On the correlation between line width and line depth of the solar HeI 1083 NM line

Abstract

Seven sets of spectra in the region of the HeI 1083 nm absorption line of the active region NOAA 7558 + 7562 near the west limb, obtained at Norikura Solar Observatory, National Astronomical Observatory, Japan have been analysed to determine the line parameters at each of the 255 * 100 pixels covering the active region. The observed range in the linewidth is similar to that seen earlier for the quiet sun, while the observed line depth is enhanced by almost an order of magnitude. This can be reconciled by considering the helium absorption to occur in unresolved chromospheric inhomogenieties with an enhanced filling factor accounting for the enhanced absorption in active regions. The line broadening can be explained in terms of a range in finite optical thickness of the elements. In this case, one would expect broader profiles to be associated with greater helium absorption, and a scatter plot of line width versus line depth indeed shows a weak positive correlation.