STORMS

The solar winds measured in situ have been classified in at least two types based on their bulk properties: “fast” (>500 km/s) wind from coronal holes and the “slow” (`<`500 km/s) solar wind (SSW) of enigmatic origin. The winds originate in the solar corona, a multi-species, multi-temperature magnetised plasma heated by yet undetermined processes. The trace elements (minor heavy ions) transported in the winds exhibit peculiar properties that are suspected signatures of the fundamental plasma processes at play during the heating of the corona and the release process of the wind. For example, spectroscopic observations show that the abundance of a subset of these minor ions, those with low First Ionisation Potential (FIP) (such as Mg, Si, Fe), is much higher in the solar corona than in the photosphere.
Since the magnitude of this ‘FIP effect’ in closed field regions of the corona is similar to that measured in the SSW, it has been argued that the SSW forms by the release onto open magnetic field lines of plasma initially confined to closed field regions of the corona (dynamic theory of the SSW). An alternative theory (here termed ‘quasi-steady state’) suggests that the SSW exists regardless of these magnetic reconfigurations and is formed along open magnetic flux tubes that undergo strong expansion between the photosphere and the upper corona. The fractionation of elements according to their ionisation potential is also seen in other stellar coronae and the process reduces as one moves to later spectral types with measured inverse-FIP effects detected seldomly in the solar corona and frequently on M-type stars.