The prototype of a new solar patrol telescope in New Mexico recorded a tsunami-like shock wave rolling across the visible face of the Sun following a major flare even on Wednesday, Dec. 2, 2006, at 18:28 Universal Time (11:28 MST). The shock wave, known as a Moreton wave, also destroyed or compressed two filaments of cool gas at opposite sides of the solar hemisphere.
"These large scale 'blast' waves occur infrequently, however, are very powerful. They quickly propagate in a matter of minutes covering the whole Sun, sweeping away filamentary material," said Dr. K. S. Balasubramaniam, of the National Solar Observatory (NSO) in Sunspot, NM, who is studying these and other phenomena. "It is unusual to see such powerful waves encompassing the whole sun from ground based observatories. Its significance comes from the fact that these waves are occurring near solar minimum, when intense activity is yet to pick up."
This large, "naked eye" sunspot region had an outburst just a day ago, and seems to have built-up tremendous energy in a short period of time.
The Optical Solar Patrol Network (OSPAN) telescope at NSO/Sunspot, was developed by NSO and the Air Force Research Laboratory as a major advance over the older Solar Optical Observing Network in operation since the 1970s. OSPAN monitors the Sun in H-alpha (656.3 nm), a deep red color emitted and absorbed by hydrogen gas in the chromosphere above the visible surface of the Sun. H-alpha is valuable for tracking solar active regions. OSPAN also observes in continuum (a stand-in for white light) and near-infrared.
OSPAN's images clearly show a large plage, or bright area, around a sunspot about the size of Earth at the heart of active region AR 10930. The plage indicates extremely hot gas and thus a likelihood of flare activity.
Observers were not disappointed. At 18:28 UT the region brightened and a large flare blew off. A shock wave propagated like the splash from a rock thrown into a pond. This was seen as a brightening from compressed and heated hydrogen gas. The flare also had a small white light component at one corner of the sunspot that brightened to about 100 times the average brightness of a Sun for a few minutes.
Some distance away from the flare, though, the wave caused a different effect: two filaments or ribbons, in the northeast and south central regions, winked out of view for a few minutes. Filaments are regions of cool hydrogen gas straddling magnetic field lines stretching from regions of north polarity to south polarity. They appear dark because they absorb H-alpha light.
As the shock wave swept across them they disappeared for a few minutes. Scientists are unsure whether the filaments were blown off or were compressed so they were temporarily invisible.
A valuable aspect of this event is that is occurred during solar minimum. That means that the wave's effects can be studied in H-alpha and other wavelengths, such as X-rays collected by satellites, without having to account for other active regions.
"Solar flares and mass ejections have consequences on the near-Earth space environment and impact communications," Balasubramaniam noted.
The National Solar Observatory is operated under a cooperative agreement between the Association of Universities for Research in Astronomy and the National Science Foundation. OSPAN is a project of the U.S. Air Force Research Laboratory and NSO.
Images and movies are available at http://www.nso.edu/staff/dooling/tsunami/