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Astron. Astrophys. 341, 954-961 (1999)

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Spectral properties of a hybrid thermal-nonthermal isotropic plasma

D. Lin and E.P. Liang

Rice University, Space Physics and Astronomy Department, MS 108, 6100 S. Main Street, Houston, TX 77005-1892, USA

Received 31 August 1998 / Accepted 9 October 1998

Abstract

The spectral properties of a hybrid steady thermal-nonthermal isotropic plasma are studied. The involved emission mechanisms are thermal bremsstrahlung, nonthermal bremsstrahlung, cyclotron and nonthermal synchrotron. Formulas are derived to calculate the emissivities and absorption coefficients of all these processes. Calculations show that the nonthermal synchrotron and the thermal bremsstrahlung dominate over most of the frequency range and that the cyclotron and the nonthermal bremsstrahlung are important only at the two ends of the spectrum. The combination of the two dominant processes could generate the synchrotron self-absorbed spectrum, the Rayleigh-Jeans spectrum and an absorbed flatter spectrum which is seen in some solar flare radio spectra. Absorbed spectra steeper than the three types of spectra are rare. The conditions for these three possible spectral types are given. Cyclotron harmonic features are seen in the emergent spectra from the hybrid plasmas. These harmonic features, useful in determining the magnetic field, exist only under certain conditions which are explored in this paper.

Key words: plasmas – radiation mechanisms: non-thermal – radiation mechanisms: thermal

Send offprint requests to: D. Lin

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998
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