Thermal structure of penumbra

The thermal structure of the penumbra below its visible surface has important implications for our present understanding of sunspots and their penumbrae: which magneto-convective mode is transporting energy, and how magneto-acoustic wave modes are converted in sunspot seismology.

Temperatures in two different regions: quiet Sun (top) and penumbra (bottom). Results at optical depths τ=1 and τ=3 are displayed in the left and middle panels, respectively. τ=3 corresponds to a layer about 75 km deeper than τ=1. Right panels show the normalized continuum intensity images. White contours in the bottom right panel (continuum intensity in the penumbra) enclose bright penumbral filaments, whereas white regions in the penumbral temperature maps (left and middle bottom panels) indicate the umbra. All values of the temperature are given in thousands of Kelvin. Temperatures were obtained from the inversion of polarimetric data of neutral iron spectral lines located in the near-infrared (1565 nm) observed with the GRIS instrument attached to the 1.5-meter German telescope GREGOR located at El Teide Observatory in Tenerife (Spain).

We analyzed spectropolarimetric data (i.e., Stokes profiles) in three iron lines located at 1565 nm observed with the GRIS instrument attached to the 1.5-meter solar telescope GREGOR. The  data are corrected for the smearing effects of wide-angle scattered light and then subjected to an inversion code for the radiative transfer equation in order to retrieve, among others, the temperature as a function of optical depth. We find that the mean temperature difference between the two layers separated by some 75km is approximately 1057 K in bright penumbral filaments, whereas in the quiet Sun it is 1390 K. This indicates that the temperature gradient below the visible surface of the bright penumbra filaments is smaller than in the quiet Sun and it implies that in the sub-photospheric layersthe penumbral temperature diverges from that of the quiet Sun. We interpret these results as evidence of a thick penumbra, whereby the magnetopause is not located near its visible surface.

 

Reference:
Borrero, J.M., Franz, M., Schlichenmaier, R., Collados, M. \& Asensio Ramos, A. "Penumbral thermal structure below the visible surface" (2017), A\&A, 601, L8.