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Kinetic exospheric models revealed that the solar wind is accelerated by anambipolar electric field up to supersonic velocities. The presence ofsuprathermal Strahl electrons at the exobase can further increase the velocityto higher values, leading to profiles comparable to the observations in thefast and slow wind at all radial distances. Such suprathermal electrons areobserved at large distances and recently at low distances as well. Thosesuprathermal electrons were introduced into the kinetic exospheric model usingKappa distributions. Here, the importance of the exobase's altitude is alsounderlined for its ability to maintain the electric potential to a higher levelfor slower winds, conversely to what is induced through the effect of a lowerkappa index only. In fact, the exobase is located at lower altitude in thecoronal holes where the density is smaller than in the other regions of thecorona, allowing the wind originating from the holes to be accelerated fromlower distances to higher velocities. The new observations of Parker SolarProbe (PSP) and Solar Orbiter (SolO) from launch to mid-2023 are here used todetermine the characteristics of the plasma in the corona so that the modelfits best to the averaged observed profiles for the slow and fast winds. Theobservations at low radial distances show suprathermal electrons already wellpresent in the Strahl in the antisunward direction and a deficit in the sunwarddirection, confirming the exospheric feature of almost no incoming particles.

Acceleration of the Solar Wind by Ambipolar Electric Field