Ionization state of the interplanetary plasma

Ionization processes in an expanding solar corona are discussed. Integration of the ionization equations demonstrates that the ionization state of the expanding coronal plasma ‘freezes in’ deep in the corona and remains unchanged in transit through interplanetary space. The comparison with Vela satellite observations of solar wind oxygen ions makes possible the determination of the temperature of the coronal region from which the solar wind has emanated. For one particular example this leads to the identification of an active region on the solar disc as the source of the solar wind. Charge exchange with interplanetary neutral hydrogen is suggested to explain the observed presence of He+ and O+5 in the solar wind.