Abstract
Observations of solar flares with sunquakes by space- and ground-based instruments reveal essentially different dynamics of seismic events in different flares. Some sunquakes are found to be closely associated with the locations of hard X-ray (HXR) and white-light (WL) emission, while others are located outside either of them. In this article we investigate possible sources causing a seismic response in a form of hydrodynamic shocks produced by the injection of mixed (electron plus proton) beams, discuss the velocities of these shocks, and the depths where they deposit the bulk of their energy and momentum. The simulation of hydrodynamic shocks in flaring atmospheres induced by electron-rich and proton-rich beams reveals that the linear depth of the shock termination is shifted beneath the level of the quiet solar photosphere on a distance from 200 to 5000 km. The parameters of these atmospheric hydrodynamic shocks are used as initial condition for another hydrodynamic model developed for acoustic-wave propagation in the solar interior (Zharkov, Mon. Not. Roy. Astron. Soc. 431, 3414, 2013). The model reveals that the depth of energy and momentum deposition by the atmospheric shocks strongly affects the propagation velocity of the acoustic-wave packet in the interior. The locations of the first bounces from the photosphere of acoustic waves generated in the vicinity of a flare are seen as ripples on the solar surface, or sunquakes. Mixed proton-dominated beams are found to produce a strong supersonic shock at depths 200 – 300 km under the level of the quiet-Sun photosphere and in this way produce well-observable acoustic waves, while electron-dominated beams create a slightly supersonic shock propagating down to 5000 km under the photosphere. This shock can only generate acoustic waves at the top layers beneath the photosphere since the shock velocity very quickly drops below the local sound speed. The distance \(\Delta\) of the first bounce of the generated acoustic waves is discussed in relation to the minimal phase velocities of wave packets defined by the acoustic cutoff frequency and the parameters of atmospheric shock termination beneath the photosphere.
Highlights
During the past decade it became well established that flares can significantly impact the solar interior, as first predicted by Wolff (1972)
For the cases presented in this paper we find that the hydrodynamic theory in the solar interior with acoustic cut-off frequency (Zharkov, 2013) can logically explain the generation of the observable acoustic waves, or sunquakes, in solar flares observed in the datacubes of 180x180 Mm2
We have combined for the first time the two hydrodynamic models associated with solar flares to derive the conditions for favourable observation of acoustic waves, or sunquakes
Summary
During the past decade it became well established that flares can significantly impact the solar interior, as first predicted by Wolff (1972). The question remains as to how exactly these shocks deposit their energy into the solar interior (depths, timescale) and how they can be accounted for from a detailed comparison with seismic observations These are closely related to the other implications of particle precipitation into a flaring loop, like the formation of magnetic transients and non-thermal plasma ionization leading to prolonged white light emission from solar flares (Zharkova, 2008). In the case of the Ni 6768 ̊A line used by the instruments of the Global Oscillation Network Group (GONG) and MDI aboard the SOHO satellite to make magnetic measurements, the non-LTE simulations have shown that thermal heating occurring at the start of the impulsive phase is insufficient to turn this line into emission when the transient magnetic changes are recorded Instead, it can be caused by a large increase of non-thermal excitation and ionization caused by precipitating electrons, i.e. intense particle bombardment (Zharkova and Kosovichev, 2002).
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