A Search for Seismic Effects of Solar Flares, Mass Ejections and Comets

Lead Investigator: A.G. Kosovichev

Other Team Members: V.V. Zharkova

Abstract/Technical Summary

Use the SOI Dynamics data to detect signal from large flares, mass ejections and comets by measuring variations of amplitude of p-modes and by studying outgoing waves generated by the events.

Investigation Plan

Solar flares, coronal mass ejections, and comets which collide with the Sun are the most strongly localized disturbances on the solar surface, that generate seismic waves propagating into the Sun. They may contribute to the excitation of the solar oscillations (Wolff, 1972, Isaak, 1981).

Investigation of seismic response to solar flares is one of the primary objectives of the SOI. During the impulsive phase a high-energy electron beam heats the upper layers of the chromosphere, resulting in explosive evaporation of chromospheric plasma at supersonic velocities (e.g. Zarro et al., 1988; Zharkova and Brown, 1994). This upward motion is balanced by recoil of the lower part of the chromosphere downward into the Sun that excites propagating waves in the solar interior. A similar rebound effect may result from mass ejections in the chromosphere.

There are two principle effects to look for: 1) an increase of amplitudes of oscillation modes, and 2) waves traveling away from the flare.

There were at least two attempts to detect the response of the five-minute oscillations to solar flares. Haber et al (1988a, b) found a substantial increase in power of p modes of radial order 5 on the day after a major white-light flare. However, the power of the modes other than p₅ did not change significantly. They also found a substantial (19%) increase of power in outward traveling waves during the flare. In contrast, Braun and Duvall (1990) who observed another flare concluded that the power increase was below 10%.

Theoretical studies by Kosovichev & Zharkova (1995) have shown that for a typical impulsive energy-release event the momentum of the downflowing plasma is about 10²² gcms^-1 and the maximum amplitude increase of individual p modes will not exceed 1 mms^-1. Therefore, a seismic response to only a very large flare with multiple energy sources can possibly be detected in oscillation power spectra. It may be possible to detect a different kind of seismic response due to a coherent signal of outgoing waves, the amplitude of which can reach 1 ms^-1 for the typical event. Observations of seismic response to solar flares will provide important information about the flare mechanism and the subphotospheric structure of active regions.

A comet with the mass 10¹⁷ g, which is about the mass of Comet Halley, carries the momentum 7 x 10²³ gcm/s. Therefore, the cometary impact is 70 times stronger than the flare one. It should be observable in the power spectrum of high-degree modes, amplitude increase of which can reach 2 cm/s. However, the amplitudes of low-degree modes which are observed in whole-disk measurements (Isaak et al., 1984) will be only 0.5 cm/s higher after the impact. The seismic response from a comet could be also seen as the outgoing wave.

The investigation plan is

1. develop a data analysis technique to study outgoing waves from localized sources; this technique is similar to the time-distance method;

2. develop a Campaign Program for high-resolution observations of sites of potential strong flares;

3. study theoretical models of the seismic response to flares and mass ejections;

4. study amplitude variations of the oscillation power spectra after cometary impacts.

Bibliography

Braun, D.C., and Duvall, T.L., Jr., 1990, Solar Phys., 129, 83

Haber, D.A., Toomre, J., and Hill, F. 1988a, in IAU Symposium 123, Advances in Helio- and Asteroseismology., J. Christensen-Dalsgaard and S. Frandsen, eds., 59-62

Haber, D.A., Toomre, J., Hill, F. & Gough, D.O., 1988b, in: Seismology of the Sun and Sun-like Stars, ESA

Isaak, G.R., 1981, Solar Phys., 74, 43-49

Isaak, G.R., van der Raay, H.B., Pallé, P.L., and Roca Cortés, T. 1984, Mem. Soc. Astron. Ital., 55, 263-265

Kosovichev, A.G. and Zharkova, V.V., 1995, in: Proc. 4th SOHO Workshop, Helioseismology, ESA SP-376, ESTEC, Noordwijk

Wolff, C.L., 1972, Astrophys. J., 176, 833

Zarro, D.M., Canfield, R.C., Strong, K.T., Metcalf, T.R., 1988, Astrophys. J., 324, 582

Zharkova, V.V., and Brown, J.C., 1994, in: Proc. of the Third SOHO Workshop, Estes Park, Colorado, USA, 26-29 September, (ESA SP-373, December 1994), 61-65.