|Caption: The figure shows seismic images of the active region, NOAA AR 8194 just before (left frame, above) and during (center) its passage across the far-side solar meridian, on 27 and 28 March 1998, respectively. The right frame shows an NSO/Kitt Peak magnetogram of AR 8194 ten days later, when the region has passed onto the visible disk of the Sun.|
Charlie Lindsey and Doug Braun (Solar Physics Research Corporation) recently applied computational seismic holography to helioseismic observations from the SOHO spacecraft to obtain seismic images of active regions on the far surface of the Sun. The first images of active regions on the far side of the Sun are the most recent result of a long and fruitful collaborative effort between NSO and SPRC in the development of "local helioseismology" as a major new field of solar research during the 1990s.
These results open the door for a practical, inexpensive monitor of large active regions on the Sun's far side for general synoptic and space-weather-forecasting purposes. Active regions are the centers of energetic phenomena such as solar flares and coronal mass ejections whose electromagnetic and particle radiation interfere with telecommunications and power transmissions on Earth and threaten space-walking astronauts and spacecraft. Because the Sun rotates rapidly, with a synodic period of 27 days, flaring regions that appear suddenly on its east limb can affect conditions in the terrestrial neighborhood as they pass across the near solar surface. Real-time seismic imaging of the far side of the Sun will now allow us to anticipate large active regions one week or more before the flaring regions arrive at the east limb.
SPRC's program to detect images on the far side of the Sun was largely motivated by ongoing research at NSO/GONG that explained the frequency shifts of global modes over the solar cycle by active regions. Because the waves that are used to reconstruct the far-side images travel from the near side of the Sun to the far side and back, they interfere with their own multiple reflections in the Sun's interior. The result is a standing wave with a sharply defined frequency, called a p mode, similar to the harmonics that resonate in an organ pipe. An active region can be likened to a subtle dent in the organ pipe, slightly reducing its internal volume and thereby slightly raising its resonant frequency. As in the organ pipe, the resonant frequencies of solar p modes can essentially be regarded as independent of which side of the resonant cavity the active region is on. The same acoustic perturbations that are largely, perhaps entirely, responsible for shifting the resonant frequencies of solar p modes locate images of active regions on the far side of the Sun. The far-side images reinforce a growing consensus that reduced sound travel times in active regions may explain the entirety of the frequency shifts of global p modes with the solar cycle.
The work of Lindsey and Braun is described in some detail in articles that appear in the 10 March 2000 issue of Science.