Neutral Hydrogen and its Emission Lines in the Solar Corona

TL;DR

This study evaluates hydrogen emission lines in the solar corona using non-LTE modeling to assess their potential for diagnostics and polarimetry, comparing results with observations and emphasizing the need for narrow bandpass filters.

Contribution

It provides detailed non-LTE calculations of hydrogen emission lines in the corona for different structures, enhancing understanding of their diagnostic potential.

Findings

H emission lines can be used for coronal diagnostics.

Narrow bandpass filters are necessary for ground-based Halpha polarimetry.

Results show rough agreement with previous observations.

Abstract

Since the Lalpha rocket observations of (Gabriel, Solar Phys. 21, 392, 1971), it has been realized that the hydrogen (H) lines could be observed in the corona and offer an interesting diagnostic for the temperature, density, and radial velocity of the coronal plasma. Moreover, various space missions have been proposed to measure the coronal magnetic and velocity fields through polarimetry in H lines. A necessary condition for such measurements is to benefit from a sufficient signal-to-noise ratio. The aim of this article is to evaluate the emission in three representative lines of H for three different coronal structures. The computations have been performed with a full non-local thermodynamic equilibrium (non-LTE) code and its simplified version without radiative transfer. Since all collisionnal and radiative quantities (including incident ionizing and exciting radiation) are taken into account, the ionization is treated exactly. Profiles are presented at two heights (1.05 and 1.9 solar radii, from Sun center) in the corona, and the integrated intensities are computed at heights up to five solar radii.We compare our results with previous computations and observations (e.g. Lalpha from UVCS) and find a rough (model-dependent) agreement. Since the Halpha line is a possible candidate for ground-based polarimetry, we show that in order to detect its emission in various coronal structures, it is necessary to use a very narrow (less than 2 A wide) bandpass filter.