The Off-Limb Behavior of the First Ionization Potential Effect in T>5×10⁵ K Solar Plasmas

Abstract

We investigate the behavior of the solar first ionization potential (FIP) effect (the abundance enhancement of elements with first ionization potential of less than 10 eV in the corona with respect to photospheric values) with height above the limb in a region of diffuse quiet corona observed by the SUMER instrument on SOHO, with emphasis on so-called upper transition region lines. Previous disk observations have shown different abundance patterns in emission from lines at temperatures above and below ~8×10⁵ K, with an FIP effect clearly visible at the higher temperatures and less so at the lower temperatures. Our initial aim is to determine whether such a difference is also visible in off-limb observations. We find a low-FIP element enhancement of a factor of 3-4 indicated in all line ratios. The Mg VII 868.11/Ne VII 895.17 ratio is also seen to decrease toward a photospheric value when tracked down from the corona to the limb. This is markedly different from the behavior of higher temperature line ratios and may be related to the differing heating and mass supply mechanisms for plasmas at temperatures above and below ~8×10⁵ K.

An additional unexpected feature of our observations is that in low-FIP/high-FIP line ratios formed at temperatures close to the freeze-in temperature of the fast solar wind (~10⁶ K), there is also a small diminution of the FIP enhancement at the highest altitudes observed in this study. We discuss the possible relevance of this to the origin of the fast solar wind.