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Astron. Astrophys. 364, 835-844 (2000)


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FIP effect measurements in the off-limb corona observed by SUMER on SOHO

A. Mohan 1, E. Landi 2 and B.N. Dwivedi 1,2

1 Banaras Hindu University, Institute of Technology, Department of Applied Physics, Varanasi 221005, India
2 Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany

Received 26 June 2000 / Accepted 20 October 2000

Abstract

Systematic differences between elemental abundances in the corona and in the photosphere have been long acknowledged in the Sun; these abundance anomalies are correlated with the first ionization potentials (FIPs) of the elements. This correlation is called FIP effect. Using spectra obtained from the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory), we investigate electron density, temperature and the abundance anomalies in the off-limb solar corona. In particular, we present the behaviour of the solar FIP effect with height above an active region observed at the solar limb, with K/Ar, Si/Ar and S/Ar ratios of lines present in the recorded UV spectra.

We find that the K FIP bias seems to be higher than for the other low-FIP elements, and that the Si and K FIP biases are dependent on the distance from the photosphere. Studying the Ne/Mg line ratio in the active region, we also investigate the presence of the FIP effect in active region structures.

Key words: Sun: abundances – Sun: atmosphere – Sun: UV radiation

Send offprint requests to: E. Landi (enricol@arcetri.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001

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