Seismic Emission from A M9.5-Class Solar Flare

Abstract

Following the discovery of a few significant seismic sources at 6.0 mHz from the large solar flares of October 28 and 29, 2003, we have extended SOHO/MDI helioseismic observations to moderate M-class flares. We report the detection of seismic waves emitted from the β γ δ active region NOAA 9608 on September 9, 2001. A quite impulsive solar flare of type M9.5 occurred from 20:40 to 20:48 UT. We used helioseismic holography to image seismic emission from this flare into the solar interior and computed time series of egression power maps in 2.0 mHz bands centered at 3.0 and 6.0 mHz. The 6.0 mHz images show an acoustic source associated with the flare some 30 Mm across in the East – West direction and 15 Mm in the North – South direction nestled in the southern penumbra of the main sunspot of AR 9608. This coincides closely with three white-light flare kernels that appear in the sunspot penumbra. The close spatial correspondence between white-light and acoustic emission adds considerable weight to the hypothesis that the acoustic emission is driven by heating of the lower photosphere. This is further supported by a rough hydromechanical model of an acoustic transient driven by sudden heating of the low photosphere. Where direct heating of the low photosphere by protons or high-energy electrons is unrealistic, the strong association between the acoustic source and co-spatial continuum emission can be regarded as evidence supporting the back-warming hypothesis, in which the low photosphere is heated by radiation from the overlying chromosphere. This is to say that a seismic source coincident with strong, sudden radiative emission in the visible continuum spectrum indicates a photosphere sufficiently heated so as to contribute significantly to the continuum emission observed.