A STUDY OF SOLAR PROMINENCES NEAR LAMBDA=1 MILLIMETER

The 10.4 m Leighton telescope at the Caltech Submillimeter Observatory was used to produce full-disk and partial area raster maps of the Sun during 1991 July 9-11 in advance of the total eclipse of the Sun on J uly 11. Maps were made at a wavelength of 850 mum with an angular reso lution of 20.6'' and at 1250 mum with an angular resolution of 30''. W e have analyzed the 850 mum brightness associated with Halpha filament s present on the disk and find that (1) they are all associated with r egions possessing a brightness comparable to or below that of the quie t Sun at the center of the disk; and (2) because of their lack of cont rast with the background brightness distribution, Halpha filaments are optically thin at 850 mum. We have also analyzed contemporaneous obse rvations of a prominence above the solar limb at both 850 and 1250 mum . We show that (1) the optical depth of the prominence is tau almost-e qual-to 0.12 at 850 mum, confirming the conclusion drawn from the anal ysis of Halpha filaments; and (2) the line-of-sight emission measure i s [n(e)2L] = 1.3-2.0 x 10(29) cm-5, and the electron number density is n(e) approximately 1-3 x 10(10) cm-3. We argue that the reduced 850 m um brightness associated with Halpha filaments and, more generally, ma gnetic neutral lines is due neither to the presence of an overlying co ronal cavity nor to the presence of absorbing material, as has been su ggested previously. We instead suggest that the reduced brightness is due to the structure of the underlying chromosphere where, in the case of filament channels, the predominantly horizontal magnetic field lea ds to a reduction of heating and/or a smaller density scale height.