Astronomy and Astrophysics – Astronomy
Scientific paper
May 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988taui.rept.....s&link_type=abstract
M.S. Thesis Tel-Aviv Univ. (Israel). School of Physics and Astronomy.
Astronomy and Astrophysics
Astronomy
Carbon, Chromosphere, Iron, Magnesium, Neon, Oxygen, Photosphere, Silicon, Solar Corona, Solar Flares, Abundance, Ionic Diffusion, Photoionization, Solar Magnetic Field, Thermal Diffusion
Scientific paper
Element abundances in the solar chromosphere were deduced from analysis of the gamma ray disk flare of 27 April 1981. Abundances of Ne, Mg, Si and Fe correlated with the local galactic abundance but C and O were 3 to 4 times less apparent. A comparison between the gamma ray and coronal abundances indicated an excess of Ne in the chromosphere. A similar 3-fold reduction in the coronal and solar wind abundances of He, C, N, O, Ne, S and Ar relative to the photosphere was explained by a high first ionization potential (greater than 9 V), keeping these elements in a neutral state in the photosphere and lower chromosphere. Gravitational, magnetic and thermal effects then lead to separation of the elements. A model was developed for a selective ionization process occurring only in the flaring area, at the preflare and impulsive phase. Soft X radiation at 1-3 A could penetrate the photospheric flare's footpoint and ionize the Ne there. In this range of wavelengths the photoionization cross section of C is an order of magnitude smaller than that of Ne, so that there is a region in which C remains neutral. O also remains neutral through the highly efficient charge exchange process with H, which is neutral in this region. Ionized Ne is exposed to vertical acceleration processes, which cause its relative enrichment during a 5 to 30 min. period. The vertical magnetic field structure at the flare's footpoint prevents horizontal diffusion of the upwardly moving ions. According to this model, Ne enrichment will occur mainly in gradual gamma ray flares which also have gradual continuous X radiation. The high Ne/O ratio would be expected in all such flares, while the Ne/C ratio, which is more susceptible to the intensity and duration of the soft X radiation, should vary among flares.
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