Why Do Temperature and Velocity Have Different Relationships in the Solar Wind and in Interplanetary Coronal Mass Ejections?

Abstract

In-situ observations of the solar wind (SW) show temperature increasing with the wind speed, whereas such a dependence is not observed in interplanetary coronal mass ejections (ICMEs). The aim of this paper is to understand the main origin of this correlation in the SW and its absence in ICMEs. For that purpose both the internal-energy and momentum equations are solved analytically with various approximations. The internal-energy equation does not provide a strong link between temperature and velocity, but the momentum equation does. Indeed, the observed correlation in the open magnetic-field configuration of the SW is the result of its acceleration and heating close to the Sun. In contrast, the magnetic configuration of ICMEs is closed, and moreover the momentum equation is dominated by magnetic forces. This implies no significant correlation between temperature and velocity, as observed.