6. Is the FIP bias dependent on FIP value?
The FIP biases for S, Si and K measured in the present work show a different behaviour in two senses:
The sulphur FIP bias is almost negligible, showing that S abundance is photospheric within uncertainties, and does not change with height. Si instead shows a typical FIP bias magnitude, that seems to become smaller as height increases (although the most distant three values contradict this). The change with height is 30% at maximum, and is slightly greater than the uncertainties given by the combination of experimental and ion fraction uncertainties.
The K FIP bias is very high, higher than the values quoted in the literature for the other low-FIP elements. Feldman & Laming (2000) suggest that the FIP bias for the very low-FIP elements such as K could be higher than for the normal low-FIP elements such as Mg, Si and Fe, so that the FIP effect for low-FIP elements could be dependent on the FIP itself. Feldman & Laming (2000) base their suggestion on a number of measurements, whose uncertainties however are as large as this effect itself. The present work seems to support the Feldman & Laming (2000) suggestion with new measurements from active region SUMER measurements, but it is not possible to draw any firm and definitive conclusion because of the presence of a blending line, whose contribution to the total intensity is very difficult to determine. It is possible that the presence of this could significantly reduce the K FIP bias to lower levels, but it is difficult to determine whether the K FIP bias is higher than, or of the order of, the FIP bias of the other low-FIP elements.
It is recommended that further studies on K lines, and more accurate and complete atomic models for the calculation of line intensites are carried out, in order to address this problem.
The evident height dependence of the K FIP bias is a new observational fact whose magnitude (a factor around 2) is larger than the combined experimental and theoretical uncertainties. Of further interest, the Si FIP bias seems also to be dependent on height, decreasing with the distance from the solar limb. This behaviour has been already noted by Laming et al. (1999) in quiet corona; the authors suggest that this decrease with height might be due to emission from fast solar wind plasma present in the field of view. Since the fast solar wind plasma composition resembles the photospheric one, its emission would have the effect of decreasing the FIP bias of low-FIP elements with increasing height. However, the height dependence of the K FIP bias goes in the opposite direction, apparently contradicting Laming et al. (1999).
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001