Abstract
view Abstract Citations (39) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Formation of the Solar He II 1640.4 Angstrom Emission Line Wahlstrom, Cathrine ; Carlsson, Mats Abstract We explain the formation of the solar He II 1640.4 A Balmer-alpha emission line. Non-LTE (non-local thermodynamic equilibrium) modeling is performed including the effects of an incoming coronal radiation field, the overlapping emission profile of He II 304 A with the He I continuum and the pumping effect of hydrogen Lyman-alpha. We find that the line is formed by the photoionization from the ground state of He II, mainly by the incident coronal radiation, followed by cascade recombination into the n = 3 levels. The peak of the contribution functions is at low temperatures, 7000-10,000 K, with a very small contribution from collisional excitation at a temperature of 70,000 K. We obtain good agreement with the observed line widths, the observed fine structure and the relative strength of the fine structure components with the blue peak about 10% stronger than the red peak. It is shown that these results are insensitive to the assumptions of geometry in the line forming regions and to the filling factor. The relative intensities only depend on an optically thin 1640.4 A line and optically thick He II Lyman lines. The observed asymmetry also imposes an upper limit to the density of 30 times the density at 8000 K in a plane parallel model. It is shown that the total intensity is very sensitive to the assumed geometry and filling factors. This sensitivity is parameterized in terms of a multiplicative factor to the assumed coronal radiation field. In a plane-parallel model the total intensity comes out a factor of 4 too low with the adopted coronal radiation field but already an enhancement factor of 2 of the incident radiation field gives a total intensity in agreement with observations. Note that this enhancement factor is only a quantification of the disagreement with observations. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174654 Bibcode: 1994ApJ...433..417W Keywords: Balmer Series; Emission Spectra; Solar Atmosphere; Solar Spectra; Star Formation; Stellar Models; Ultraviolet Astronomy; Ultraviolet Spectra; Chromosphere; Lyman Alpha Radiation; Mathematical Models; Solar Corona; Solar Radiation; Solar Physics; LINE: FORMATION; LINE: PROFILES; SUN: ATMOSPHERE; SUN: CHROMOSPHERE; SUN: UV RADIATION full text sources ADS |
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