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 A 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, <-shaped profiles. In most of the penumbra, the individual bisectors are sharply inclined, with a shape of / or \, indicative of a highly suppressed convective flow. The intensity and velocity data show that a new family of penumbral filaments rises in the middle penumbra. Bisector intensity-velocity relationships display opposite gradients in the inner and outer penumbra, showing the rising and falling parts of curved penumbral flux tubes. Some clustering of the bisector intensity-velocity relationship is perhaps due to the fluted nature of flux tubes.