Simulating the Solar Minimum Corona in UV Wavelengths with Forward Modeling II. Doppler Dimming and Microscopic Anisotropy Effect
Jie Zhao, S. E. Gibson, Silvano Fineschi, Roberto Susino, R. Casini, Steven R. Cranmer, L. Ofman, Hui Li
Abstract
Abstract In ultraviolet (UV) spectropolarimetric observations of the solar corona, the existence of a magnetic field, solar wind velocity, and temperature anisotropies modify the linear polarization associated with resonant scattering. Unlike previous empirical models or global models, which present blended results of the above physical effects, in this work, we forward-model expected signals in the H i Ly α line (121.6 nm) by adopting an analytic model that can be adjusted to test the roles of different effects separately. We find that the impact of all three effects is most evident in the rotation of the linear polarization direction. In particular, (1) for magnetic fields between ∼10 and ∼100 G, the Hanle effect modifies the linear polarization at low coronal heights, rotating the linear polarization direction clockwise (counterclockwise) when the angle between the magnetic field and the local vertical is greater (less) than the van Vleck angle, which is consistent with the result of Zhao et al.; (2) solar wind velocity, which increases with height, has a significant effect through the Doppler dimming effect at higher coronal heights, rotating the linear polarization direction in an opposite fashion to the Hanle effect; and (3) kinetic temperature anisotropies are most significant at lower heights in open nonradial magnetic field regions, producing tilt opposite to isotropic Doppler dimming. The fact that the three effects operate differently in distinct spatial regimes opens up the possibility for using linear polarization measurements in UV lines to diagnose these important physical characteristics of the solar corona.