Wednesday, March 6, 2013

1303.1126 (G. Stellmacher et al.)

Spectroscopy of Solar Prominences Simultaneously from Space and Ground    [PDF]

G. Stellmacher, E. Wiehr, I. E. Dammasch
We present a comprehensive set of spectral data from two quiescent solar prominences observed in parallel from space and ground: with the VTT, simultaneous two-dimensional imaging of H-beta 4862 and Ca II 8542 yields a constant ratio, indicating small spatial pressure variations over the prominences. With the Gregory, simultaneous spectra of Ca II 8542 and He I 10830 were taken, their widths yielding 8000< T_kin< 9000 K and v_nth<8 km/s. The intensity ratio of the helium triplet components gives an optical thickness of tau < 1.0 for the fainter and tau < 2.0 for the brighter prominence. The tau_0 values allow to deduce the source function for the central line intensities and thus the relative population of the helium 3S and 3P levels with a mean excitation temperature T_ex = 3750 K. With SUMER, we sequentially observed 6 spectral windows containing higher Lyman lines, 'cool' emission lines from neutrals and singly charged atoms, as well as 'hot' emission lines from ions like O IV, O V, N V, S V and S VI. The EUV lines show pronounced maxima in the main prominence body as well as 'side-locations' where the 'hot' lines are enhanced with respect to the 'cool' lines. The line radiance of 'hot' lines blue-wards of the Lyman series limit (lambda<912 A) appear reduced in the main prominence body. This absorption is also visible in TRACE images of Fe IX/X 171 as fine dark structure which covers only parts of the main ('cool') prominence body. The Lyman lines show a smooth decrease of line widths and radiance with increasing upper level k = 5 through 19. For k= 5 through 8 the level population follows a Boltzmann distribution with T_ex >6 *10^4 K; higher levels k > 8 appear more and more overpopulated. The larger widths of the Lyman lines require high non-thermal broadening close to that of 'hot' EUV lines. In contrast, the He II emission is more related to the 'cool' lines.
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