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