First Sunquake of Solar Cycle 24 Observed by Solar Dynamics Observatory
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Introduction
The X2.2-class solar flare of February 15, 2011, produced a powerful "sunquake" event, representing a seismic response to the flare impact. The impulsively excited seismic waves formed a compact wavepacket traveling through the solar interior and appeared on the surface as expanding wave ripples. The Helioseismic and Magnetic Imager (HMI), instrument on SDO, observes variations of intensity, magnetic field and plasma velocity (Dopplergrams) on the surface of Sun almost uninterruptedly with high resolution (0.5 arcsec/pixel) and high cadence (45 sec). The flare impact on the solar surface was observed in the form of compact and rapid variations of the HMI observables (Doppler velocity, line-of-sight magnetic field and continuum intensity). These variations, caused by the impact of high-energy particles in the photosphere, formed a typical two-ribbon flare structure. The sunquake can be easily seen in the raw Dopplergram differences without any special data processing. The source of this quake was located near the outer boundary of a very complicated complicated sunspot region, NOAA 1158, in a sunspot penumbra and at the penumbra boundary. This caused an interesting plasma dynamics in the impact region. I present some preliminary results of analysis of the near-real-time data from HMI, and discuss properties of the sunquake and the flare impact sources.
"Sunquakes", the helioseismic response to solar flares, are caused by strong localized hydrodynamic impacts in the photosphere during the flare impulsive phase. The helioseismic waves are observed directly as expanding circular-shaped ripples on the solar surface, which can be detected in Dopplergram movies and as a characteristic ridge in time-distance diagrams or by calculating integrated acoustic emission. Solar flares are sources of high-temperature plasma and strong hydrodynamic motions in the solar atmosphere. Perhaps, in all flares such perturbations generate acoustic waves traveling through the interior. However, only in some flares is the impact sufficiently localized and strong to produce the seismic waves with the amplitude above the convection noise level. The sunquake events with expanding ripples are relatively rare, and observed only in some high-M and X-class flares. The last previous observation of the seismic waves was during X1.2 flare of January 15, 2005.
It has been found in the initial July 9, 1996, flare observations that the hydrodynamic impact follows the hard X-ray flux impulse, and hence, the impact of high-energy electrons. Thus, it was suggested that the mechanism of sunquakes can be explained by a hydrodynamic thick-target model. Several other mechanisms, including impact by [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=26 high-energy protons and back-warming heating], and also due to magnetic field variations. However, the mechanism, which converts a part of the flare energy and momentum into the seismic acoustic waves, is currently unknown. It is also unknown why only some flares generate such waves.
Most of the previous observations of sunquakes were obtained with the Michelson Doppler Imager instrument on SOHO. However, these observations did not provide uninterrupted coverage. Generally, the full-disk observations with the full 2 arcsec/pixel resolution were obtained only for 2 months a year. Thus, many flares were not observed, and the statistics of sunquakes and their relation to the flare properties were not established.
Except short eclipse periods in March and September, the HMI instrument provides uninterrupted observations of the Sun, and will help us to solve the mystery of `sunquakes'. The flare of February 15, 2011, was the first X-class flare of the new solar cycle, 24, and the first flare observed by HMI. I present some preliminary results of initial analysis, which reveal a sunquake event. This event had curious properties, which make it different from the previously observed sunquakes.
Results
The X2.2 flare of February 15, 2011, occurred in the central sunspot of active region NOAA 1158, which had a $\delta$-type magnetic configuration (Fig.~\ref{fig1}).
According to the GOES-15 soft X-ray measurements, the flare started at 01:44, reached maximum at 01:56 and ended at 02:06~UT. The flare signals are clearly detected in all HMI observables, and show that the flare had a typical two-ribbon structure with the ribbons located on both sides of the magnetic neutral line. This is particularly well seen in the running difference movie of continuum intensity (Fig.~\ref{fig2}).
References
[1] X-ray flare sparks quake inside Sun [2] Gasdynamics of a flare region heated by a stream of high-velocity electrons
