Acceleration of the Solar Wind by Ambipolar Electric Field

Abstract

Kinetic exospheric models revealed that the solar wind is accelerated by an ambipolar electric field up to supersonic velocities. The presence of suprathermal Strahl electrons at the exobase can further increase the velocity to higher values, leading to profiles comparable to the observations in the fast and slow wind at all radial distances. Such suprathermal electrons are observed at large distances and recently at low distances as well. Those suprathermal electrons were introduced into the kinetic exospheric model using Kappa distributions. Here, the importance of the exobase's altitude is also underlined for its ability to maintain the electric potential to a higher level for slower winds, conversely to what is induced through the effect of a lower kappa index only. In fact, the exobase is located at lower altitude in the coronal holes where the density is smaller than in the other regions of the corona, allowing the wind originating from the holes to be accelerated from lower distances to higher velocities. The new observations of Parker Solar Probe (PSP) and Solar Orbiter (SolO) from launch to mid-2023 are here used to determine the characteristics of the plasma in the corona so that the model fits best to the averaged observed profiles for the slow and fast winds. The observations at low radial distances show suprathermal electrons already well present in the Strahl in the antisunward direction and a deficit in the sunward direction, confirming the exospheric feature of almost no incoming particles.