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Neutral Hydrogen and its Emission Lines in the Solar Corona  

Jean-Claude Vial   Submitted: 2016-09-16 08:16

Since the Lα rocket observations of (Gabriel, Solar Phys. 21, 392,1971), it has been realized that the hydrogen (H) lines could be observed in the corona and offer an interesting diagnostic for the temperature, density, and radial velocity of the coronal plasma. Moreover, various space missions have been proposed to measure the coronal magnetic and velocity fields through polarimetry in H lines. A necessary condition for such measurements is to benefit from a sufficient signal-to-noise ratio. The aim of this article is to evaluate the emission in three representative lines of H for three different coronal structures. The computations have been performed with a full non-local thermodynamic equilibrium (non-LTE) code and its simplified version without radiative transfer. Since all collisionnal and radiative quantities (including incident ionizing and exciting radiation) are taken into account, the ionization is treated exactly. Profiles are presented at two heights (1.05 and 1.9 solar radii, from Sun center) in the corona, and the integrated intensities are computed at heights up to five radii. We compare our results with previous computations and observations (e.g. Lα from UVCS) and find a rough (model-dependent) agreement. Since the Hα line is a possible candidate for ground-based polarimetry, we show that in order to detect its emission in various coronal structures, it is necessary to use a very narrow (less than 2 A wide) bandpass filter.

Authors: Jean-Claude Vial, Martine Chane-Yook
Projects: None

Publication Status: Solar Physics (accepted)
Last Modified: 2016-09-17 07:04
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Jean-Claude Vial   Submitted: 2007-10-20 08:13

Ly α and Ly beta line profiles in a solar prominence were observed with high spatial and spectral resolution with SOHO/SUMER. Within a 60 arcsec scan, we measure a very large variety of profiles: not only reversed and non-reversed profiles but also red-peaked and blue-peaked ones in both lines. Such a spatial variability is probably related to both the fine structure in prominences and the different orientations of mass motions. The usage of integrated-intensity cuts along the SUMER slit, allowed us to categorize the prominence in three regions. We computed average profiles and integrated intensities in these lines which are in the range (2.36 ? 42.3) W m-2 sr-1 for Ly α and (0.027 ? 0.237) W m-2 sr-1 for Ly beta. As shown by theoretical modeling, the Ly α Ly beta ratio is very sensitive to geometrical and thermodynamic properties of fine structure in prominences. For some pixels, and in both lines, we found agreement between observed intensities and those predicted by onedimensional models. But a close examination of the profiles indicated a rather systematic disagreement concerning their detailed shapes. The disagreement between observations and thread models (with ambipolar diffusion) leads us to speculate about the importance of the temperature gradient between the cool and coronal regions. This gradient could depend on the orientation of field lines as proposed by Heinzel, Anzer, and Gun?ar (2005).

Authors: J.-C. Vial, H. Ebadi, A. Ajabshirizadeh
Projects: SoHO-SUMER

Publication Status: Accepted in Solar Physics
Last Modified: 2007-10-21 08:32
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