The hot prominence periphery in EUV lines

Abstract

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 7500 <= T_kin <= 8000 K and 2.5 <xi <5.0 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 D_3 and He 3888 lines. If this is due to optical depth effects in the EUV lines, one obtains tau_0 (584) ~ 8*10(3) and tau_0 (537) ~ 2*10(3) , 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(2*) ster)] for the 584, 537, and D_3 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_3)/ E(He 584)~0.3 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 10(4) and 10(5) K.