Helioseismic response to the X2.2 solar flare of 2011 February 15

The X2.2-class solar flare of 2011 February 15 produced a powerful "sunquake" event, representing a helioseismic response to the flare impact in the solar photosphere, which was observed with the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO). The impulsively excited acoustic waves formed a compact wave packet traveling through the solar interior and appearing on the surface as expanding wave ripples. The initial flare impacts were observed in the form of compact and rapid variations of the Doppler velocity, line-of-sight magnetic field, and continuum intensity. These variations formed a typical two-ribbon flare structure, and are believed to be associated with thermal and hydrodynamic effects of high-energy particles heating the lower atmosphere. The analysis of the SDO/HMI and X-ray data from RHESSI shows that the helioseismic waves were initiated by the photospheric impact in the early impulsive phase, observed prior to the hard X-ray (50-100keV) impulse, and were probably associated with atmospheric heating by relatively low-energy electrons (6-50keV) and heat flux transport. The impact caused a short motion in the sunspot penumbra prior to the appearance of the helioseismic wave. It is found that the helioseismic wave front traveling through a sunspot had a lower amplitude and was significantly delayed relative to the front traveling outside the spot. These observations open new perspectives for studying the flare photospheric impacts and for using the flare-excited waves for sunspot seismology.