Spectropolarimetric signatures of anisotropic velocity distributions of optically thin coronal UV lines

¹ Dipartimento di Fisica e Astronomia, SASS, Università degli Studi di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
e-mail: khan@arcetri.astro.it
² INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, Pino Torinese (TO), Italy

Received: 5 March 2012
Accepted: 6 June 2012

Abstract

Context. Many interpretations of observations with the Ultraviolet Coronagraph Spectrometer (UVCS) operating aboard the Solar and Heliospheric Observatory (SOHO) have suggested that there are variations in particle velocity distributions. In this paper, we investigate their spectropolarimetric signatures.

Aims. We uncover the spectropolarimetric signatures of anisotropic velocity distribution functions after a line-of-sight (LOS) integration of both the 1215.16 Å Lα line and the 1032 Å O vi line.

Methods. We perform a forward modelling of the resonance scattering of neutral hydrogen atoms and O⁵⁺ ions in the presence of anisotropic velocity distribution functions using a self-consistent 2.5-dimensional, magnetohydrodynamics global coronal model. We analyse the most important observables in spectropolarimetry, i.e., the rotation of the plane of linear polarisation, and de-or hyperpolarisation after a LOS integration.

Results. The spectropolarimetric signatures of anisotropic velocity distributions do survive the LOS integration, and are to be found in the region where there are non-radial solar outflows, i.e., from mid-way between the poles and the equator and down to the equator, in some cases starting from the photosphere all the way out to almost 2 R_⊙. We consider the cases of w_⊥ = 2w and w_∥ = w or w _⊥  = w and w_∥ = 2w, where w is the local thermal velocity of either neutral hydrogen atoms or O⁵⁺ ions, and where w_⊥ and w_∥ characterise the perpendicular and parallel distributions of the components of the velocity with respect to the magnetic field, respectively. We find that the rotation angles reach values of roughly  ± 15 degrees, and should in principle be measurable.

Conclusions. Our results show that it should be possible to distinguish anisotropic velocity distribution functions on the condition that one samples as extensive a region of the plane of the sky as possible. The effects of the anisotropy are in most cases seen all the way out to 2 R_⊙, and we therefore recommend that the observations be made as far away from the photosphere as possible in order to avoid the possible contamination by active regions. It will in most cases, however, be extremely hard to determine the sense of the anisotropy using only spectropolarimetry.