Abstrakt
Relative properties of solar wind protons and alpha particles are often used as indicators of a source region on the solar surface, and analysis of their evolution along the solar wind path tests our understanding of physics of multicomponent magnetized plasma. The paper deals with the comprehensive analysis of the difference between proton and alpha particle bulk velocities at 1 au with a special emphasis on interplanetary coronal mass ejections (ICMEs).
A comparison of about 20 years of Wind observations at 1 au with Helios measurements closer to the Sun (0.3-0.7 au) generally confirms the present knowledge that (1) the differential speed between both species increases with the proton speed; (2) the differential speed is lower than the local Alfven speed; (3) alpha particles are faster than protons near the Sun, and this difference decreases with the increasing distance. However, we found a much larger portion of observations with protons faster than alpha particles in Wind than in Helios data and attributed this effect to a preferential acceleration of the protons in the solar wind.
A distinct population characterized by a very small differential velocity and nearly equal proton and a particle temperatures that is frequently observed around the maximum of solar activity was attributed to ICMEs. Since this population does not exhibit any evolution with increasing collisional age, we suggest that, by contrast to the solar wind from other sources, ICMEs are born in an equilibrium state and gradually lose this equilibrium due to interactions with the ambient solar wind.