Observation of a Helioseismically Active Solar Flare with a Low Hard X-ray Flux up to 50 keV

Abstract: We consider the M1.1-class solar flare occurred on July 5, 2012, at UT. This event is unique in that a helioseismic perturbation was detected in it despite its low hard X-ray flux in the 25–50 keV energy band and its very soft hard X-ray spectrum. As a rule, most of the known sunquakes have been detected in solar flares with large hard X-ray fluxes at high energies (at least up to 100–300 keV). The event under consideration contradicts the popular hypothesis about the generation of sunquakes by beams of accelerated high-energy electrons. An analysis of the available RHESSI X-ray spectra shows that they can be explained in two ways. The X-ray spectrum in the 25–50 keV energy band is explained by a power-law distribution of accelerated electrons with an power-law index of 7–9 or by the presence of a superhot plasma with a temperature –60 MK. In both cases, we are dealing with electrons of relatively low energies that either were responsible for the sunquake generation or should be considered as a secondary (accompanying) phenomenon with respect to the true cause of the photospheric perturbation. The results of a joint analysis of the X-ray and microwave spectra are presented for the first time for a helioseismically active solar flare. Our analysis shows that the spectra in both ranges can be well explained by the emission of a superhot magnetized plasma and not by accelerated electrons with a soft spectrum. However, the explanation of the spectra when considering partially magnetically trapped accelerated electrons is also possible. We have estimated the parameters of the thermal plasma, accelerated electrons, and energy fluxes of various types. We analyze the dynamics of ultraviolet and X-ray emission sources. We also present an analysis of the magnetic field structure based on vector magnetograms and the nonlinear force-free coronal magnetic field extrapolation. We discuss the mechanisms for the generation of the helioseismic perturbation during the solar flare under consideration. An eruptive process could probably be both primary and secondary causes of the sunquake. The appearance of a superhot plasma in the corona could give rise to propagating thermal fronts into the lower layers of the solar atmosphere, where helioseismic waves are excited. Our analysis does not allow the possibility of the sunquake generation by accelerated electrons with a soft spectrum to be ruled out either.