EDP Sciences
Free access
Volume 506, Number 3, November II 2009
Page(s) 1437 - 1443
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361/200912616
Published online 27 August 2009
A&A 506, 1437-1443 (2009)
DOI: 10.1051/0004-6361/200912616

Electron beam – plasma system with the return current and directivity of its X-ray emission

M. Karlický and J. Kašparová

Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
    e-mail: [karlicky;kasparov]@asu.cas.cz

Received 2 June 2009 / Accepted 29 July 2009

Aims. An evolution of the electron distribution function in the beam-plasma system with the return current is computed numerically for different parameters. The X-ray bremsstrahlung corresponding to such an electron distribution is calculated and the directivity of the X-ray emission is studied.
Methods. For computations of the electron distribution functions we used a 3-D particle-in-cell electromagnetic code. The directivity of the X-ray emission was calculated using the angle-dependent electron-ion bremsstrahlung cross-section.
Results. It was found that the resulting electron distribution function depends on the magnetic field assumed along the electron beam propagation direction. For small magnetic fields the electron distribution function becomes broad in the direction perpendicular to the beam propagation due to the Weibel instability and the return current is formed by the electrons in a broad and shifted bulk of the distribution. On the other hand, for stronger magnetic fields the distribution is more extended in the beam-propagation direction and the return current is formed by the electrons in the extended distribution tail. In all cases, the anisotropy of the electron distribution decreases rapidly due to fast collisionless processes. However, the magnetic field reduces this anisotropy decrease. The X-ray directivity shows the same trend and it is always closer to the isotropic case than that in a simple beaming model.

Key words: Sun: flares -- Sun: particle emission -- Sun: X-rays, gamma rays

© ESO 2009