Submillimeter Radiometry of Sunspots

We use observations of sunspots by the 15 m James Clerk Maxwell Telescope (JCMT) for radiometry of sunspot umbrae and penumbrae The observations reported here, taking account of the effects of the far wings of the JCMT's beam, show that sunspot umbrae and penumbrae vary considerably in brightness between one another. The sunspot umbra is typically considerably dimmer than the quiet Sun but surrounded by a penumbra that may be fully as bright as surrounding plage. Moreover, the vertical brightness temperature gradients of sunspot umbrae and penumbrae appear to be uniformly positive, roughly equivalent to that of the quiet Sun. This gradient substantiates the operation of nonradiative heating in the low chromospheres of the strongest magnetic regions visible on the Sun's surface, i.e., sunspot umbrae.