Ionization state and magnetic topology of coronal mass ejections

Abstract

Charge state distributions of heavy solar wind ions measured in interplanetary space can be used to probe the physical conditions in the solar corona. This paper presents a study of the charge state distributions and the magnetic topology of 56 coronal mass ejections (CMEs) observed in interplanetary space by the Ulysses spacecraft. The analysis of the data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument and the Vector Helium Magnetometer (VHM) experiment onboard Ulysses shows a clear correlation between the charge state distributions and the magnetic topology of CMEs. Almost all CMEs whose charge state distributions are shifted to higher charge states with respect to the ambient solar wind have the structure of magnetic clouds, whereas CMEs with the same charge state distributions as the surrounding solar wind do not show magnetic cloud structure. This correlation is found for CMEs observed at low, mid, and high solar latitudes. On the basis of the numerical solution of the ionization/recombination equations for oxygen and silicon, it is investigated which changes of the electron temperature, electron density, and the speed of the ions in the source region of the CMEs can reproduce the observations. It is shown that the main reason for the observed enhancement of higher charge states in the cloud CMEs is an increased electron temperature. However, the evolution of the density and velocity of the CMEs before the charge states freeze in cannot be neglected.