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

Abstract

The effect of a viewing angle on the hydroden Hα-line impact polarisation is investigated in a plane vertical atmo- sphere 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 Hα 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) Hα-line linear polarisation, caused by moderate electron beams, is shown to be 2−20% 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 50 ◦ the Hα-line impact polarisation is negative increasing up to −10% towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane B × K where B is the magnetic field induction and K is the photon momentum vector. For viewing angles greater than 60 ◦ the measured impact polarisation becomes positive, sharply increasing up to 20% towards the limb. In the range of 50−60 ◦ 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 Hα-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.