It is well established that in the solar corona, in the slow solar wind and among solar energetic particles the abundances of elements with first ionization potential (FIP) below 10 eV are enhanced relative to those with higher first ionization potential when compared to the abundance ratio measured in the photosphere. This variation of relative solar abundances in different atmospheric regions is called the FIP effect (see, e.g., Meyer 1985; von Steiger & Geiss 1989; Reames et al. 1994; Feldman & Laming 1994; Sheeley 1996; Bochsler 1998; Feldman 1998; Geiss 1998, for reviews).
There is as yet no single generally accepted physical mechanism for the FIP effect. However, the idea underlying most theoretical approaches is that the segregation of high and low FIP elements takes place in the chromosphere or possibly the transition region (e.g., Henoux 1998). Nevertheless, it is worthwhile also to consider alternatives.
Here we investigate the possibility that the segregation has (partly or completely) already taken place in the photospheric layers of magnetic features, so called magnetic elements, thought to be composed of magnetic flux tubes (e.g., Solanki 1993). Such an effect could reproduce the observed FIP effect, since in the chromosphere the magnetic elements expand, forming a magnetic canopy (Giovanelli & Jones 1982, Solanki & Steiner 1990), and finally fill the whole upper atmosphere, so that the gas exhibiting the FIP effect is connected in the photosphere almost exclusively to the magnetic elements and to sunspots. Note that even if photospheric flux tubes exhibit a FIP effect of the same magnitude as witnessed in the upper solar atmosphere, we would still not expect it to be noticeable in standard photospheric abundance determinations, because magnetic elements cover only 1% or less of the solar surface in photospheric layers.
Even if magnetic elements were to exhibit no FIP effect, an analysis of the elemental abundances in magnetic elements still provides a firm lower boundary condition for any model of FIP segregation in the chromosphere. In fact, to our knowledge elemental abundances have never been determined in flux tubes before (with the exception of sunspots, which are not studied here).
In the present paper we determine abundances of elements inside magnetic features relative to the iron abundance there. As we shall explain in Sect. 3, it is currently not possible to obtain absolute abundances. However, this is not a serious disadvantage in the context of the FIP effect, which concerns relative abundance differences.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999