He [CSC]i[/CSC] 10830 Å Wing Asymmetry in Polar Coronal Holes: Evidence for Radial Outflows

Abstract

Imaging spectroscopy of the Sun, carried out across the the solar poles, yielded several thousand profiles of the He I 10830 A chromospheric absorption line with effective spatial pixels of 1.1 × 2 arcsec2. Profiles of He I 10830 A show the relative blue-wing absorption is stronger in the coronal holes than in the quiet Sun, creating an asymmetric profile indicative of mass outflow. Within the coronal holes, blueshifted line wings are found where He I absorption is weak, corresponding to the center of supergranular cells. However, in the quiet Sun, there is no line wing shift in supergranular centers. Spatially compact regions of strong red-wing absorption also occur across the disk. Within the polar coronal holes, the amplitude of the wing shift shows a linear dependence with cos θ (where the angle θ is measured with respect to an outward normal to the Sun's surface), suggesting that a radial outflow occurs with a characteristic speed of ~8 km s-1. These observations represent the first detection of systematic outflows near the chromosphere transition region interface that appear to mark the origin of the high-speed wind acceleration from the solar surface.