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Astron. Astrophys. 334, 280-288 (1998)

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The hot prominence periphery in EUV lines

C.R. de Boer 1, G. Stellmacher 2 and E. Wiehr 3

1 Max-Planck Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany (cboer@osf1.mpae.gwdg.de)
2 Institut d'Astrophysique, [FORMULA] Boulevard d'Arago, F-75014 Paris, France (stell@iap.fr)
3 Universitäts-Sternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany (ewiehr@uni-sw.gwdg.de)

Received 26 November 1997 / Accepted 6 February 1998


Two sets of He I and metallic lines were observed with the EUV spectrograph SUMER in a quiescent prominence. H, He, and Ca II lines were observed simultaneously with both German telescopes on Tenerife. The visible lines from elements with different atomic weights yield thermal and non-thermal broadening parameters of [FORMULA]  K and [FORMULA]  km/s for the cool prominence body. The EUV lines, however, show line widths which correspond to much higher temperatures and non-thermal velocities. If the calculated formation temperature for every individual ion is assumed, the observed line widths require non-thermal velocities of 14 - 25 km/s. The narrowest reduced widths of the EUV He I 584 and He I 537 lines are 3.1 and 2.9 times broader than those of the visible He D3 and He 3888 lines. If this is due to optical depth effects in the EUV lines, one obtains [FORMULA] and [FORMULA], respectively.

The emission ratios of the Ca II-to-Balmer lines vary little inside the prominence, indicating a largely constant gas pressure. The ratios of the visual He-to-Balmer lines as well as those of the EUV He-to-metallic lines show a significant branching between peripheral and central prominence regions. The total emissions in the main prominence body amount to 13, 0.3, and 4 [Watt/ (m [FORMULA] ster)] for the 584, 537, and D3 lines, respectively. The observed emission ratio E(He 584)/E(He 537)= 45 agrees with model calculations whereas their total emissions are about 37 times higher than calculated. The observed ratio E(He D [FORMULA] / E(He 584) [FORMULA] is about 15 times smaller than model predictions. The observations indicate that the emissions of different ions originate from individual (isothermal) threads with different temperatures between [FORMULA] and [FORMULA]  K.

Key words: Sun: prominences – Sun: UV radiation

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998