The viewing angle effect on H_α-line impact polarisation in impulsive solar events

¹ Cybernetics Department, Bradford University, Bradford BD7 1DP, UK e-mail: v.v.zharkova@brad.ac.uk
² Institute of Solar Terrestrial Physics SD RAS, PO Box 4026, Irkutsk, 664033 Russiae-mail: lkk@iszf.irk.ru

Received: 12 July 2004
Accepted: 14 October 2004

Abstract

The effect of a viewing angle on the hydroden H_α-line impact polarisation is investigated in a plane vertical atmosphere arbitrary located on the solar disk. The impact polarisation is assumed to be caused by precipitating beam electrons with pitch-angular anisotropy steadily injected into the flaring atmosphere from its top. The polarisation is calculated for a 3 level plus continuum hydrogen atom affected by Zeeman splitting in a moderate magnetic field taking into account depolarising effects of diffusive radiation and collisions with thermal electrons. The H_α polarisation profiles are affected by electron beams only in the line cores whereas the wings are fully depolarized by the collisions with thermal electrons despite the extended wing emission, or “moustaches”, caused by beam electrons. The full (integrated in wavelength) H_α-line linear polarisation, caused by moderate electron beams, is shown to be  and either negative or positive depending on the position of a flaring loop on the solar disk and the direction of an emitted photon from the local magnetic field. The polarisation plane is projected onto a viewing angle ψ, being a superposition of the flare location on a solar disk and the magnetic field deviation from vertical on the solar surface. For viewing angles less then the H_α-line impact polarisation is negative increasing up to  towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane where is the magnetic field induction and is the photon momentum vector. For viewing angles greater than  the measured impact polarisation becomes positive, sharply increasing up to  towards the limb. In the range of the observed impact polarisation goes through a zero point despite the actual presence of beam electrons in the flaring atmosphere. The theoretical predictions of the dependence of polarisation degree on viewing angle fit remarkably well the observations of H_α-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.