Effect of the electron density stratification on off-limb O VI line profiles: How large is the velocity distribution anisotropy in the solar corona?

Abstract

Coronagraphic spectral observations carried out with UVCS on SoHO have shown that the velocity distribution in the solar corona is highly anisotropic. Here we examine the influence of the density stratification on the interpretation of such observations. In particular, we investigate the profiles of O VI lines emitted in the corona by employing an analytical 2-D model of the large scale coronal magnetic field and solar wind. We concentrate on the polar coronal holes and take into account the integration along the line of sight. We find that at distances greater than 1 R ○. from the solar surface the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. In particular, the densities deduced by Doyle et al. (1999a,b) from SoHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS/SoHO although only isotropic velocity distributions are employed. Hence we expect that the magnitude of anisotropy of the velocity distribution deduced from UVCS data depends strongly on the adopted density profile.