Doppler Shifts of the Hα Line and the Ca ii 854.2 nm Line in a Quiet Region of the Sun Observed with the FISS/NST

Abstract

The characteristics of Doppler shifts in a quiet region of the Sun are compared between the Hα line and the Ca ii infrared line at 854.2 nm. A small area of 16″×40″ was observed for about half an hour with the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope (NST) at Big Bear Solar Observatory. The observed area contains a network region and an internetwork region, and identified in the network region are fibrils and bright points. We infer Doppler velocity v m from each line profile at each individual point with the lambdameter method as a function of half wavelength separation Δλ. It is confirmed that the bisector of the spatially averaged Ca ii line profile has an inverse C-shape with a significant peak redshift of + 1.8 km s−1. In contrast, the bisector of the spatially averaged Hα line profile has a C-shape with a small peak blueshift of − 0.5 km s−1. In both lines, the bisectors of bright network points are significantly redshifted not only at the line centers, but also at the wings. The Ca ii Doppler shifts are found to be correlated with the Hα ones with the strongest correlation occurring in the internetwork region. Moreover, we find that here the Doppler shifts in the two lines are essentially in phase. We discuss the physical implications of our results in view of the formation of the Hα line and Ca ii 854.2 nm line in the quiet region chromosphere.