Study of the Solar Wind Plasma Thermodynamics in the Solar Corona Based on the Charge State of Heavy Ions

The solar wind (SW) plasma thermodynamics in the solar corona is determined by the energy exchange with external sources and can be studied if information about physical plasma parameters, such as the SW temperature, density, flow velocity, etc., is known. Previously, Parker showed that within the one-fluid model the SW plasma state could be described by a polytropic function in which the pressure \(p\) and density \(\rho\) are related by the relation \(p/\rho^{\gamma}=\textrm{const}\) with the polytropic index \(\gamma\). In present-day MHD models the application of a polytropic function instead of an approximate description of the plasma heating mechanisms speeds up the computation considerably. The polytropic index \(\gamma\) can be estimated using the SW plasma parameters, but for the SW flows moving toward the Earth measuring such parameters presents certain difficulties. In this paper we consider a method to determine the polytropic index \(\gamma\) for the SW flows at the stage of expansion in the corona from the SW plasma ion parameters measured in situ: the mean Fe ion charge \(\langle\textrm{Q}_{\text{Fe}}\rangle\) and the \(\textrm{O}^{7+/}\) \(\textrm{O}^{6+}\) ion density ratio. The relation between the ion parameters and the polytropic index \(\gamma\) is established by solving the balance equations for the ionization and recombination processes in the SW plasma. The mean values of \(\gamma\) in the corona at heights of \({\approx}\)1–7 solar radii for the flows of the slow SW, the fast SW from coronal holes, and interplanetary coronal mass ejections have been obtained from the histograms of the SW ion parameters measured with the ACE/SWICS instrument in 2010.