Imaging Spectroscopy of a Sunspot: Thermal and Velocity Structure

Abstract

The purpose of this paper is (1) to confirm and establish the working of a dual-etalon Fabry-Perot imaging spectroscopy system at the National Solar Observatory/Sacramento Peak Dunn Solar Telescope and (2) to use this system to extend previous work by many authors and understand the structure and dynamics of sunspots. A detailed investigation of the thermal and velocity structure in an isolated sunspot, using the Fe I 5576 Angstrom spectral line, is presented. The concept of flowless maps is incorporated, to separate velocity and intensity effects. The resulting intensities are used to generate thermal maps of the sunspot along the height of formation of a spectral line, followed by a thermal span map. The thermal span in penumbral regions is in the range of 1350 1580 K. It is a factor of 2 smaller in the umbra. Using spectral line bisectors, we extend the concept of a velocity span to a sunspot, following Gray. The velocity span is used to study the velocity gradients across a sunspot. The velocity span maximizes in the middle of the sunspot penumbra and falls off on either side. The Doppler-neutralized mean bisectors from the disk-side and limb-side penumbra show more sharply inclined gradients, when compared with the C-shaped photospheric bisectors. The mean umbral bisectors show sharp,