Astron. Astrophys. 320, L13-L16 (1997)

1. Introduction

An interpretation of simultaneous observations on board of the SMM satellite of impulsive solar events in different hard X-ray bands (MacKinnon et al. (1985), Duijveman et al. (1982)) and in microwave emission (Kosugi (1986), Schmahl et al. (1986)) revealed some discrepancies in the explanation by a pure collisional model of electron beam precipitation. The footpoint X-ray fluxes, obtained by HXIS in a range of 16-30 keV, were found to be only 15-28 of the fluxes, extrapolated to the same energy range from the HXRBS spectra, observed at higher energies of 25-300 keV. A possibile flattening of the spectrum at energies below 30 keV could explain this result, but the HXIS emission has much bigger spectral index than the HXRBS spectrum.

Similar time profiles of X-ray and microwave emission are likely to be provided by a common particle acceleration, but there is a noticeable decrease (up to 3 orders of the magnitude) in total microwave fluxes, than those in X-rays. This allows to suggest that microwave emission is caused by higher energy electrons with keV, whose abundance is 3 orders of magnitude lower than those for lower energy electrons. From comparison of hard X-ray emission, obtained by HXRBS, and microwave radiation, observed by VLA, the hard X-ray indices are higher by 1-2 units than those deduced from microwaves (see Kosugi, (1986, and Schmahl et al. (1986)). More recent observations with the Nobeyama Radioheliograph and their comparison with the Yohkoh Hard X-ray Telescopes, done by Nishio et al. (1995), led to similar conclusions. An interpretation of microwave and hard X-ray observations by non-thermal electron beams in a pure collisional approach showed that the microwave spectra normally are flatter (harder) by 1-2 units than those derived from hard X-rays.

In order to understand these discrepancies, more advanced kinetic simulations were required and these have been carried out in the past decade. An electron beam dynamics was considered taking into account anisotropic scattering in electric (Diakonov and Somov (1988, Emslie (1980)) and converging magnetic fields (Karlicky et al., (1990, Leach and Petrosian (1981), McClements (1992), Syniavskii and Zharkova (1994), Zharkova et al. (1995)). In collisional model with Ohmic heating, return current energy losses in the fully ionised coronal plasma were shown to have a noticeable effect on the injected beam dynamics in depth, particularly at the chromospheric level (Emslie (1980)). They reduce a penetration distance of beam electrons, responsible for bremsstrahlung emission, and, therefore, increase total energy flux, required for the production of this emission (LaRosa and Emslie (1988), Diakonov and Somov (1988)).

The return current losses were overestimated in the partially ionized ambient plasma taking into account anisotropic scattering, and were found to vary strongly in depth (Syniavskii and Zharkova (1994, hereafter Paper 1). At low coronal and upper chromospheric levels, where the ambient plasma is completely ionised, the induced electric field governs completely an injected beam dynamics. Less powerful beams with smaller upper energy limit do not precipitate to the chromosphere, but lose their energy at higher levels, transforming into electrons of return current with nearly a Maxwellian distribution. More powerful beams precipitate to chromospheric levels with wider angular distributions and smaller abundances of low energy electrons than in the initial beams. These results emphasize the importance of return current effects, but still could not give an explanation of the observational discrepancies.

A self-consistent solution of the equation for induced electric field and of kinetic equation, for beam electrons with anisotropic scattering in a presence of electric and converging magnetic fields (Zharkova et al. (1995), confirmed the previous results for coronal levels. But at the transition region and chromosheric levels, where the ambient plasma is partially ionized, the return current effect was considerably decreased. It results that the full electron beam thermalization, caused by return current, is occurred at lower column depth but at higher upper energy limit . At the chromospheric level, instead of the steep fall at lower energies found in the model with collisions and Ohmic losses, electron distributions revealed an energy depression (dip) at lower energies (22-25 keV) which divides electron energy distributions into 2 parts. Before the dip () the energy distributions have a single power law, but with higher spectral indices than the initial ones. The dip is followed by maximum with nearly normal distribution, and after the dip, at higher energies (), the distributions have a power law again with the initial spectral indices.

As it was shown by Emslie and Smith (1984)) these energy distributions with maximum in tail can be two stream unstable with consequent generation of Langmuir waves. However, in our models this instability takes place only for very intensive beams, and it will be discussed in Section 3.2.

In the present paper we apply the electron distributions from Paper II to the interpretation of the observational features, which can be associated with the kinetic effects of electron beam precipitation.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
helpdesk.link@springer.de