Properties of flares-generated seismic waves on the Sun

The helioseismic waves excited by solar flares ("sunquakes") are observed as circular, expanding waves on the Sun's surface. The first sunquake was observed for a flare on July 9, 1996, by the Solar and Heliospheric Observatory (SOHO) space mission. This paper presents results of new observations and a detailed qualitative analysis of the basic properties of the helioseismic waves generated by four solar flares in 2003 - 2005. For two of these flares, the X17 flare of October 28, 2003, and the X1.2 flare of January 15, 2005, the helioseismology observations are compared with simultaneous observations of flare X-ray fluxes measured from the RHESSI satellite. These observations show a close association between the flare seismic waves and the hard X-ray source, indicating that high-energy electrons accelerated during the flare impulsive phase produced strong compression waves in the photosphere, causing the sunquake. The results also reveal new physical properties such as strong anisotropy of the seismic waves, the amplitude of which varies significantly with the direction of propagation. The waves travel through surrounding sunspot regions to large distances, up to 120 Mm, without significant distortion. These observations open new perspectives for helioseismic diagnostics of flaring active regions on the Sun and for understanding the mechanisms of the energy release and transport in solar flares.