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

TL;DR

This paper investigates the helioseismic response to the February 15, 2011, X2.2 solar flare, revealing how flare impacts generate acoustic waves that provide insights into solar interior and sunspot structures.

Contribution

It presents detailed observations of flare-induced helioseismic waves and their initiation prior to hard X-ray emission, offering new methods for sunspot seismology.

Findings

Helioseismic waves were initiated before the hard X-ray impulse.

Wavefronts in sunspots had lower amplitude and were delayed.

Flare impacts caused rapid photospheric variations linked to low-energy electrons.

Abstract

The X2.2-class solar flare of February 15, 2011, produced a powerful sunquake event, representing a helioseismic response to the flare impact in the solar photosphere, which was observed with the HMI instrument on the Solar Dynamics Observatory (SDO). The impulsively excited acoustic waves formed a compact wavepacket 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 the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) shows that the helioseismic waves were initiated by the photospheric impact in the early impulsive phase, observed prior to the hard X-ray (50-100 keV) impulse, and were probably associated with atmospheric heating by relatively low-energy electrons (~6-50 keV) 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.