Why proton temperature and velocity are correlated in the SW and not in ICMEs?

Abstract

The in situ correlation of the proton temperature with the bulk plasma velocity of the solar wind (SW) is a firm result confirmed by several spacecraft, while in interplanetary coronal mass ejections (ICMEs) the proton temperature is both cooler and not correlated with the velocity. For some specific heating mechanism, the internal energy equation alone could provide a temperature increasing with velocity, but the increase remains much weaker than observed in the SW. In fact, it is shown here that the observed correlation is generically obtained from a global energy conservation in thermally driven winds. It results from the conversion of thermal to kinetic energy close to the Sun. The absence (or even anti‐) correlation observed in situ between electron temperature and SW velocity, together with the correlation found for proton temperature, show that protons have a dominant role in the SW acceleration. In contrast, in ICMEs the plasma is contained by the magnetic field since ICMEs have both a closed configuration and a low plasma β. It implies no significant correlation between temperature and velocity, as observed. In conclusion, a different dominating term in the impulsion equation is the main origin of the different relation observed between the proton temperature and the bulk velocity in SW and ICMEs.