Sunquake sources and wave propagation

Solar flares generate sunquakes observed as ripples on the solar surface. This helioseismic response to solar flares is caused by energetic particles penetrating into the lower atmosphere. The sunquake sources are observed directly in the MDI Dopplergrams as localized high-velocity impulses. The seismic sources are typically located in are ribbons and correlate very well with sources of hard X-ray emission produced by high-energy electrons and observed from RHESSI. Detailed analysis of SOHO/MDI data showed that the structure of sunquake sources can be quite complicated in space and time. The analysis of several sunquake events in 2003-5 revealed new important features such as the strong anisotropy and ellipticity of the seismic wave fronts, and it also showed much smaller than expected distortion of the fronts when the waves propagate through sunspots. The helioseismic waves tend to have the greatest amplitude in the direction of expansion of the are ribbons. This phenomenon is somewhat similar to the fault rupture effect in earthquakes, and can be explained by theoretical calculations of sunquake waves. In some cases, the wave anisotropy can be also caused by subsurface structures and organized flows. Investigation of sunquakes provides new insight into the physics of solar flares and new means for local helioseismic diagnostics.