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Astron. Astrophys. 328, 390-398 (1997)

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1. Introduction

Hard X-ray observations demonstrate that energetic flare-electrons frequently exhibit an energy distribution which can be approximated by a power law (Lin 1974). The gyrosynchrotron radiation from these electrons trapped in magnetic fields of solar active regions can be regarded as the main source of microwave bursts at frequencies [FORMULA] 1 GHz (cf. Kundu & Vlahos 1982). Another important burst component which is intimately connected with flare electrons is formed by the decimetric continuum comprising mainly type IVdm bursts, but also a number of substructures like pulsations, spikes, zebra patterns etc. in the frequency range between about 200 MHz and 1 or 2 GHz (Zheleznyakov 1970; Krüger 1979; Benz 1985, 1986, 1996; Isliker & Benz 1994). This connection is evident from the near-coincidence of the time profiles of microwave bursts and decimetric continua (with a small delay of the latter, cf., e.g., Aschwanden et al. 1990) and the high correlation between decimetric spikes and hard X-ray bursts (Aschwanden & Güdel 1992). The presence of peculiarities in the dynamical spectrum of the decimetric continuum such as zebra patterns and sudden reductions has lead to the assumption that this continuum is generated by plasma waves at the upper hybrid frequency [FORMULA], excited by fast electrons having a loss-cone anisotropy (Kuijpers 1974; Zaitsev & Stepanov 1975; Benz & Kuijpers 1976) while interaction with lower hybrid waves has been invoked to explain dm-pulsations (Benz 1980).

There arises the question whether the electrons generating the microwave bursts via gyrosynchrotron emission and those electrons generating the decimetric continuum by the plasma-wave mechanism represent two populations of power-law electrons with similar velocity distributions and the same origin.

In the present paper we consider the instability of a power-law distribution with a loss-cone anisotropy and show that such a distribution can generate plasma waves at all reasonable values of the mirror ratio and of the exponent of the power-law distribution function (Sect. 2). Furthermore, we calculate the energy density of the plasma waves excited by the loss-cone instability and relate it to the parameters of the power-law electron energy distribution and properties of the trapping magnetic loop (Sect. 3). In Sect. 4 we investigate the possibility of the generation of both, decimetric continua and microwave bursts by one original population of flare-generated electrons with a power-law energy spectrum and assume that the same power-law distribution (with different number densities) exists in both sources. In Sect. 5 we discuss the results and in Sect. 6 the conclusions are summarized.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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