Fe Ion Composition in Solar-Wind Streams in the Solar Corona and Heliosphere

Abstract

Analysis of the distributions of Fe ions over the stages of ionization in solar-wind (SW) plasma provides valuable information on the formation of SW streams and plasma heating processes, as well as for identifying SW sources on the Sun. When passing SW plasma through the corona, its ion composition evolves and finally “freezes” at distances of the order of several solar radii from the solar surface, remaining further practically unchanged in the heliosphere. This makes it possible to obtain information about the physical conditions in its source and the solar corona from the SW charge state. Average charge QFe, which is usually used to characterize the distributions of Fe ions, does not take into account all the features of the evolution of the ion composition, which does not allow one to extract more detailed information about the state of SW plasma. In this study, to describe the charge state of Fe ions, three parameters q4, q8, and q12 are introduced, which characterize the relative fractions of ions with charges Z = 0–7, 8–11, and 12–20, respectively, and conditionally correspond to “cold,” “medium,” and “hot” SW plasma components. According to the measurements of the Fe ion composition in 2010 on the STEREO-A spacecraft, the characteristic values of these parameters for different types of SW streams are given. The problem of modeling ion distributions in SW plasma based on diagnostic data on the parameters of coronal sources is considered. Using the example of the event associated with coronal-mass ejection on August 18, 2010, it is shown that the parameters of the charge state of Fe ions calculated from the model distributions are in good agreement within errors with the measurement data.