http://arxiv.org/abs/1310.5449
The solar active region NOAA 11158 produced a series of flares during its passage through the solar disk. The first major flare (of class X2.2) of the current solar cycle occurred in this active region on 2011 February 15 around 01:50 UT. We have analyzed the Dopplergrams and magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) instrument onboard Solar Dynamics Observatory (SDO) to examine the photospheric velocity and magnetic field changes associated with this flare. The HMI instrument provides high-quality Doppler and magnetic maps of the solar disk at 0.5 arcsec spatial scale at a cadence of 45 s along with imaging spectroscopy. We have identified five locations of velocity transients in the active region during the flare. These transient velocity signals are located in and around the flare ribbons as observed by Hinode in Ca II H wavelength and the footpoints of hard X-ray enhancement in the energy range 12-25 keV from RHESSI. The changes in shape and width of two circular polarization states have been observed at the time of transients in three out of five locations. Forward modeling of the line profiles shows that the change in atmospheric parameters such as magnetic field strength, Doppler velocity and source function could explain the observed changes in the line profiles with respect to the pre-flare condition.
Date added: Tue, 22 Oct 13