Electron Properties and Coulomb Collisions in the Solar Wind at 1 AU:WindObservations

Abstract

The question of what controls the electron properties in the solar wind has been the subject of several extensive analyses over the past 20 years. We analyze here the electron properties of the solar wind observed by the Wind satellite at 1 AU in the ecliptic plane, during 50 days close to the last minimum of solar activity. The electron temperature anisotropy Te∥/Te⊥, which seems to depend on the wind speed Vsw, the density Np, the heliomagnetic latitude λm, or the time, actually depends mainly on the Coulomb collisions. The collisional age Ae is the number of transverse collisions suffered by a thermal electron during the expansion of the wind over the scale of the density gradient. The Ae depends on Vsw, on Np, and thus on λm; it also depends on the time because it changes strongly at the crossing of a stream interface. We show that Te∥/Te⊥ is strongly correlated with 1/Ae. The effect of Coulomb collisions on the electron heat flux are also investigated. We find that the total electron heat flux Qe displays an upper bound that is inversely proportional to the collisional age, in favor of a regulation of the heat flux by Coulomb collisions. The observed heat flux is then compared to the collisional heat flux of the classical Spitzer-Harm (SH) theory. Although earlier observations have shown that the electron heat flux in the solar wind at 1 AU is well below the values given by the SH theory, we find that the observed heat flux reaches the SH limit for the lowest values of the electron mean free paths. The Coulomb collisions thus seem to play a part in the regulation of the electron heat flux in the solar wind.