Extreme-ultraviolet observations of sunspots with the Harvard spectrometer on the Apollo Telescope Mount

Abstract

EUV spectroheliograms show that the areas directly above sunspot umbrae are the brightest features in an active region by an order of magnitude in the chromospherecorona transition region (1 K < T < 108 K.) Rarios of density-sensitive lines in the transition region show a significant decrease in gas density over the umbra relative to surrounding plage. We deduce that the temperature gradient in the transition region over the spot is decreased by an order of magnitude or more, relative to the plage. S#ject headings: chromosphere, solar - corona, solar - spectroheliograms - sunspots