle. Using photosynthesis, plants use the Sun’s energy to convert water and carbon dioxide into carbohydrates, which releases the oxygen animals breath. Animals also eat these plants for nourishment. The warmth in every animal’s body was once sunlight. We all owe our lives to this savage Sun.

(page 1 of 8)

1 2 3 4 5 6 7 8 [next >>]

Page 4

We can look inside the Sun by observing the slow, rhythmic, in-and-out motions of the photosphere. These widespread throbbing oscillations are caused by internal sound waves. When the sounds strike the photosphere and rebound back down, they disturb the gases there, causing them to move in and out with a period of about five minutes.

The sound waves are trapped inside the Sun and cannot travel through the near vacuum of space. Nevertheless, when these sounds propagate upward to the photosphere they disturb the gases there and cause them to rise and fall, producing widespread throbbing motions (Fig. 4.1). These vertical oscillations can be tens of thousands of meters high and travel a few hundred meters per second. Such movements are imperceptible to the eye, but sensitive instruments on the ground and in space routinely pick them out. They are detected as tiny, periodic changes in the wavelength of a well-defined spectral line, or as miniscule variations in the Sun’s total light output.

When oscillations move part of the photosphere toward Earth, the wavelength of light emitted from that region becomes shorter, the wave fronts or crests appear closer together, and the light therefore becomes bluer. This shift occurs because each successive wave has a shorter distance to travel than the one before it did in order to reach Earth, so the distance between waves, the wavelength, becomes shorter. When the oscillations carry localized regions away from Earth, the wavelength becomes longer and the light redder. Each wave has farther to travel than the one before it did. The magnitude of the wavelength change, in either direction, establishes the velocity of motion along the line of sight, which is called the radial velocity.

(page 1 of 3)

Page 5

The entire Sun is a giant mass of incandescent gas, unlike anything we know on Earth. The Sun has no surface; its gas just becomes more tenuous the farther out you go. Although we cannot see it with our eyes, very diffuse solar gas extends all the way to the Earth and beyond.

The diaphanous solar atmosphere includes, from its deepest part outward, the photosphere, chromosphere and corona. The Sun’s magnetism plays an important role in molding, shaping and heating the coronal gas.

The visible photosphere, or sphere of light, is the part of the Sun we can watch each day. It is the level of the solar atmosphere from which we get our light and heat. The photosphere contains sunspots, thousands of times more magnetic than the Earth, and the number and position of sunspots varies over the 11-year cycle of solar magnetic activity.

The visible sharp edge of the photosphere is something of an illusion. It is merely the level beyond which the gas in the solar atmosphere becomes thin enough to be transparent. The chromosphere and corona are so rarefied that we look right through them, just as we see through the Earth’s clear air.

The chromosphere is very thin, but the Sun does not stop there. Its atmosphere extends way out in the corona, to the edge of the solar system. The corona’s temperature is a searing million degrees Kelvin, so hot that the corona is forever expanding into space.

The entire solar atmosphere is permeated by intense magnetic fields generated inside the Sun, rooted in the photosphere, and extending into the chromosphere and corona.

(page 1 of 3)

Page 6

Total Eclipse Of The Sun

Still higher, above the chromosphere, is the corona, from the Latin word for “crown”. The corona becomes momentarily visible to the unaided eye when the Sun’s bright disk is blocked out, or eclipsed, by the Moon and it becomes dark during the day. During such a total solar eclipse, the corona is seen at the limb, or apparent edge, of the Sun, against the blackened sky as a faint, shimmering halo of pearl-white light (Fig. 6.1). But be careful if you go watch an eclipse, for the light of the corona is still very hazardous to human eyes and should not be viewed directly.

A total eclipse of the Sun occurs when the Moon passes between the Earth and the Sun, and the Moon’s shadow falls on the Earth. In an incredible cosmic coincidence, the Moon is just the right size and distance to blot out the bright photosphere when properly aligned and viewed from the Earth. In other words, the apparent angular diameters of the Moon and the visible solar disk are almost exactly the same, so that under favorable circumstances the Moon’s shadow can reach the Earth and cut off the light of the photosphere. The Moon then acts just like your thumb with your arm stretched out and pointed at the Sun. At that distance from your eye, a thumb subtends an angle of about 30 arc-minutes, roughly the same as that of the Moon and the Sun.

A total eclipse does not happen very often. Since the Moon and the Earth move along different orbits whose planes are inclined to each other, the Moon only passes directly between the Earth and the Sun about three times every decade on average. Even then, a total eclipse occurs along a relatively narrow region of the Earth’s surface, where the tip of the Moon’s shadow touches the Earth (Fig. 6.2). At other nearby places on the Earth, the Sun will be partially eclipsed, and at more remote locations you cannot see any eclipse.

The low corona, that is close to the photosphere, shines by visible sunlight scattered by electrons there. This electron-scattered component of the corona’s white light has been named the K corona. It emits a continuous spectrum without absorption lines, and the K comes from the German Kontinum.

The amount of observed coronal light is proportional to the electron density integrated along the line of sight, so we can use observations of the K corona to infer the density of electrons there (Fig. 6.3). At the base of the corona there are almost a million billion (1015) electrons per cubic meter. These coronal electrons are so tenuous and rarefied that a million, billion cubic meters would only weigh one kilogram. Since protons are 1836 times more massive than electrons, they supply most of the corona’s mass. Since the corona is made from hydrogen atoms, there is one proton for every electron in the hot gas. The mass density in the low corona is about 10-12 kilograms per cubic meter.

The F corona is the more distant component of the corona’s white light. It extends from about two or three solar radii to far beyond the Earth (Fig. 6.3), and is caused by sunlight scattered from solid dust particles in interplanetary space. Unlike the K corona, the spectrum of the F corona includes dark Fraunhofer absorption lines, so the F stands for Fraunhofer. The faint light of the F corona is not polarized, with any preferred direction, but the K corona is, so polarization is another way to distinguish between the two components.

White-light coronal photographs show that the electrons can be confined within helmet streamers (Fig. 6.4), which are peaked like old-fashioned, spiked helmets once fashionable in Europe. At the base of helmet streamers, electrified matter is densely concentrated within magnetized loops rooted in the photosphere. Further out in the corona, the streamers narrow into long stalks that stretch tens of billions of meters into space. These extensions confine material at temperatures of about two million degrees Kelvin within their elongated magnetic boundaries.

Near the maximum in the activity cycle, the shape of the corona and the distribution of the Sun’s extended magnetism can be much more complex. The corona then becomes crowded with streamers that can be found close to the Sun’s poles (Fig. 6.5). At times of maximum magnetic activity, the width and radial extension oaf a streamer is smaller and shorter than at activity minimum. Near solar maximum, the global dipolar magnetic field of the Sun swaps its north and south magnetic poles, so a much more volatile corona can exist then.

Natural eclipses of the Sun occur every few years, and can then be seen from only a few, often remote places on the globe. So, scientists decided to make their own artificial eclipses by putting occulting disks in their telescopes to mask the Sun’s face and block out the photosphere’s intense glare. Such instruments are called coronagraphs, since they let us see the corona. The first coronagraph was developed in 1930 by the French astronomer Bernard Lyot, and the corona is now routinely observed with coronagraphs at mountain sites.

As Lyot realized, coronagraph observations are limited by the bright sky to high-altitude sites where the thin, dust-free air scatters less sunlight. He therefore installed one at the Pic du Midi observatory in the Pyrennes. The higher and clearer the air, the darker the sky, and the better we can detect the faint corona around the miniature moon in the coronagraph. They work best in space, where almost no air is left. Modern solar satellites, such as the Solar and Heliospheric Observatory or SOHO, use coronagraphs to get clear, edge-on views of the corona from outside our atmosphere (Fig. 6.6). Such satellites also use ultraviolet and X-ray telescopes to view the low corona across the face of the Sun, a development that followed the realization of the corona’s million-degree temperature.

The Corona’s Searing Heat

The solar corona defies expectations, for it is hundreds of times hotter than the underlying photosphere, which is closer to the Sun’s energy generating core. The Sun’s temperature rises to more than one million degrees Kelvin just above the photosphere at a temperature of 5,780 degrees Kelvin. Heat simply should not flow outward from a cooler to a hotter region. It violates the laws of thermodynamics, the branch of physics that deals with the movement and transfer of heat. These laws indicate that it is physically impossible to transfer thermal energy by conduction from the underlying photosphere to the much hotter corona. The high temperature of the corona also defies common sense; after all, when you sit farther away from a fire it becomes colder, not hotter. It is as if a cup of coffee was put on a cold table and suddenly began to boil.

The linkage between the chromosphere and the corona occurs in a very thin transition region, less than 100 thousand meters thick, where both the density and temperature change abruptly (Fig. 6.7). In the transition region, the temperature shoots up from 10,000 to more than a million degrees Kelvin, but the density decreases as the temperature increases in such a way to keep the gas pressure spatially constant. The corona then thins out and slowly cools with increasing distance from the Sun.

The Ultraviolet And X-Ray Sun

For studying the corona and identifying its elusive heating mechanism, scientists look at ultraviolet and X-ray radiation. This is because very hot material – such as that within the corona – emits most of its energy at these wavelengths. Also, the photosphere is too cool to emit intense radiation at these wavelengths, so it appears dark under the hot gas. As a result, the hot corona can be seen all across the Sun’s face, with high spatial and temporal resolution, at ultraviolet and X-ray wavelengths.

Since ultraviolet and X-rays are partially or totally absorbed by the Earth’s atmosphere, they must be observed through telescopes in space. This has been done using a soft X-ray telescope on the Yohkoh spacecraft, and with ultraviolet and extreme ultraviolet telescopes aboard the SOlar and Heliospheric Observatory, or SOHO for short, and the Transition Region and Coronal Explorer, or TRACE.

Observations at different temperatures can also be used to focus on different layers of the solar atmosphere. As an example, the spectral line of singly ionized helium, He II, at 30.4 nanometers, is thought to be formed at 60,000 degrees Kelvin, and it is therefore used to image structure in the lower part of the transition region, near the chromosphere (Fig. 6.8). The Sun is mottled all over in this ultraviolet perspective, like a cobbled road or a stone beach. Each stone is a continent-sized bubble of hot gas that flashes on and off in about 10 minutes. The whole Sun seems to sparkle in the ultraviolet light emitted by these localized brightening, known as blinkers. About 3,000 of them are seen erupting all over the Sun, including the darkest and quietest places at the solar poles.

Images at extreme-ultraviolet and X-ray wavelengths have shown that the hottest and densest material in the low corona is concentrated in magnetic loops. Indeed, Yohkoh’s soft X-ray images have demonstrated that the entire corona is stitched together by thin, bright, magnetized loops that shape, mold and constrain the million-degree gas (Fig. 6.9). Wherever the magnetism in these coronal loops is strongest, the coronal gas in them shines brightly at soft X-ray wavelengths (Fig. 6.10, Fig 6.11).

High-resolution TRACE images at the Fe IX, Fe XII and Fe XV lines, respectively formed at 1.0, 1.5 and 2.0 million degrees, have demonstrated that there is a great deal of fine structure in the coronal loops (Fig. 6.12). They have pointed toward a corona comprised of thin loops that are naturally dynamic and continually evolving. These very thin loops are heated in their legs on a time span of minutes to tens of minutes, after which the heating stops or changes, suggesting the injection of hot material from somewhere near the loop footpoints in the photosphere or below. The erratic changes in the rate of heating forces the loops to continuously change their internal structure.

The ultraviolet and X-ray emission of the Sun vary significantly over the 11-year cycle of magnetic activity. The ultraviolet emission doubles from activity minimum to maximum, while the X-ray brightness of the corona increases by a factor of 100. At the cycle maximum, when the sunspots and their associated active regions are most numerous, bright coronal loops dominate the X-ray Sun; at activity minimum the bipolar sunspots and their connecting magnetic loops have largely disappeared, and the Sun is much dimmer in X-rays (Fig. 6.13). However, the corona still stays hot at a minimum in its 11-year activity cycle when active regions go away; the million-degree gas is just a lot more rarefied and less intense.

Not all magnetic fields on the Sun are closed loops. Some of the magnetic fields extend outward, within regions called coronal holes. These extended regions have so little hot material in them that they appear as large dark areas seemingly devoid of radiation at extreme-ultraviolet and X-ray wavelengths (Fig. 6.14). Coronal holes are nearly always present at the Sun’s poles, and are sometimes found at lower solar latitudes. They are routinely detected by instruments aboard the SOHO, TRACE and Yohkoh spacecraft.

Solving The Heating Crisis

The temperature of the million-degree corona is not supposed to be hotter than the cooler photosphere immediately below it. Heat should not emanate from a cold object to a hot one any more than water should flow uphill. For more than half a century, scientists have been trying to identify the elusive heating mechanism that transports energy from the photosphere, or below, out to the corona. We know that sunlight will not do the trick, for the corona is transparent to most of it.

Magnetic waves provide a method of carrying energy into the corona. The ever-changing coronal loops are always being jostled, twisted and stirred around by motions deep down inside the Sun where the magnetism originates. A tension acts to resist the motions and pull the disturbed magnetism back, generating waves that propagate along magnetic fields, somewhat like a vibrating string. They are often called Alfvén waves after Hannes Alfvén who first described them mathematically. He pioneered the study of the interaction of hot gases and magnetic fields, receiving the Nobel Prize in Physics in 1970 for his discoveries in it.

However, once you get energy into an Alfvén wave, it is difficult to get it out. So there may be a problem in depositing enough magnetic-wave energy into the coronal gas to heat it up to the observed temperatures. Like radiation, the Alfvén waves seem to propagate right through the low corona without being noticeably absorbed or dissipated there.

Magnetic loops can heat the corona in another way – by coming together and releasing stored magnetic energy when they make contact in the corona. Internal motions twist and stretch the magnetic fields, slowly building up their energy. When these magnetic fields are pressed together in the corona, they merge, join and self destruct at the place where they touch, releasing their pent-up energy to heat the gas.

The SOHO spacecraft has provided direct evidence for such a transfer of magnetic energy from the solar photosphere into the low corona. Images of the photosphere’s magnetism, taken with SOHO, reveal tens of thousands of pairs of opposite magnetic polarity, each joined by a magnetic arch that rises above them. They form a complex, tangled web of magnetic fields low in the corona, dubbed the magnetic carpet (Fig. 6.15). The small magnetic loops rise up out of the photosphere and then disappear within hours or days. But they are continuously replenished by the emergence of new magnetic loops, rising up to form new magnetic connections all the time and all over the Sun.

The idea of powerful energy release during magnetic reconnection is not a new one. It was proposed decades ago to account for sudden, brief, intense explosions on the Sun, called solar flares, that can release energies equivalent to billions of terrestrial nuclear bombs. Converging flows in solar active regions apparently press oppositely-directed field lines together, releasing magnetic energy at the place that they join. The new, reconnected field lines can snap apart, accelerating and hurling energetic particles out into interplanetary space and down into the Sun.

Such bi-directional, collimated and explosive jets of material have been observed in ultraviolet images of the chromosphere outside active regions (Fig. 6.15). The magnetic interaction of coronal magnetic loops, driven together by underlying convective motions, also energizes at least some of the bright “points” found in X-ray images of the Sun. Unlike sunspots and active regions, the X-ray bright “points” are uniformly distributed over the Sun, appearing at the poles and in coronal holes, some almost as large as the Earth. Hundreds and even thousands of them come and go, fluctuating in brightness like small flares, apparently energized by magnetic reconnection.

(page 1 of 5)

Page 7

Active regions are the sites of sudden and brief explosions, called solar flares, that rip through the atmosphere above sunspots with unimagined intensity. In just 100 to 1,000 seconds, the disturbance can release an energy of 1024 Joule. A single flare then creates an explosion equivalent to 2.5 million nuclear bombs on Earth, each with a destructive force of 100 Megatons of trinitrotoluene, or TNT. All of this power is created in a relatively compact explosion, comparable in total area to a sunspot, and occupying less than one ten thousandth (0.01 percent) of the Sun’s visible disk.

For a short, while a flare can be the hottest place on the Sun, heating Earth-sized active regions to tens of millions of degrees Kelvin. The explosion floods the solar system with intense radiation across the full electromagnetic spectrum, from the shortest X-rays to the longest radio waves.

Although flares appear rather inconspicuous in visible light, they can briefly produce more X-ray and radio radiation than the entire non-flaring Sun does at these wavelengths. We can use this radiation to watch the active-region atmosphere being torn asunder by the powerful explosions; and then view the lesion being stitched together again.

During the sudden and brief outbursts, electrons and protons can be accelerated to nearly the speed of light. Protons and helium nuclei are thrown down into the chromosphere, generating nuclear reactions there. The high-speed electrons and protons are also hurled out into interplanetary space where they can threaten astronauts and satellites. Shock waves can be produced during the sudden, violent release of flare energy, ejecting masses of hot coronal gas into interplanetary space. Some of the intense radiation and energetic particle emission reaches the Earth where they can adversely affect humans.

Since flares occur in active regions, their frequency follows the 11-year magnetic activity cycle. The rate of solar flares of a given energy increases by about an order of magnitude, or a factor of ten, from the cycle minimum to its maximum. At the cycle maximum, scores of small flares and several large ones can be observed each day. Yet, even at times of maximum solar activity, the most energetic flares remain infrequent, occurring only a few times a year; like rare vintages, they are denoted by their date. Flares of lesser energy are much more common. Those with half the energy of another group occur about four times as often.

Solar flares are always located near sunspots and occur more often when sunspots are most numerous. This does not mean that sunspots cause solar flares, but it does suggest that solar flares are energized by the powerful magnetism associated with sunspots. When these magnetic fields in a solar active region become contorted, they want to release their pent-up energy, and when they do it is often in the form of a solar flare. This energy is suddenly and explosively released at higher levels in the solar atmosphere just above sunspots.

Flares in the Chromosphere

Our perceptions of solar flares have evolved with the development of new methods of looking at them. Despite the powerful cataclysm, most solar flares are not, for example, detected on the bland white-light face of the Sun. They are only minor perturbations in the total amount of emitted sunlight; every second the photosphere emits an energy of 3.86 x 1026 Joule, far surpassing the total energy emitted by any solar flare by at least a factor of one hundred.

Routine visual observations of solar explosions were made possible by tuning into the red emission of hydrogen alpha, designated Ha, at a wavelength of 656.3 nanometers, and rejecting all the other colors of sunlight. Light at this wavelength originates just above the photosphere, in the chromospheric layer of the solar atmosphere. For more than half a century, astronomers throughout the world have used filters to isolate the Ha emission, carrying out a vigilant flare patrol that continues today. Most solar observatories now have automated Ha telescopes, and some of them are used to monitor the Sun for solar flares by capturing images of the Sun every few seconds.

When viewed in this way, solar flares appear as a sudden brightening, lasting from a few minutes to an hour, usually in active regions with strong, complex magnetic fields. The Ha light is not emitted directly above sunspots, but is instead located between regions of opposite magnetic polarity in the underlying photosphere, near the line or place marking magnetic neutrality. They often appear on each side of the magnetic neutral line as two extended parallel ribbons (Fig. 7.1). The two ribbons move apart as the flare progresses, and the space between them is filled with higher and higher shining loops while the lower ones fade away.

The powerful surge of flaring hydrogen light is also detected by spacecraft observing at ultraviolet wavelengths. Detailed magnetic filaments have been resolved in the Lyman alpha, or Ly a, transition of hydrogen at 121.6 nanometers (Fig. 7.2).

X-Ray Flares

Since solar flares are very hot, they emit the bulk of their energy at X-ray wavelengths. For a short while, a large flare can outshine the entire Sun in X-rays (Fig. 7.3). The hot X-ray flare then dominates the background radiation of even the brightest magnetic loops in the quiescent, or non-flaring, solar corona. Because any X-rays coming from the Sun are totally absorbed in the Earth’s atmosphere, X-ray flares must be observed from satellites orbiting the Earth above our air.

Researchers describe X-rays by the energy they carry. There are soft X-rays with relatively low energy and modest penetrating power. The hard X-rays have higher energy and greater penetrating power. The wavelength of radiation is inversely proportional to its energy, so hard X-rays are shorter than soft X-rays.

The time profiles of a solar flare depend upon the choice of observing wavelength; when combined they provide detailed information about the physical processes occurring during solar flares. Hard X-rays are emitted during the impulsive onset of a solar flare, while the soft X-rays gradually build up in strength and peak a few minutes after the impulsive emission (Fig. 7.4). The soft X-rays are therefore a delayed effect of the main flare explosion.

The energetic electrons that produce the impulsive, flaring hard X-ray emission also emit radiation at microwave (centimeter) and radio (meter) wavelengths (Fig. 7.5). The similarity in the time profiles of the microwave and hard X-ray bursts on time scales as short as a second, suggests that the electrons that produce both the hard X-rays and the microwaves are accelerated and originate in the same place. Impulsive flare radiation at both the long, hard X-ray wavelengths and the short microwave wavelengths is apparently produced by the same population of high-speed electrons, with energies of 10 keV to 100 keV.

The soft X-rays emitted during solar flares are thermal radiation, released by virtue of the intense heat and dependent upon the random thermal motions of very hot electrons. At such high temperatures, the electrons are set free from atoms and move off at high speed, leaving the ions (primarily protons) behind. When a free electron moves through the surrounding material, it is attracted to the oppositely charged protons. The electron is therefore deflected from a straight path and changes its speed during its encounter with the proton, emitting electromagnetic radiation in the process (Fig. 7.6). This radiation is called bremsstrahlung from the German word for “braking radiation”. Scientists use measurements of the flaring X-ray power to infer the density of the electrons emitting the bremsstrahlung.

Observations from the Yohkoh spacecraft, launched on 30 August 1991, have confirmed and extended this understanding of X-ray flares. They have shown exactly where both the soft X-rays and hard X-rays are coming from, and confirmed the overall Neupert effect/chromospheric evaporation scenario. According to this picture, solar flare energy release occurs mainly during the rapid, impulsive phase, when charged particles are accelerated and hard X-rays are emitted. The subsequent, gradual phase, detected by the slow build up of soft X-rays, is viewed as an atmospheric response to the energetic particles generated during the impulsive hard X-ray phase.

With Yohkoh, the double-source, loop-footpoint structure of impulsive hard X-ray flares was confirmed with unprecedented clarity. It established a double-source structure for the hard X-ray emission of roughly half the flares observed in the purely non-thermal energy range above 30 keV. The other half of the flares detected with Yohkoh were either single sources, that could be double ones that are too small to be resolved, or multiple sources that could be an ensemble of double sources. As subsequently discussed, a third hard X-ray source is sometimes detected near the apex of the magnetic loop joining the other two; this loop-top region marks the primary energy release site and the location of electron acceleration due to magnetic interaction.

Two white-light emission patches were also detected by Yohkoh during at least one flare, at the same time and place as the hard X-ray sources (Fig. 7.7). This shows that the rarely-seen, white-light flares can also be produced by the downward impact of non-thermal electrons, and demonstrates their penetration deep into the chromosphere.

Gamma Rays From Solar Flares

Protons and heavier ions are accelerated to high speed during solar flares, and beamed down into the chromosphere where they produce nuclear reactions and generate gamma rays, the most energetic kind of radiation detected from solar flares. Thus, for a few minutes, during an impulsive phase of a solar flare, nuclear reactions occur in the low, dense solar atmosphere; they also occur all the time deep down in the Sun’s energy-generating core that is even denser and hotter. Like X-rays, the gamma rays are totally absorbed in our air and must be observed from space.

The protons slam into the dense, lower atmosphere, like a bullet hitting a concrete wall, shattering nuclei in a process called spallation. The nuclear fragments are initially excited, but then relax to their former state by emitting gamma rays. Other abundant nuclei are energized by collision with the flare-accelerated protons, and emit gamma rays to get rid of the excess energy (Fig. 7.8). The excited nuclei emit gamma rays during solar flares at specific, well-defined energies between 0.4 and 7.1 MeV. One MeV is equivalent to a thousand keV and a million electron volts, so the gamma rays are ten to one hundred times more energetic than the hard X-rays and soft X-rays detected during solar flares.

Solar Radio Bursts

The radio emission of a solar flare is often called a radio burst to emphasize its brief, energetic and explosive characteristics. During such outbursts, the Sun’s radio emission can increase up to a million times normal intensity in just a few seconds, so a solar flare can outshine the entire Sun at radio wavelengths. Although the radio emission of a solar flare is much less energetic than the flaring X-ray emission, the solar radio bursts provide an important diagnostic tool for specifying the magnetic and temperature structures at the time. They additionally provide evidence for electrons accelerated to very high speeds, approaching that of light, as well as powerful shock waves.

Radio bursts do not occur simultaneously at different radio frequencies, but instead drift to later arrival times at lower frequencies. This is explained by a disturbance that travels out through the progressively more rarefied layers of the solar atmosphere, making the local electrons in the corona vibrate at their natural frequency of oscillation, called the plasma frequency.

With an electron density model of the solar atmosphere (Fig. 7.9), the emission frequency can be related to height, and combined with the time delays between frequencies to obtain the outward velocity of the moving disturbance. Electron beams that produce type III radio bursts are moving at velocities of up to half the velocity of light, or 150 million meters per second. Outward moving shock waves that generate type II radio bursts move at a slower speed, at about a million meters per second.

Australian radio astronomers pioneered this type of investigation in the 1950s, using swept-frequency receivers to distinguish at least two kinds of meter-wavelength radio bursts. Designated as type II and type III bursts, they both show a drift from higher to lower frequencies, but at different rates. The most common bursts detected at meter wavelengths are the fast-drift type III bursts that provide evidence for the ejection of very energetic electrons from the Sun, with energies of about 100 keV. (One keV is equivalent to one thousand electron volts, and to an energy of 1.6 x 10-16 Joule.) These radio bursts last for only a few minutes at the very onset of solar flares and extend over a wide range of radio frequencies (Fig. 7.10).

The high-speed electrons that emit type III or type IV bursts spiral around the magnetic field lines in the low corona, moving rapidly at velocities near that of light, and send out radio waves called synchrotron radiation after the man-made synchrotron particle accelerator where it was first observed (Fig. 7.11). Unlike the thermal radiation of a very hot gas, the non-thermal synchrotron radiation is most intense at long, invisible radio wavelengths rather than short X-rays.

A giant array of radio telescopes, located near Socorro, New Mexico and called the Very Large Array, can zoom in at the very moment of a solar flare, taking snapshot images with just a few seconds exposure (Fig. 7.12). It has pinpointed the location of the impulsive decimeter radiation and the electrons that produce it. These radio bursts are triggered low in the Sun’s atmosphere, unleashing their vast power just above the apex of magnetic arches, called coronal loops, that link underlying sunspots of opposite magnetic polarity. Some of the energetic electrons are confined within the closed magnetic structures, and are forced to follow the magnetic fields down into the chromosphere. Other high-speed electrons break free of their magnetic cage, moving outward into interplanetary space along open magnetic field lines and exciting the meter-wavelength type III bursts.

(page 1 of 4)

Page 8

Neutrinos from the Sun

Neutrinos, or little neutral ones, are very close to being nothing at all. They move at or very near the velocity of light, have no electric charge, and have so little mass that until very recently scientists were not sure if neutrinos have any mass at all. Lacking any bonds to matter, neutrinos are extraordinarily antisocial. They just don’t like to interact with anything in the material world.

To neutrinos, the Sun is transparent, and large amounts of them move out into all directions of space at nearly the speed of light. About 3 million billion solar neutrinos enter every square meter of the Earth’s surface facing the Sun every second, and pass out through the opposite surface unimpeded. Each second there are about 100 billion ghostly solar neutrinos passing through the tip of your finger, and every other square centimeter of your body, whether you are indoors or outdoors, or whether it is day or night, and without your body noticing them, or them noticing your body. At night they go through the entire Earth before reaching you.

Detecting Neutrinos

The only way to detect solar neutrinos is through their exceptionally rare collisions with ordinary matter. Although the vast majority of neutrinos pass right through matter more easily than light through a window pane, there is a finite chance that a neutrino will interact with a sub-atomic particle. When this slight chance is multiplied by the enormous quantities of neutrinos flowing from the Sun, we conclude that once in a great while a solar neutrino will score a direct hit, and the resulting blast of nuclear debris can signal the existence of the otherwise invisible neutrino.

The Sun produces neutrinos with a range of energies, and both the amount and energy of solar neutrinos depend on the particular reactions that produced them. Their expected flux at the Earth is calculated using large computers that produce theoretical models, culminating in the Standard Solar Model that best describes the Sun’s luminous output, size and mass at its present age. The results of these calculations (Fig. 3.1) indicate that the great majority of solar neutrinos have the lowest energy, and that they are generated during the nuclear fusion of two protons, the reaction that initiates the proton-proton chain. Smaller amounts of high-energy neutrinos are produced from the decay of boron 8 during a rare termination of the proton-proton chain. Different neutrino detectors are sensitive to different energy ranges (Fig. 3.1), and so tell us about different nuclear reactions in the Sun.

Fortunately for science, a miniscule proportion of the Sun’s neutrinos do collide with more palpable sub-atomic particles, and when such a collision occurs inside a massive detector its effects reveal the neutrino’s presence. The neutrino detectors contain large amounts of material, literally tons of it, to measure even a few of the solar neutrinos. The massive tanks must also be placed deep underground, beneath a mountain or inside a mine, so that only neutrinos can reach them (Fig. 3.2). The thick layers of intervening rock are transparent to neutrinos, but they filter out other energetic particles, generated by cosmic rays, and shield the detectors from their confusing signals. So, all of the solar neutrino hunters work deep underground where the Sun never shines.

Massive, subterranean detectors have been snagging just a handful of elusive neutrinos for more than a quarter century. The first such experiment, constructed by Raymond Davis Jr. in 1967, is a 615-ton tank containing 100 thousand gallons of cleaning fluid, technically called perchloroethylene or “perc”.

This pioneering chlorine detector measured solar neutrinos for more than a quarter century, always detecting about one third of the expected amount (Fig. 3.3). It measures an average neutrino flux at the Earth of 2.55 ± 0.25 SNU. In contrast, the Standard Solar Model predicts that it should observe a flux of about 8.0 ± 1.0 SNU. This discrepancy between the number of detected neutrinos and the number predicted is known as the solar neutrino problem.

A second experiment, that began operating in 1987, was located 1,000 meters down in the Kamioka zinc mine under Mt. Ikena in the Japanese Alps. It was also limited to the rare, high-energy solar neutrinos, but it could uniquely tell where the neutrinos come from. The detector consisted of a 3,000-ton tank of pure water.

The Kamiokande experiment was updated in 1996 to a high-tech, $100 million Super Kamiokande status, also located deep underground in the Kamioka zinc mine. The new detector is a stainless steel cylinder, roughly 40 meters in diameter and height, that contains 50,000 metric tons, or 12.5 gallons, of ultra-pure water. About 13,000 light sensors are uniformly arrayed on all the inner walls of the cistern (Fig. 3.4). These photo-multiplier tubes are so sensitive that they can detect a single photon of light – a light level approximately the same as light visible on Earth from a candle on the Moon. Since there are practically no impurities in the very clean water, light can travel for almost 100 meters without being noticeably attenuated; for ordinary water its less than 3 meters. This means that the light sensors can monitor the entire water volume for the bluish Cherenkov light generated by an electron recoiling from a direct hit by a neutrino.

The Sudbury Neutrino Observatory, or SNO pronounced “snow”, is located 2,000 meters underground in a working nickel mine near Sudbury Ontario. Like the previous water detectors, it will see only boron-8 solar neutrinos with energies above 5 MeV. But unlike Super Kamiokande, the heart of the SNO detector contains heavy water. One thousand tons of heavy water, with a value of $300 million, has been placed in a central spherical cistern with transparent acrylic walls (Fig. 3.5, Fig. 3.6). Since the scientists cannot afford its cost, the heavy water has been borrowed from Atomic Energy of Canada Limited, who have stockpiled it for use in its nuclear power reactors – the heavy water moderates neutrons created by uranium fission in the reactors.

A geodesic array of 9,500 photo-multiplier tubes surrounds the vessel to detect the flash of light given off by heavy water when it is hit by a neutrino. Both the light sensors and the central tank are enveloped by a 7,800-ton jacket of ordinary water (Fig. 3.6), to shield the heavy water from weak natural radiation, gamma rays and neutrons, in the underground rocks. As with the other neutrino detectors, the overlying rock blocks energetic particles generated by cosmic rays. If the detector was put on the surface of the Earth, the high-energy, cosmic-ray particles would make the detector glow like a giant neon sign.

Solving the Solar Neutrino Problem

In one attractive solution to the solar neutrino problem, the neutrinos are produced at the Sun's center in the quantity predicted by the Standard Solar Model, but the neutrinos change form and switch identity as they propagate out from the Sun. The neutrinos have an identity crisis on their way to us from the center of the Sun, transforming themselves into a form that we have not yet detected from the Sun and a flavor that we have not yet tasted.

The resolution of the solar neutrino problem rests with the refinement of our detection techniques. Experiments currently under way should reveal new secrets of the Sun’s core, and settle the question of whether solar neutrinos switch identities while travelling to Earth. They will also tell us about neutrinos themselves, providing stringent limits to, or measurements of, the neutrino mass. This may yield an improved estimate of the mass of the Universe.

(page 1 of 1)

Page 9

The Sun is a magnetic variable star

Our lives depend on the Sun’s continued presence and steady output. It illuminates our days, warms our world, and makes life on Earth possible.The total amount of the Sun’s life-sustaining energy is called the “solar constant”, perhaps because no variations could be detected in it for a very long time. Yet, as reliable as the Sun appears, it is an inconstant companion. Its luminous output varies in tandem with the Sun’s 11-year magnetic activity cycle.

Stable detectors placed aboard satellites above the Earth’s atmosphere have been precisely monitoring the Sun’s total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant (Fig. 9.1). It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.

The Earth’s varying Sun-layered atmosphere

Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes, but it is not a simple fall-off. It falls and rises in two full cycles as we move off into space (Fig. 9.2).

The temperature decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for turning. Visible sunlight passes harmlessly through this region to warm the ground below. The temperature above the ground tends to fall at higher altitudes where the air expands in the lower pressure and becomes cooler. The temperature increases at greater heights within the next atmospheric layer, named the stratosphere. The Sun’s invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.

The threat of dangerous and even lethal ultraviolet rays caused world-wide concern when it was discovered that everyday, man-made chemicals are punching a hole in the ozone layer (Fig. 9.3). The chemicals, called chlorofluorocarbons or CFCs for short, were therefore completely banned by international agreement in 1990. Still, the ozone layer is not expected to regain full strength until well into the latter half of the twenty-first century.

The mesosphere, from the Greek meso for intermediate, lies just above the stratosphere. The temperature declines rapidly with increasing height in the mesosphere, reaching the lowest levels in the entire atmosphere. The main reason for the decreasing temperatures is the falling ozone concentration and decreased absorption of solar ultraviolet.

The temperature then begins to rise again with altitude in the ionosphere, a permanent, spherical shell of electrons and ions, reaching temperatures that are hotter than the ground. The ionosphere is created and heated by absorbing the extreme ultraviolet and X-ray portions of the Sun’s energy. This radiation tears electrons off the atoms and molecules in the upper atmosphere, thereby creating ions and free electrons that are not attached to atoms.

Solar X-rays and extreme ultraviolet radiation both produce and significantly alter the Earth’s ionosphere. Their greater intensity near the maximum of the 11-year magnetic activity cycle produces increased ionization, greater heat, and expansion of our upper atmosphere. At a given height, the temperature, the density of free electrons, and the density of neutral, unionized atoms all rise and fall in synchronism with solar activity over its 11-year cycle (Fig. 9.4). This Sun-induced change in the content and structure of the ionosphere affects its ability to mirror radio waves.

(page 1 of 2)

Page 10

Invisible magnetic fields emanate from the Earth, as well as the Sun. As early as 1600, William Gilbert, physician to Queen Elizabeth I of England, demonstrated that our planet is itself a great magnet, which explains the orientation of compass needles. It is as if there was a colossal bar magnet at the center of the Earth, with magnetic fields that emerge out of the south geographic polar regions, loop through nearby space, and re-enter at the north polar regions (Fig. 8.1). Since the geographic poles are located near the magnetic ones, a compass needle always points north or south. The magnetic fields are produced by electrically conducting currents in the Earth’s molten core, so the acts like it has a magnet buried at its center.

The dipolar (two poles) magnetic configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the Sun’s perpetual wind. The energy-laden, electrically-charged solar wind blows out from the Sun in all directions and never stops, carrying with it a magnetic field rooted in the star. Although it is exceedingly thin, far less substantial than a terrestrial breeze or even a whisper, the solar wind is powerful enough to mold the outer edges of the Earth’s magnetosphere into a changing asymmetric shape (Fig. 8.2), like a tear drop falling toward the Sun.

The solar wind pushes the magnetic field toward the Earth on the day side that faces the Sun, compressing the outer magnetic boundary and forming a shock wave. It is called a bow shock because it is shaped like waves that pile up ahead of the bow of a moving ship. The Sun’s wind drags and stretches the terrestrial magnetic field out into a long magnetotail on the night side of Earth. The magnetic field points roughly toward the Earth in the northern half of the tail and away in the southern. The field strength drops to nearly zero at the center of the tail where the opposite magnetic orientations lie next to each other and currents can flow (Fig. 8.2).

Thus, the Earth’s magnetosphere is not precisely spherical. It has a bow shock facing the Sun and a magnetotail in the opposite direction. The term magnetosphere therefore does not refer to form or shape, but instead implies a sphere of influence. The magnetosphere of the Earth, or any other planet, is that region surrounding the planet in which its magnetic field dominates the motions of energetic charged particles such as electrons, protons and other ions. It is also the volume of space from which the main thrust of the solar wind is excluded.Yet, some of the energetic particles in space do manage to penetrate the Earth’s magnetic defense.

The merging between the magnetic fields of the solar wind and the Earth is most effective if they are pointing in opposite directions. With this orientation, the two fields become linked, just as the opposite poles of two toy magnets stick together, and the solar wind particles can enter the magnetosphere.

The wind’s magnetic field will be dragged by the flow of the wind behind the Earth into its magnetotail, wrapping and clinging around the magnetosphere like saran wrap (Fig. 8.3). The magnetosphere can then be punctured in the tail, providing a back door entry that funnels some of the wind into the magnetosphere. The passing solar wind is slowed down by the connected fields and decelerates in the vicinity of the tail. Energy is extracted from the solar wind and drives a large-scale circulation, or convection, of charged particles within the magnetosphere (Fig. 8.3).

When solar wind electrons and protons enter the Earth’s domain, they also become trapped within it and cannot easily get out. In fact, the inner magnetosphere is always filled with a veritable shooting gallery of electrons and protons, trapped within two torus-shaped belts that encircle the Earth’s equator but do not touch it (Fig. 8.4). These regions are often called the inner and outer Van Allen radiation belts, named after James A. Van Allen who discovered them in 1958. Van Allen used the term “radiation belt” because the charged particles were then known as corpuscular radiation; the nomenclature is still used today, but it does not imply either electromagnetic radiation or radioactivity.

More than half a century before the discovery of the radiation belts, Carl Størmer showed how electrons and protons can be trapped and suspended in space by the Earth’s dipolar magnetic field. An energetic charged particle moves around the magnetic fields in a spiral path that becomes more tightly coiled in the stronger magnetic fields close to a magnetic pole. The intense polar fields act like a magnetic mirror, turning the particle around so it moves back toward the other pole.

Thus, the electrons and protons bounce back and forth between the north and south magnetic pole (Fig. 8.5). It takes about one minute for an energetic electron to make one trip between the two polar mirror points. The spiraling electrons also drift eastward, completing one trip around the Earth in about half an hour. There is a similar drift for protons, but in the westward direction. The bouncing can continue indefinitely for particles trapped in the Earth’s radiation belts, until the particles collide with each other or some external force distorts the magnetic fields.

(page 1 of 3)

Page 11

Page 12

Page 13

Page 14

Page 15

Page 16

Page 17

Page 18

Page 19

Page 20

The atmosphere colors the sky blue and makes sunsets red.
Oxygen, carbon dioxide and water are cycled around the globe.
The ocean waters were supplied to the young Earth by colliding comets and asteroids or by volcanoes.
The seasons are due to changes in the incident sunlight, caused when the relevant hemisphere is tilted toward or away from the Sun.
Invisible carbon dioxide and water vapor in our atmosphere help to warm the Earth by trapping the Sun’s heat and preventing some of it from being reflected back into space. This process, commonly known as the greenhouse effect, keeps the oceans from freezing and the Earth from becoming a frozen ball of ice.
The Earth’s upper atmosphere is heated and ionized by the Sun’s variable X-ray and ultraviolet radiation, respectively producing the ionosphere and stratosphere.
The ionosphere is hotter than the ground.
The ozone layer in our stratosphere is both created and modulated by solar ultraviolet radiation, while also protecting us from dangerous ultraviolet sunlight.
An ozone hole forms above the South Pole in the local spring. Chlorine compounds known as chlorofluorocarbons, which have been released into the atmosphere by past human activity, are destroying the ozone. Such compounds have now been outlawed by international agreement.
The Sun is a variable star, with a brightness that increases, decreases and increases again every 11 years.
The total solar irradiance of the Earth, the so-called solar constant, rises and falls in step with the 11-year magnetic activity cycle, but with a total recent change of only about 0.1 percent.
When sunspots cross the visible solar disk, they produce, in themselves, a brief dimming of the Sun’s radiative output, amounting to a few tenths of one percent for just a few days; a brightness increase caused by faculae and plage exceeds the overall sunspot decrease at times of high solar activity.
Over the Sun’s 11-year magnetic activity cycle, there are small changes in the amount of visible light emitted by the Sun, but there are huge variations in the Sun’s X-ray and ultraviolet output during the 11-year cycle.
Solar X-rays and extreme-ultraviolet radiation both produce and significantly alter the Earth’s ionosphere. The solar X-rays fluctuate in intensity by two orders of magnitude, or a factor of one hundred, during the Sun’s 11-year magnetic activity cycle. Near activity maximum greater amounts of X-rays produce increased ionization, greater heat, and expansion of our upper atmosphere, altering satellite orbits and disrupting communications.
During the past one thousand years, there have been century-long periods of very low solar activity.
Sun-like stars can vary in brightness by more than the solar constant changes observed so far.
The Earth is now hotter than any time during the previous 1,000 years. This global warming is attributed to carbon dioxide and other heat-trapping greenhouse gases, such as methane and nitrous oxide, which have been released into the atmosphere by humans. These gases have been increasing in the atmosphere for more than a century as wastes from industry, cars and trucks are added to the air.
By burning coal and oil, humans have increased the amount of carbon dioxide in the Earth’s atmosphere by 30 percent since the industrial revolution.
Rising air and ocean temperatures, retreating glaciers, rising sea levels, climate changes, and many alterations of physical and biological systems indicate that global warming is increasing and the added heat is here.
Incomplete knowledge of the role of clouds and the oceans in global warming make the computations uncertain.
If current emissions of carbon dioxide and other greenhouse gases go unchecked over the next 100 years, global warming could produce agricultural disasters in the world’s poorest countries, and rising seas with coastal flooding throughout the world.
The Kyoto Protocol limits the production of greenhouse gases, but the United States has not signed it and the developing countries are exempt from the agreement.
The major ice ages, which repeat every one hundred thousand years, are caused by astronomical rhythms that alter the angles and distances from which sunlight strikes the Earth.
The Milankovich cycles that produce variations in the amount of sunlight incident on Earth include changes in the Earth’s rotational wobble, axial tilt, and orbit shape, with respective periodicities of 23,000, 41,000 and 100,000 years.
Analysis of the deep ice core taken from Vostok, Antarctica reveal the roughly 100,000-year periodicity of the ice ages over the past 420,000 years, and indicate that transitions from glacial to warm epochs are accompanied by an increase in the atmospheric concentration of the three principal greenhouse gases – carbon dioxide, methane and nitrous oxide. A relativity small increase in the intensity of incident solar radiation, associated with a 100,000-year periodic change in the Earth’s orbit, may be amplified by an increase in greenhouse gases to produce a warm, interglacial epoch.

Page 21

A neutrino, or little neutral one, has no electrical charge, almost no mass, and travels at nearly the velocity of light.
The neutrino was proposed in 1933 by Wolfgang Pauli to explain radioactive beta decay, and first observed in 1956 by Clyde Cowan and Frederick Reines in Project Poltergeist involving a nuclear reactor on Earth. Reines received the 1995 Nobel Prize in Physics for this accomplishment.
The fusion reactions that generate the Sun’s energy also create great quantities of electron neutrinos that pass effortlessly through both the Sun and the Earth; billions of solar neutrinos are streaking right through you every second.
The amount and energies of the neutrinos produced by the Sun depend on the nuclear fusion reactions in its core.
Computer models are used to determine the flux of solar neutrinos expected at the Earth.
Massive subterranean neutrino detectors have captured a small number of the electron neutrinos generated by nuclear reactions in the Sun. These instruments detected only one-third to one-half the expected amount of electron neutrinos, a discrepancy known as the solar neutrino problem.
Solar neutrinos were first detected in 1967, in an experiment designed by Raymond Davis, Jr. It consisted of a huge vat of cleaning fluid placed deep underground in the Homestake Gold Mine near Lead, South Dakota. Solar neutrinos occasionally convert chlorine in the cleaning fluid into argon, but the amount of argon produced, and therefore the number of solar neutrinos, is less than that expected. Davis received the 2002 Nobel Prize in Physics for his pioneering detection of solar neutrinos.
A second solar neutrino detector, located in the Kamioka zinc mine in Japan, uses pure water to detect the neutrinos and to show that they come from the Sun. The 2002 Nobel Prize in Physics was shared by Masatoshi Koshiba for this experiment, which also detected a very few neutrinos from the supernova SN 1987A.
By measuring the internal velocity of sound waves, scientists have taken the temperature of the Sun’s energy-generating core, showing that it agrees with model predictions, at 15.6 million kelvin, and apparently ruling out any astrophysical solution to the solar neutrino problem.
Observations with the Super Kamiokande neutrino detector indicate that atmospheric neutrinos, generated by cosmic rays, change type and oscillate between flavors, therefore possessing a small mass.
The solar neutrino problem has now been resolved by observations at the Sudbury Neutrino Observatory, which uses heavy water. These observations indicate that solar neutrinos switch between types, or flavors, on their way from the center of the Sun to the Earth. Some of the Sun’s electron neutrinos transform into other types of neutrinos, the muon or tau neutrinos, which were undetectable by the pioneering neutrino experiments.
Beams of neutrinos generated by nuclear reactors on Earth have been sent through the planet and detected on its other side, confirming that the solar electron neutrino deficit is due to the transformation of some electron neutrinos into other types of neutrinos.

Page 22

There are three types of explosive phenomena resulting from magnetic reconnection in the low solar corona – they are solar flares, eruptive prominences and coronal mass ejections.
The relatively calm solar atmosphere can be torn asunder by sudden, brief outbursts called solar flares, the most powerful explosions in the solar system. In 100 to 1,000 seconds, they release energy equivalent to millions of 100-megaton hydrogen bombs exploding at the same time, and raise the temperature of Earth-sized regions in the low corona up to 20 million kelvin.
Large looping arches of magnetism, containing relatively cool material with a temperature of about 10,000 kelvin, can suddenly expand out into space; such erupting prominences or filaments are often associated with coronal mass ejections.
Coronal mass ejections rip out billions of tons of material from the corona, hurling it into interplanetary space in expanding magnetic bubbles that rapidly rival the Sun in size.
Solar flares occur in solar active regions, above sunspots where the magnetic fields are strong.
Solar flares are brief, sudden and unpredictable.
Solar flares are more frequent and powerful at the maximum of the Sun’s 11-year magnetic activity cycle.
Most solar flares cannot be seen in the visible sunlight that we detect with our eyes. Rare visible solar flares, detected with optical telescopes, are called white-light flares.
Solar flares can outshine the entire Sun at X-ray and radio wavelengths.
Flares are detected in the chromosphere by tuning into the red spectral line of hydrogen, the Balmer alpha transition at 656.3 nanometers. These chromospheric hydrogen-alpha flares often exhibit two parallel ribbons of light, attributed to energetic particles beamed down into the chromosphere along newly linked coronal loops.
Flare ribbons have been detected at extreme-ultraviolet wavelengths from TRACE, with high spatial and temporal resolution. It has been used to measure the separation speed of loop-footpoint ribbons and the rate of magnetic reconnection above them.
Hard X-rays are detected during the impulsive phase of solar flares, when electrons are accelerated to high velocities and hurled down to the footpoints of coronal loops.
Soft X-rays are emitted during the decay phase of solar flares, when material from the chromosphere rises up to fill newly linked coronal loops in a process called chromospheric evaporation.
Solar-flare X-rays are emitted by bremsstrahlung, or braking radiation, when electrons encounter protons.
The slow, smooth rise of the flare emission at soft X-rays resembles the time integration of a flare’s impulsive hard X-ray radiation. This Neupert effect suggests that the electron beams that give rise to the hard X-rays produce chromospheric evaporation.
Hard X-rays are emitted as the bremsstrahlung of high-speed, non-thermal electrons, accelerated in the low corona during the impulsive flare phase and beamed down toward the photosphere. Less energetic thermal electrons give rise to soft X-ray bremsstrahlung.
The Bragg Crystal Spectrometer aboard Yohkoh has been used to show that flaring gas is heated in the chromosphere from 10,000 to 20 million kelvin, flowing up into flaring coronal loops that produce the flare soft X-ray emission.
Yohkoh’s Hard X-ray Telescope has been used to discover non-thermal, loop-top hard X-ray sources located just above flaring loops detected by the Soft X-ray Telescope, suggesting that very energetic electrons are accelerated above the flare loops.
A flare particle acceleration site that is located about 50% higher than the soft X-ray flare loop heights has been inferred from electron time-of-flight measurements with the Compton Gamma Ray Observatory and from ground-based observations of upward and downward moving radio bursts.
Rare white-light flares correlate well with the flare hard X-ray emission, in both space and time, suggesting a similar origin.
Flare-associated protons and heavier ions can be beamed down into the lower solar atmosphere, producing brief, non-sustainable nuclear reactions with the emission of gamma-ray spectral lines and neutrons that move nearly at the speed of light.
Gamma-ray lines emitted during solar flares include the 0.511 MeV electron-positron annihilation line and the 2.223 MeV neutron capture line.
The first gamma-ray images of solar flares, in the 2.223 MeV neutron-capture line, have been obtained from an instrument aboard RHESSI, showing that the flaring gamma-ray sources can be spatially displaced from the hard X-ray ones.
Solar flares result from magnetic energy stored in the low solar corona.
Solar flares originate near the apex of coronal loops.
A solar flare is triggered by magnetic interaction and reconnection.
Explosive solar flares and CMEs can be ignited when magnetized coronal loops come together and reconnect in the low solar corona. Stored magnetic energy is released rapidly at the place where the magnetic fields touch and merge together.
During coronal magnetic reconnection, the stressed coronal loops partially annihilate each other; release magnetic energy stored in them, and reconnect into less-energetic, more stable configurations.
Signatures of magnetic reconnection include soft X-ray cusps and hard X-rays emitted in the low solar corona.
A filament is a region of relatively cool gas embedded in the corona, seen in absorption on the solar disk and suspended by a long, low-lying core magnetic loop. A prominence is a filament seen in emission at the edge, or limb, of the Sun.
An erupting prominence, or filament, can be caused by excessive shear of its low-lying, core magnetic loop, which can rise to open the overlying magnetic arches that straddle it.
Coronal Mass Ejections, abbreviated CMEs, are seen edge-on with a white-light coronagraph. It has an occulting disk that blocks the intense glare of the photosphere and permits the corona to be seen.
Many thousands of coronal mass ejections have been observed since the 1970s from space-borne coronagraphs. The LASCO instrument aboard SOHO has recorded images of more than ten thousand coronal mass ejections, and incidentally resulted in the discovery of more than a thousand comets.
At the minimum in the Sun’s 11-year activity cycle, coronal mass ejections appear near equatorial latitudes with an average rate of 0.5 events detected per day; near cycle maximum they appear at all latitudes, including the poles, with an average rate of 6 events per day.
Coronal mass ejections can exhibit a three-part structure – a bright outer front, followed by a darker, underlying cavity, surrounding a brighter core; but not all coronal mass ejections display these nearly circular shapes.
CMEs are huge, much bigger than solar flares or active regions; the CMEs rapidly become larger than the Sun.
Coronal mass ejections hurl between 1 and 50 billion tons, or 1012 to 5 x 1013 kilograms, of material out from the Sun at apparent speeds near the Sun of up to 3,400 kilometers per second. But most coronal mass ejections exhibit apparent speeds of between 300 and 500 kilometers per second.
CMEs generate shock waves that accelerate high-energy particles and may cause intense geomagnetic storms when directed toward Earth.
Coronal mass ejections are often associated with, and followed by, eruptive prominences that can reform into an arcade of magnetic loops detected in soft X-rays or at extreme ultraviolet wavelengths.
Coronal loops are sent into quivering oscillations in the aftermath of solar flares. Such oscillations can be used to infer the physical properties of the loops, in a technique known as coronal seismology.
Coronal mass ejections can generate waves that propagate across the entire Sun, which have been detected with the EIT instrument on SOHO.
Coronal mass ejections are associated with depleted regions of the corona, which have been detected as reductions in the soft X-ray emission and a related dimming of the Sun’s extreme-ultraviolet radiation.
The rate of occurrence of solar flares, erupting prominences and coronal mass ejections varies in step with the 11-year cycle of solar magnetic activity, becoming more frequent near the cycle maximum and suggesting an origin related to strong magnetic fields.
Coronal mass ejections and solar flares may be a manifestation of a similar energy-release process in the solar corona. Exceptionally fast and energetic coronal mass ejections are commonly accompanied by solar flares, and vice versa, but each type of solar outburst can occur without the other one and there appears to be no general cause-effect relation between the two.
Coronal loops can remain stable for days; weeks or even months, and then explode, perhaps when the magnetic loops become twisted into an unstable configuration.
When large, twisted sigmoid (S or inverted S) shapes appear in soft X-ray images of the Sun, they can give advance notice, by a few days, of mass ejections that might collide with the Earth.
Solar flares and coronal mass ejections most likely arise from coronal loops that have been sheared and twisted into non-potential configurations with free magnetic energy available to power the outbursts. This twist can be detected as active-region sigmoid structures in soft X-rays, and in images of both eruptive prominences and coronal mass ejections.
A twisted magnetic flux tube that emerges into the corona can become unstable to form an erupting prominence or coronal mass ejections. There are at least two models for the outbursts, and one of them includes the kink instability that sets in when there is too much twist.
GONG and SOHO MDI data have been used with the techniques of local helioseismology to detect deep swirling flows that circulate horizontally around solar active regions, and to show that the strength of this twisting flow is correlated with the intensity of X-ray flares emitted from active regions.
Helioseismic holography is used with SOHO MDI or GONG data to determine the location and magnitude of active regions anywhere on the backside of the Sun days before they rotate into view on the visible solar disk. Daily images of these unseen active regions are available on the web, and can be used to give advance warning of possible solar flares and coronal mass ejections when the active regions rotate to face the Earth.

Page 23

The Sun is the closest star to the Earth and the Sun provides the light and heat necessary to sustain life on our planet.
The mean distance between the Earth and the Sun is one astronomical unit, or 1 AU for short.
The AU is equal to 149.6 million kilometers.
The time for light to travel from the Sun to the Earth at the speed of light is 499 seconds, or about 8.3 minutes.
The time for light to travel from the next nearest star, Proxima Centauri, to the Earth is 4.24 years.
Electromagnetic radiation propagates by the interplay of electric and magnetic waves.
Radiation has a wavelength and a frequency.
The product of wavelength and frequency is equal to the speed of light c = 299.8 million meters per second.
All radiation travels at the speed of light in empty space.
The radiation we see with our eyes is called visible light.
Invisible radiation that we do not see with our eyes includes x-rays, gamma rays, infrared rays, ultraviolet rays and radio waves.
Visible light and radio waves emitted by cosmic objects reach the ground and are not absorbed in the Earth’s atmosphere.
Cosmic x-rays, gamma rays, infrared rays, and ultraviolet rays are absorbed in the Earth’s atmosphere.
The unit of energy is the Joule, and the unit of luminosity is the Watt, or 1 Joule per second.
The solar constant is the amount of solar radiant energy per unit time per unit area reaching the top of our atmosphere.
The Sun’s luminosity L = 3.828 x 1026 J s-1.
The radius of the Sun is R = 6.955 x 108 m = 695.5 million meters, or 109 times the size of the Earth.
The Sun’s visible disk has an effective temperature of 5,780 K.
Any hot gas emits thermal radiation.
The maximum intensity of thermal radiation occurs at a wavelength that is inversely proportional to the temperature.
The luminosity of thermal radiation increases the fourth power of the temperature and the square of the radius; this is known as the Stefan-Boltzmann law.
Bigger telescopes gather more radiation, enabling the detection of fainter objects. Bigger telescopes also detect finer details with greater angular resolution.
Visible light telescopes collect light using a mirror or a lens. They are respectively known as reflectors and refractors.
An interferometer uses two connected telescopes to achieve a resolution comparable to that of a single telescope as big as the separation of the two telescopes.

Page 24

Artists, writers and religions have revered the Sun.
Without the Sun's light and heat, life would quickly vanish from our planet.
The Sun drives our weather, warms our globe and energizes plants by photosynthesis.
When compressed, dead organic material that was originally energized by the Sun has been turned into fuel for our lights, cars and trucks.
Radiation from the Sun includes all the colors of sunlight, which make white light when combined, as well as invisible infrared, ultraviolet, radio and X-ray radiation.
The Sun is the only star close enough to resolve details in its atmosphere.
Physical conditions capable of sustained nuclear reactions occur within the Sun and not on Earth.
A spectrum is a display of radiation intensity as a function of wavelength.
From long to short waves, the colors of sunlight are red, orange, yellow, green, blue and violet.
Features in the spectrum of sunlight are known as absorption lines; they are used to identify the chemical ingredients of the Sun.
The absorption lines seen in the Sun's light are generated as emission lines when individual elements are vaporized in the terrestrial laboratory.
The absorption, or emission, lines act as a bar code that identifies, or fingerprints, the element that produced them.
Hydrogen is the most abundant element in the Sun. Hydrogen is also the most abundant element in most other stars and interstellar space.
Helium, the second most abundant element in the Sun, is so rare on Earth that it was first discovered in the Sun.
All of the hydrogen and most of the helium in the Universe were produced in the Big Bang that sent the galaxies fleeing in an expanding Universe.
The Sun is a second generation star, containing elements heavier than helium that were synthesized inside former stars that seeded space with these elements during their explosive death.
The carbon, oxygen and iron inside your body were synthesized inside other stars before the Sun was born, but your hydrogen was produced in the Big Bang.
The Sun's radiation carries energy out in every direction.
Different types of electromagnetic radiation differ in their wavelength, although they propagate at the same speed, known as the velocity of light and usually denoted by the lower-case letter c, with a value of about 300,000 kilometers per second.
The light travel time is equal to the distance divided by the velocity of light. for the Sun-Earth distance, the light travel time is 8.3 minutes or 499 seconds, while it takes 4.29 years for light to travel to us from the nearest star other than the Sun.
The observed wavelength of radiation increases or decreases when the motion of the source is respectively away or toward the observer; this Doppler effect also applies to sound waves.
The product of the wavelength and the frequency of radiation is equal to the velocity of light, c. So longer wavelengths correspond to lower frequencies, and shorter wavelengths correspond to higher frequencies.
The ability of radiation to interact with matter is determined by the energy of its photons.
The photon energy of radiation increases with decreasing wavelength. So short waves have more energy.
The Sun emits invisible radiation at short ultraviolet and X-ray wavelengths, detected from above the Earth's atmosphere, and at long radio wavelengths observed with gigantic radio telescopes on the ground. But the total intensity of the Sun's radiation emitted at any of these wavelengths is less than that emitted in visible sunlight.
A hotter gas emits its most intense radiation at shorter wavelengths. Intense X-rays are emitted from a million-degree gas, visible sunlight from a 5780-kelvin gas like the visible solar disk, and radio waves from interstellar space at 100 kelvin.
The development of visible light, or optical, spectroscopy permitted investigation of the magnetic fields, atmospheric motions and composition of the Sun.
The coronagraph uses an occulting disk to block out the intense glare of the photosphere, and permit observation of the faint light of the Sun's outer atmosphere, known as the corona.
Radio telescopes can be used to study violent activity on the Sun, and interferometric radio telescopes can have an angular resolution that is better than that obtainable with ground-based visible light, or optical, telescopes.
The development of artificial satellites and other spacecraft allowed scientists to study the Sun above the Earth's obscuring atmosphere, permitting a full and continuous view of the Sun's ultraviolet and X-ray radiation, and direct sampling of energetic particles and magnetic fields flowing from the Sun.
Sensitive and precise instruments aboard nine solar missions, named Yohkoh, Ulysses, Wind, SOHO, ACE, TRACE, RHESSI, Hinode and STEREO, detect otherwise invisible particles, ultraviolet and X-ray radiation and magnetic fields, tracing the flow of energy and matter from down inside the Sun to the Earth and beyond.

Page 25

In one second the Sun emits enough energy to supply civilization’s power requirements for a million years.
The Sun has been keeping the Earth warm enough to sustain life for at least 3.5 billion years.
Your grandparents had no clue as to why the Sun could shine so brightly for billions of years.
If the Sun shines, at its present rate, by converting gravitational potential energy into heat, it can last for only 31 million years.
The Sun is big and massive; it is 109 times the Earth’s diameter and 333,000 times its mass.
The mean mass density of the Sun is just a quarter of the average mass density of the Earth, indicating that the Sun is mainly composed of lighter material.
The Sun is about 4.6 billion years old, but the expanding Universe is about 14 billion years old.
The solar material must become hotter, and move faster, at deeper depths in the Sun to support the overlying weight. At the visible solar disk the temperature is 5780 kelvin and at the Sun center it is 15.6 million kelvin.
The Sun is mainly composed of hydrogen, and each hydrogen atom consists mostly of empty space with a single proton in the nucleus and a single orbiting electron.
Hydrogen atoms are torn into pieces by collisions inside the Sun, so their nuclear protons and orbiting electrons are free to move about in an ionized gas called plasma.
Plasma is the fourth state of matter, in addition to solid, liquid and gas. All stars and most of the Universe are plasma.
The material at greater depths in the Sun is compressed to larger densities, up to 13 times the density of solid lead at the center of the Sun. It is so hot down there that the protons and electrons behave like a gas in spite of the large density.
The Sun’s energy comes from fusing protons together within the hot, dense core of the Sun, thereby converting mass into energy.
Two protons tunnel through the electrical barrier produced by the repulsion of their like charges. This tunneling is a quantum mechanical thing, made possible because protons do not have exact positions and energies, and the tunneling does not happen very often.
The sequence of nuclear reactions in the Sun, called the proton-proton chain, converts four hydrogen nuclei, or four protons, into one helium nucleus.
Hans Bethe was awarded the Nobel Prize in Physics in 1967 for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production of stars like the Sun.
A helium nucleus is just 0.7 percent less massive than the four protons that went into making it, but this mass loss, denoted by ?m, liberates an awesome amount of energy, E, given by Einstein’s celebrated equation E = ? m c2, where c is the velocity of light.
When two protons fuse together, they produce a positron, the anti-particle of the electron, and an electron neutrino. The anti-matter positrons immediately collide with the material electrons, disappearing in a puff of energetic gamma-ray radiation, but the neutrinos pass effortlessly through both the Sun and the Earth.
The helium nucleus contains two neutrons and two protons, so two of the four protons that went into making a helium nucleus have to be neutralized.
Two positrons and two neutrinos are made each time a single helium nucleus is synthesized.
Only the core of the Sun is hot enough and dense enough to generate energy by sustained nuclear fusion reactions; they cannot occur within other parts of the Sun.
Energy generated in the core of the Sun slowly works its way out to the visible solar disk by radiation and convection.
Just outside the core, energy is transported by radiation, taking 170,000 years to travel through the radiative zone and arrive at the bottom of the convective zone.
As the radiation works its way out, it loses energy and is changed from short wavelength, energetic gamma rays to longer wavelength, less energetic visible sunlight.
Radiation is blocked by the convective zone, where energy is transported by currents of heated material, taking about 10 days to go from the bottom of the convective zone to its top.
The convective zone is capped by a thin layer of gas, the photosphere, where radiation is emitted as the sunlight we see.
High-resolution, white-light images of the photosphere reveal the tops of gases rising out of the Sun by convection. Known as the granulation, they resemble Benard convection formed when a gas or liquid is heated from below, as with a pot of boiling water.
At any moment, about a million granules can be seen in the white light of the visible solar disk, the photosphere. They mark the tops of gases rising out of the Sun by convection, each granule lasting for about 15 minutes before another one replaces it.
Velocity images of the photosphere reveal a large convective pattern, named the supergranulation, which moves mainly in the horizontal direction across the visible solar disk. The granules are superposed on this larger cellular supergranulation.
Our star began it life shining with only 70 percent of its present luminosity, slowing growing in luminous intensity as it aged. Assuming an unchanging terrestrial atmosphere, with the same composition and reflecting properties as today, the decreased solar luminosity would have caused the Earth’s global surface temperature to drop below the freezing point of water during its first 2.5 billion years. Yet, there is clear geological evidence that the Earth was never this cold, and must have had a warm climate in its early history. The discrepancy between the Earth’s warm climatic record and an initially dimmer Sun has come to be known as the faint-young-Sun paradox.
The faint-young-Sun paradox can be resolved if there was a stronger atmospheric concentration of greenhouse gases, such as carbon dioxide, methane or ammonia, in the Earth’s early history; the greater heating of the enhanced greenhouse effect could have kept the oceans from freezing. Another solution is that the young Sun was a bigger, brighter, hotter and more massive star than it is now, subsequently losing much of that mass in strong solar winds associated with its youth.
The Sun is getting brighter as time goes on because there is a steady increase in the amount of helium in its core, resulting in hotter temperatures there. Sunlight will be hot enough in three billion years to boil the Earth’s oceans away.
In about seven billion years, the Sun will use up the hydrogen fuel in its core, and it will then balloon into a red-giant star, melting the Earth's surface and turning the frozen satellites of the outer planets into seas of liquid water.

Page 26

The Sun’s unseen depths are explored by tracing the in-and-out vertical oscillations of the visible solar gases.
The oscillations detected in the visible solar disk have a period of about five minutes. They are caused by sound waves generated in the convective zone and trapped inside the Sun. The sounds are too low-pitched to hear even if they could get out of the Sun.
The five-minute oscillations are detected by the Doppler effect of a spectral line seen in the visible sunlight of the photosphere. When the pulsation is out toward us, the wavelength is shortened, and when the motion is into the Sun and away from us the wavelength becomes longer.
Observations of the five-minute solar oscillations have been used to detect low-pitched sound waves that travel to different depths within the Sun, enabling determination of the inner structure and dynamics of the Sun by the techniques of helioseismology.
Different sound waves follow different paths inside the Sun, penetrating to different depths.
Only certain notes are played inside the Sun; they are the sounds that hit the photosphere at the same place, over and over again. These sound waves are said to be moving within a resonant cavity.
Significant helioseismology results have been obtained with instruments aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, which has had a continued, uninterrupted view of the Sun for more than ten years. The Sun has also been followed 24 hours a day for years by a worldwide network of ground-based observatories known as the Global Oscillations Network Group, or GONG for short.
The Sun’s visible oscillations are the combined effect of about 10 million separate notes. They can be used to look deep inside the Sun, in much the same way that an ultrasonic scanner can peer inside a mother’s womb and map out the shape of an unborn infant.
The velocity of a sound wave depends on the temperature and composition of the material it travels in.
A small but definite change in sound speed has pinpointed the bottom of the convective zone. It is located at a radius of 0.713 ± 0.003 of the radius of the visible solar disk.
The abundance of elements in the Sun is inferred by comparing solar models of internal opacity and sound wave propagation to the helioseismology results, with small but important differences from the observed solar abundances.
By measuring the internal velocity of sound waves, helioseismologists have taken the temperature of the Sun’s energy-generating core, showing that it agrees with model predictions, at 15.6 million K, and apparently ruling out any astrophysical solution to the solar neutrino problem.
Motion splits the sound-wave velocity, as well as the oscillation periods, like the speed of an airplane traveling with or against the wind.
The visible solar disk rotates at different speeds at different solar latitudes, with a faster rate at the solar equator than the solar poles and a smooth variation in between.
Regions near the Sun’s visible poles rotate with slow speeds and rotation periods of 35 days, while the visible equatorial regions spin rapidly with rotation periods of 25 days.
Differential rotation persists through the convective zone to about a third of the way inside the Sun, where the rotation becomes uniform from pole to pole.
Just below the convective zone, the Sun is a rigid rotator with rotation speeds independent of latitude, like any solid body including the Earth. The rotation of the core of the Sun is unknown.
The Sun’s magnetism is probably sustained by dynamo action at the tachocline interface between the deep interior, which rotates at one speed, and the overlying gas that spins faster in the equatorial middle.
Helioseismic tomography, or time-distance helioseismology, has been used with SOHO Michelson Doppler Imager, abbreviated MDI, data to show that poleward motions, also called meridional flows, extend deep beneath the photosphere. They move with speeds of about 20 meters per second.
Helioseismology techniques have been used to show that after average rotation is removed from both MDI and GONG data, zonal bands of slower- and faster-rotation, sometimes called torsional oscillations, are detected, which extend throughout the convective zone. The zonal rotation bands migrate in solar latitude together with belts of sunspots over the 11-year solar activity cycle.
After subtracting the contributions of differential rotation, zonal rotation bands, and poleward meridional circulation, a swirling pattern of residual flows is found; this subsurface solar weather increases in complexity with the 11-year cycle of solar magnetic activity.
The Sun is not strictly spherical, and its asphericity changes with solar activity, at least in the outer layers of the Sun.
Helioseismic tomography has been used with SOHO MDI data to look beneath sunspots, and to discover that strong converging flows push and force the sunspot magnetic fields together, keeping sunspots from expanding at their edges and dispersing into the surrounding photosphere.
A 10-year sequence of photosphere observations with SOHO’s Global Oscillations at Low Frequencies, abbreviated GOLF, instrument exhibits a periodic structure that could be due to gravity waves, or g-modes, generated in the deeper parts of the Sun. If confirmed, this could also suggest that the solar core could be rotating three to five times faster than the radiative zone.