Effect of a Dense Coronal Region on Line Observations from Coronal Holes

Abstract

The outer corona and inner solar wind region (1.5 ≦ r ≦ 8 R_s) is of particular importance for the acceleration of the solar wind, but it is difficult to measure plasma parameters in this region. The width of the resonantly scattered HI Ly-α line (1216 Å) has been used to determine the kinetic temperature of hydrogen atoms and protons (Withbroe et al., 1982a, Withbroe et al., 1985). In the present paper we shall discuss the difficulties involved in obtaining information about the temperature in coronal holes from observed line profiles. The spectral lines are optically thin in the region we consider and the measured line profiles are integrated over the line of sight (l.o.s). The width of the line is (e.g. Aller, 1963):

$$ \left( {\Delta \lambda } \right)_{\text{FWHM}} = \frac{{2.4\;\lambda }} {\text{c}}\left( {\frac{{^{2\text{k}\text{T}} \text{eff}}} {\text{m}}} \right)^{\frac{1} {2}} $$

((1))