EDP Sciences
Free access
Volume 419, Number 2, May IV 2004
Page(s) 763 - 770
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20035911

A&A 419, 763-770 (2004)
DOI: 10.1051/0004-6361:20035911

Electron cyclotron maser emission from solar coronal funnels?

C. Vocks1, 2 and G. Mann1

1  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2  now at: Space Sciences Laboratory of UC Berkeley, Berkeley, CA 94720-7450, USA

(Received 19 December 2003 / Accepted 25 February 2004)

The sun is covered by a network of supergranular cells. The convective motion of these cells leads to the formation of strong magnetic fields at the cell boundaries. At larger heights in the solar transition region and low corona, this magnetic field geometry expands rapidly within a short distance, and forms the magnetic structure of "coronal funnels". This field line geometry represents a magnetic mirror, and since the plasma density strongly increases with depth in the transition region, the electron velocity distribution function (VDF) can develop a loss cone. Within such a coronal funnel, the plasma frequency can have smaller values than the electron cyclotron frequency, $\omega_{\rm p} <
\Omega_{\rm e}$ . These are the necessary conditions for the generation of X-mode waves through the electron cyclotron maser mechanism. Since there is some observational evidence for radio emission from the supergranular network, it is of interest to investigate the possibility of this plasma wave generation in a quiet stellar atmosphere in detail. In this paper, a kinetic model is used to calculate the electron VDF in a coronal funnel. A method is derived to determine wave growth rates from the electron VDF. Its application on the coronal funnel VDF indeed results in X-mode wave growth. However, it is also found that wave absorption by higher-order resonances at larger heights in the atmosphere plays an important role.

Key words: radiation mechanisms: non-thermal -- Sun: corona -- Sun: radio radiation -- masers

Offprint request: C. Vocks, vocks@ssl.berkeley.edu

© ESO 2004