The interpretation of Balmer line profiles in solar prominences.
Abstract
In the past, it has been the usual practice to interpret prominence observations on the assumption that the observed radiation is the intrinsic radiation of the prominence itself. It has been recently pointed out, however, that a major component of the observed radiation-at least in Ha should be scattered disk radiation; the prominence thus acting as a diffuser as well as an emitter. In order to get a better understanding of the process of line formation in prominences as well as to obtain a consistent approach to the determination of prominence temperatures, high dispersion profiles have been obtained of the hydrogen Balmer lines Ha through HE and the helium lines X5876 and X447I in prominences showing a minimum of internal motions. The prominences were observed, at the limb, with the i6-inch coronagraph at Sacramento Peak. Several years ago, Conway (1952) and Ellison (1952) observed that Ha prominence line profiles could be fitted very well to theoretical profiles computed on the assumption of a source function constant in frequency across the line and constant in depth in the prominence. The same remarkable agreement has been obtained for our observations, except of course, for those cases where the Ha line is self-reversed. For subordinate lines like these, it is almost certain that the source function is in fact frequency independent. The good agreement obtained between the observed profiles, and those computed on the assumption of a source function constant in depth, however, is probably largely fortuitous. It seems that almost any monotonic source function could be fitted by one of the family of curves. In any case, the occurrence of double- peaked profiles in Ha shows that for this line the assumption is not generally valid. In order to test this assumption further, the optical depth at the center of the Ha line has been computed from the run of widths of the Balmer lines H~ through HE. This method was found to give a higher optical depth than that implied by the shape of the Ha line using the assumption of a source function constant in depth. It seems clear that the usual method of estimating prominence temperatures from the Ha profile is very unreliable. Conway, M. 1955, Contr. Dunsink Obs. No. 3. Ellison, M. A. 1952, Pub. Royal Obs. Edinburgh I, No. 5. Sacramento Peak Observatory, Sunspot, N. Mex.
- Publication:
-
The Astronomical Journal
- Pub Date:
- July 1957
- DOI:
- 10.1086/107495
- Bibcode:
- 1957AJ.....62..143J