Caption (click on the image for a larger version)
A great deal of energy is released in a large solar flare, as marked for example by RHESSI X-ray and γ-ray observations. In some flares, some of this energy is released in the form of a transient seismic wave that penetrates several thousand kilometers into the solar interior. Over the succeeding hour most of this wave is refracted back to the surface, where its arrival is manifested by an outgoing ripple, called a "sunquake" (see the movie and the information in the movie caption).
The first sunquake was discovered by Drs. A. G. Kosovichev and V. V. Zharkova, who recognized the surface ripples in seismic observations of the X2.6-class flare of 1996 July 9 spreading outward from the site of the flare during the succeeding hour. The discoverers gave at least a part of the explanation of the sunquake phenomenon: X-rays, such as those detected by RHESSI, are the signature of intense beams of high energy particles impinging into the lower solar atmosphere from the overlying corona. These particles plow into the lower solar atmosphere and heat it through a variety of mechanisms operating at various levels, causing a pressure transient that drives a seismic wave into the interior of the Sun.
It soon became evident after the flare of 1996 July 9, and perhaps somewhat of a disappointment, that most flares were seismically silent. Even those much more powerful than the flare of 1996 July 9 turned out to be seismically silent. For several years no further sunquakes were discovered, suggesting that the flare of 1996 July 9 was an exceptionally rare instance. For some reason, flares accompanied by sunquakes were special.
Following the launch of RHESSI, a comprehensive survey of seismic emission from solar flares was conducted by A.-C. Donea and D. Besliu-Ionescu. This resulted in the discovery of more than a dozen new sunquakes, all in the declining phase of Solar Cycle 23. Flares as "small" as M7 are now known to have emitted detectable seismic transients. Donea and Besliu-Ionescu used a technique called computational seismic holography in their comprehensive survey, which allows an analyst to image the source of seismic emission from the surface ripples that emanate from a seismically active flare such as that of January 15, 2005:
Observations of the same flares by RHESSI showed that the region of impulsive hard X-ray emission matched the time and location of the sources of the sunquake waves. The hard X-rays are known to be caused by high-energy particles accelerated from the corona into the underlying chromosphere of the active region. In powerful flares these particles are almost invariably focused into the penumbra of a sunspot. The sources of seismic emission from flares are now known to be regions of sudden, strong visible continuum emission by flares, what are known as white-light flares and described in an earlier RHESSI nugget.
The seismic energy radiated in a typical sunquake is estimated at several times 1027 ergs. This is approximately the energy that would be released if every man, woman and child on earth had his/her own personal city-nullifying nuclear bomb and we all contrived to detonate these in unison, a prospect that fortunately thus far remains comfortably beyond 21st century technology. The magnitude of 1027 ergs may perhaps be more readily grasped in the context of earthquakes. This amount of seismic energy in an earthquake would register not quite 12 on the Richter scale. This is nearly 10,000 times the seismic energy of the greatest earthquake of the 20th century, which measured approximately 9 on the Richter scale, devastating Valdivia and Puerto Montt, in Chile on May 22, 1960. As powerful as sunquakes are, the seismic energy released in them is only about a thousandth of the total energy radiated into space by a major solar flare in its impulsive phase alone.
The direct effects of a sunquake on earth are essentially imperceptible in practical terms, unlike other manifestations of the flares that produce them. However, sunquakes are telling us a lot about how flare work, particularly white-light flares. The strong correlation between the sources of seismic emission and white-light emission in flares suggests that sudden heating of the low photosphere during a white light flare is a major contributor to the pressure transient required to drive a sunquake. In some cases this heating may possibly be caused by high energy protons accelerated through the chromosphere and directly into the photosphere from the corona. However, we now know of instances of strong seismic emission from flares for which the signatures of high-energy protons is absent. These support a theory known as "back-warming," which proposes that the low photosphere is heated during a white-light flare by intense visible and near ultraviolet radiation shown downward from a chromosphere super heated by high-energy electrons from the corona.
Seismic transients from flares may also be useful for studying the subphotospheres underlying the active regions that produce the flares. The solar interior is transparent to seismic waves, just as glass is transparent to visible light. Helioseismologists hope that seismic emission from flares can be used as acoustic penlights to illuminate and probe thermal anomalies and flows beneath active regions.