New clues to what happens inside a solar flare have been provided by a new adaptive optics system that recently became operational at the nation’s premier telescope for high-resolution solar observing. The results are being presented today to the American Astronomical Society meeting in Denver by Thomas Rimmele and Kasiviswanathan Sankarasubramanian of the National Solar Observatory in Sunspot, NM.
The new high-order adaptive optics system (AO76) in the National Science Foundation’s Dunn Solar Telescope in Sunspot, NM, produced the data on Oct. 23, 2004, as the sunspot at the heart of active region 10486 erupted into a flare.
The AO76 system provided crisp, stable images to the new Diffraction Limited Spectropolarimeter (DLSP) and the older Universal Birefringent Filter (UBF) camera. The DLSP is a joint project of NSO and the High Altitude Observatory in Boulder, CO; Sankarasubramanian of NSO and Bruce Lites of HAO are co-principal investigators.
Using the Shack-Hartmann technique, the AO76 system divides the incoming view into 76 subimages. A computer determines how much to shift the subimages so they align with each other, then reshapes a deformable mirror 2500 times a second. This is the forerunner of a highly complex AO system, with about 1,200 subimages, that will be used by the NSO’s planned 4-meter Advanced Technology Solar Telescope. The AO76 became operational in March 2004. A few weeks later, the prototype AO24 system was upgraded to AO76 so the Dunn now has two high-order adaptive optics systems on call.
During Oct. 23-25, 2003, Rimmele and Sankarasubramanian used active region 10486 as a target during engineering tests. (The same active region produced a huge flare on Oct. 29.) The region was near the limb (18 deg. latitude, -66 deg. longitude). Observations reported in their paper were made from 18:14 to 19:34 UT, Oct. 23.
The UBF captured a movie of the flare in iron I (543.4 nm) in the upper photosphere (the lowest visible layer of the solar atmosphere) and in H-alpha (656.3 nm) in the chromosphere (the hot layer above the photosphere).
During the dynamic H-alpha core flare sequence, the observers were able to resolve fine structure in the flare ribbons at scales of 0.2 arc-second. They saw the footpoints of loops as well as individual loop tops brighten as the flare erupted.
Pre- and post-flare data from the DLSP show evidence of magnetic flux cancellation at small spatial scales that could not have been observed before the AO76 system was installed. (The DLSP only captured before-and-after data, 18:15-18:41 and 19:08-19:34 UT, because it was off line for a calibration period that coincided with the flare.) The DLSP was optimized to observe around the 630.15 and 630.25 nm iron lines associated with magnetic activity about 200 km (120 miles) above the photosphere. Analyzing the polarization and intensity of these lines determines the strength and direction of the magnetic fields at that altitude.
“The data indicate that the flare may have been triggered by these opposite polarities coming together and canceling each other out,” Rimmele said. Analysis of the observations is continuing, and that he and colleagues hope to extract more information that will lead to a better understanding of the underlying physics of a flare.
“We are looking forward to using the AO76 and DLSP for extended, high-resolution observations of new solar activities,” he added.
The observations were made during engineering test runs with two new instruments at the National Science Foundation’s Dunn Solar Telescope in Sunspot, NM. The NSO is owned and funded by the National Science Foundation, and operated under contract by the Association of Universities for Research in Astronomy.