Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...420..892m&link_type=abstract
The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 420, no. 2, p. 892-902
Astronomy and Astrophysics
Astronomy
10
Abundance, Aluminum, Calcium, Chromium, Ionization Potentials, Photosphere, Solar Corona, Black Body Radiation, Gamma Ray Astronomy, Lyman Alpha Radiation, Photoionization, Temperature Gradients, X Ray Spectra
Scientific paper
We have used X-ray spectra obtained by the SOLEX spectrometers on the P78-1 satellite to study coronal abundances of three elements with low first ionization potential (FIP): Cr, Ca, and Al. The Cr/O abundance ratio was determined from the ratio of the Cr XV 2p6 (1)S0-2p5 3d (1)P1 line to the O VIII 1s (2)S-2p (2)P unresolved doublet, a ratio that is almost independent of temperture. In a scenario in which the ratio of an element's coronal to its photospheric abundance is largely determined by its FIP, the Cr/O ratio should be reflective of the absolute Cr abundance, because the FIPs of O and H are nearly equal. We find that the Cr/O abundance ratio in coronal active regions varies by a factor of 2.7 and is probably always enhanced by at least a factor of 3 over the photospheric ratio. The Ca/O abundance ratio was determined from the ratio of a blend of Ca XV lines at around 22.75 A to the O VIII 1s (2)S-2P (2)P doublet. Because this ratio is temperature dependent, only lower limits are derived for the Ca/O abundance ratio. The lower limits range from 3.0 to 9.7 times the photospheric ratio. Hence, it is likely that both Ca and Cr are always enriched by at least a factor of 3 in coronal active regions and flares. The Al/Mg abundance ratio determination was based on the line ratio (Al XII 1 S2 (i)S0-s2p (1)P1/(Mg XI 1s2 (1)S0-1s 3p (1)P1). Because this ratio is temperature sensitive and we have only six Al/Mg spectra, the results of this study are somewhat equivocal. The evidence indicates that Al is enriched in coronal active regions, but it is uncertain whether, as expected, it is enriched as much as is Ca, which has a slightly higher FIP. In order to account for these observations, as well as previously reported determinations of the Fe/O, O/Ne, and Fe/Mg coronal abundance ratios, in terms of a mechanism that moves ions much more easily than neutrals form the chromosphere to the corona, we have calculated the fraction of ionization for the abundant elelments in the chromosphere. Photoionization by blackbody radiaiton from just below the temperature minimum region, by locally produced Ly alpha radiation, and by line and continuum radiation emitted at high temperatures play and important role in determining the chromospheric ionization structure. In particular, photoionization of O I by EUV radiation from above must be invoked to account for variations in the coronal O/Ne abundance ratio. Because H I strongly absorbs this radiation, O is unionized at chromospheric depths of more than a few hundred kilometers. The ionization pattern, as a function of FIP, in the part of the upper chromosphere where the temperature is in the range 6000-8000 K, may account for the observed coronal abundance variations and enrichments. A mechanism that imparts a few tens of eV to ions, but not to neutrals, in this region could be responsible for the coronal enrichment of low-FIP elements.
Feldman Uri
McKenzie David L.
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