7. The Violent Sun

Solar flares

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 Sunís 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.

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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.

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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

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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 Earthís 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.

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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.

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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.

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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 ďbraking radiationĒ. 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.

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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 Sunís 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.

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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 Sunís 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.

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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.

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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).

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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.

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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 Sunís 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.

Coronal mass ejections and eruptive prominences

Another dramatic, magnetically energized type of solar explosion is called a Coronal Mass Ejection, or CME for short. These are giant magnetic bubbles that rapidly expand to rival the Sun in size, and hurl billions of tons of million-degree gas into interplanetary space at speeds of about 400 thousand meters per second, on average, reaching the Earth in about four days. Their associated shocks accelerate and propel vast quantities of high-speed particles ahead of them.

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Coronal mass ejections are detected during routine visible-light observations of the corona from spacecraft such the Solar and Heliospheric Observatory, or SOHO. With a disk in the center to block out the Sunís glare, the coronagraph sees huge pieces of the corona that are blasted out from the edge of the occulted photosphere (Fig. 7.13). The mass ejections are seen as moving loop-like features in light scattered by coronal electrons.

Coronal mass ejections are huge. Their average angular span of 45 degrees along the disk edge implies a size near the Sun that is comparable to that of the visible solar disk, and they can expand to even larger sizes further out.

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Coronal mass ejections usually have expanding curvilinear shapes that resemble the cross sections of loops, shells or filled bubbles, suggesting magnetically closed regions that are sporadically blown out by the eruption. The upper portions of the magnetic loops are sometimes carried out by the highly-ionized material, while remaining attached and rooted to the Sun at both ends. In other situations, the expelled material stretches the magnetic field until it snaps, taking the coiled magnetism with it and lifting off into space like a hot-air balloon that breaks its tether. Whenever, a big, closed loop of magnetism is unable to hold itself down, a coronal mass ejection takes off.

Coronal mass ejections often exhibit a three part structure - a smooth, bright outer loop or bubble of enhanced density, followed by a dark cavity of low density, within which sits an erupted prominence. The leading bright loop or shell is the coronal mass ejection that opens up and lifts off like a huge umbrella in the solar wind, piling the corona up and shoving it out like a snowplow. About 70 percent of the coronal mass ejections are associated with, and followed by, eruptive prominences (Fig. 7.14, Fig. 7.15).

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We can see the magnetic backbone of an erupting prominence regroup and close up again in soft X-ray images, retaining a memory of its former stability. When the prominence erupts, it is replaced by a row of bright X-ray emitting loops (Fig. 7.16), aligned like the bones in your rib cage or the arched trestle in a rose garden. First observed from Skylab, the X-ray loops bridge the magnetic neutral line between opposite polarity regions in the photosphere, stitching together and healing the wound inflicted by emptying that part of the corona.

Theories for explosive solar activity

Powerful solar flares involve the explosive release of incredible amounts of energy, sometimes amounting to as much a million, billion, billion (1024) Joule in just a few minutes. A substantial fraction of this energy goes into accelerating electrons and protons to very high speeds. Comparable amounts of energy are released in expelling matter during a coronal mass ejection, or CME, and they are most likely powered by similar processes to those that drive solar flares.

To explain how solar explosions happen, we must first know where their colossal energy comes from. The only plausible source of energy for these powerful outbursts is the strong magnetic fields in the low solar corona. After all, solar flares occur in active regions where the strongest magnetic fields are found. Both solar flares and CMEs are also synchronized with the Sunís 11-year cycle of magnetic activity, becoming more frequent and violent when sunspots and intense magnetic fields are most commonly observed.

Once the source of explosive energy has been established, we must explain where and why that energy is suddenly and rapidly let go. The energy release for solar flares has to occur in the low corona where the energetic particles are accelerated and the magnetic fields are strong enough to provide the necessary energy. Coronal mass ejections similarly require intense magnetic fields to be sufficiently energized, and their enormous size suggests an origin above the photosphere and chromosphere. The free magnetic energy needed to power the solar explosions is stored in the low corona in the form of non-potential magnetic field components, or, equivalently, as electric current systems.

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The free magnetic energy accumulates in the corona, but it comes from the dynamo below. Differential rotation and turbulent convective churning shuffle the photospheric footpoints of coronal loops, and these loops become sheared, twisted and braided. All of this distortion creates large electric current densities and non-potential magnetic fields within the coronal gas.

But what triggers the instability and suddenly ignites the explosions from magnetic loops that remain unperturbed for long intervals of time? They might be triggered when magnetized coronal loops, driven by motions beneath them, meet to touch each other and connect (Fig. 7.17). If magnetic fields of opposite polarity are pressed together, an instability takes place and the fields partially annihilate each other. Nevertheless there is still some magnetism left. The magnetic field lines are never permanently broken, and they simply reconnect into new magnetic configurations. The non-potential components of the magnetic fields are destroyed in this reconnection process, and their free magnetic energy is used to energize solar explosions.

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Yohkohís SXT even revealed the probable location of the magnetic reconnection site, showing that the rounded magnetism of a coronal loop can be pulled into a peaked shape at the top (Fig. 7.18). The sharp, cusp-like feature marks the place where oppositely directed field lines stretch out nearly parallel to each other and are brought into close proximity. Here the magnetism comes together, merges and reconnects, releasing the energy needed to power a solar explosion. Many long-lived (hours), gradual soft X-ray flares show cusp-shaped loop structures suggesting magnetic reconnection, and they are often associated with coronal mass ejections.

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Evidence for magnetic reconnection during the compact, short-lived (minutes) impulsive solar flares has been obtained by observing them at the limb of the Sun. Co-aligned Yohkoh images show a compact, impulsive hard X-ray source well above and outside the corresponding soft X-ray loop structure, in addition to the double-footpoint hard X-ray emission (Fig. 7.19). The hard X-rays at the loop footpoints are produced when high-speed, non-thermal electrons collide with dense material in the chromosphere. However, there is no similar dense material out in the tenuous corona, so the hard X-ray emission out there has to be emitted during the electron acceleration process. The loop-top source most likely represents the site where oppositely-directed magnetic fields meet and electrons are accelerated to high energy. These electrons rapidly move down toward the footpoints, explaining the similarity of the time variations of all three hard X-ray sources.

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As previously noted, solar radio astronomers have, in the meantime, shown that flare energy release and the acceleration of high-energy electrons occurs near the tops of coronal loops. The demarcation region between downward-directed and upward-directed electron beams, observed during some type III radio bursts, pinpoints the acceleration site. The electron density at this place, inferred from the plasma frequency, is about 1016 electrons per cubic meter. This is one to two orders of magnitude, or 10 to 100 times, less than the density of the soft X-ray flare loops, indicating that the acceleration site is above these loops. Moreover, electron time-of-flight measurements with the Compton Gamma Ray Observatory (CGRO) satellite, confirm the existence of a coronal acceleration site for flares observed with both CGRO and Yohkoh (Fig. 7.20).

In summary, a well-developed magnetic theory for solar explosions has received substantial observational verification. These violent outbursts originate in the low solar corona, where free magnetic energy, associated with non-potential magnetic fields, is stored. Thanks to the detailed views afforded by radio telescopes on the ground and X-ray observatories in space, we now know that the explosions are frequently triggered in compact structures just above the tops of coronal loops. Magnetic fields of opposite magnetic polarity are probably driven together there.

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During an impulsive solar flare, the free magnetic energy released during magnetic reconnection is converted to charged particle kinetic energy. In less than a second, electrons are accelerated to nearly the speed of light, producing intense radio signals. Protons are likewise accelerated, and both the electrons and protons can be hurled down into the Sun and out into space. The downward moving beams strike the denser chromosphere below, producing nuclear reactions and creating X-rays and gamma rays (Fig. 7.21, 7.22, 7.23).

Predicting explosions on the Sun

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The low solar corona is in a constant state of agitation and metamorphosis. Coronal loops are magnetically reconfigured as they twist and writhe in response to internal differential rotation and convection motions (Fig. 7.24). Yet, the coiled magnetic fields hold their energy in place, and remain without substantial change for days, weeks and even months at a time, like a rattlesnake waiting to strike. Then they suddenly and unpredictably go out of control, igniting an explosion that rips the magnetic cage open and breaks its grip apart.

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Scientists may have discovered how to predict the sudden and unexpected outbursts. When the bright, X-ray emitting coronal loops are distorted into a large, twisted sigmoid (S or inverted S) configuration, a coronal mass ejection from that region becomes more likely (Fig. 7.25). In some instances, a coronal mass ejection occurs just a few hours after the magnetic fields have snaked past each other in a sinuous S-shaped feature. The mass ejection arrives at the Earth three or four days later. In the meantime, just after the mass had been expelled from the Sun, the X-ray emitting region dramatically changes shape, exhibiting the tell-tale, cusp-like signature of magnetic reconnection and a X-ray fading or dimming due to the mass removal. In other words, the magnetism gets stirred up into a complex, stressed and twisted situation before it explodes.

Now scientists can use sound waves to see right through the Sun to its hidden, normally-invisible, back side, enabling them to monitor active regions before they rotate to face the Earth. The new technique, dubbed helioseismic holography, examines a wide ring of sound waves that emanate from a region on the side of the Sun facing away from the Earth (the far side) and reach the near side that faces the Earth. When a large active region is present on the back side of the Sun, its intense magnetic fields compress the gases there, making them slightly lower and more dense than the surrounding material. A sound wave that would ordinarily take 6 or 7 hours to travel from the near side to the far side of the Sun and back again takes approximately 12 seconds less when it bounces off the compressed active region on the far side. When near-side, photosphere oscillations are examined by SOHOís Michelson Doppler Imager, or MDI, they can detect the quick return of these sound waves.