Abstract
We review our state of knowledge of coronal element abundance anomalies in the Sun and stars. We concentrate on the first ionization potential (FIP) effect observed in the solar corona and slow-speed wind, and in the coronae of solar-like dwarf stars, and the "inverse FIP" effect seen in the corona of stars of later spectral type; specifically M dwarfs. These effects relate to the enhancement or depletion, respectively, in coronal abundance with respect to photospheric values of elements with FIP below about 10~eV. They are interpreted in terms of the ponderomotive force due to the propagation and/or reflection of magnetohydrodynamic waves in the chromosphere. This acts on chromospheric ions, but not neutrals, and so can lead to ion-neutral fractionation. A detailed description of the model applied to closed magnetic loops, and to open field regions is given, accounting for the observed difference in solar FIP fractionation between the slow and fast wind. It is shown that such a model can also account for the observed depletion of helium in the solar wind. The helium depletion is sensitive to the chromospheric altitude where ion-neutral separation occurs, and the behavior of the helium abundance in the closed magnetic loop strongly suggests that the waves have a coronal origin. This, and other similar inferences may be expected to have a strong bearing on theories of solar coronal heating. Chromospheric waves originating from below as acoustic waves mode convert, mainly to fast mode waves, can also give rise to ion-neutral separation. Depending on the geometry of the magnetic field, this can result in FIP or Inverse FIP effects. We argue that such configurations are more likely to occur in later-type stars (known to have stronger field in any case), and that this explains the occurrence of the Inverse FIP effect in M dwarfs.
Highlights
Working during the early years of solar UV and X-ray spectroscopy, Pottasch (1963) found evidence for significantly higher abundances of Mg, Si, and Fe in the low solar corona than in the photosphere, and concluded, somewhat reluctantly that “the chemical composition in the solar atmosphere differs from the photosphere to the corona”
Modern models of the effect to be discussed in detail below, in which the fractionation is driven by the ponderomotive force of Alfven waves, make an intimate connection between the abundance anomaly and coronal heating mechanisms, such that the first ionization potential (FIP) effect may yield several important insights into the nature of the latter
The coronal temperature is constant at the maximum temperature of the chromospheric temperature (∼ 1.6 × 106 K) but this value is not significant since we are uninterested in the ionization balance in this region and the Alfven wave propagation only depends on the density
Summary
We have introduced an extra longitudinal pressure associated with the Alfven waves proportional to vw ave [see Eq (17)], which has the effect of causing some saturation of the FIP fractionation. We give some physical motivations for this extra term. The first is the inevitable generation of slow-mode waves by the Alfven driver. The periodic variation in magnetic pressure of the Alfven wave drives longitudinal compressional waves. These generated acoustic waves can act back on the Alfven driver, as the compressional wave introduces a periodic variation in the Alfven speed, which generates new Alfven waves. Torsional Alfven wave on a twisted magnetic flux tube are inevitably mixed with the kink mode, and are compressive. The third relates to acoustic waves propagating up to the chromosphere from the convection zone
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