7. The Violent Sun
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.
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).
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.
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 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.
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.
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.
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 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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University