Authors: Chadi Salem (University of California Berkeley), John Bonnell (University of California Berkeley) and Marc Pulupa (University of California Berkeley)

We present a statistical analysis of solar wind electrons at 1AU using several years of accurate core, halo and strahl electron parameters from the Wind spacecraft to investigate the properties and variability of the spacecraft floating potential as well as the total photoelectron current on Wind.

This work is based on the comprehensive analysis of electron data from the EESA electrostatic analyzers of the Wind 3D-Plasma experiment.  

We developed a comprehensive and sophisticated algorithm for a non-linear fitting analysis of the structure of the eVDF in the solar wind using data from both 3DP/EESA-Low and 3DP/EESA-High electrostatic analyzers, with the aim of producing large data sets of accurate and reliable total and core-halo-strahl electron parameters over the entire Wind mission: the first ever such analysis for that instrument suite (Salem et al., A&A, 2023).

The important component of the technique is how we estimate the spacecraft potential of the Wind spacecraft, a crucial unknown parameter that distorts the thermal part of the distribution severely. The spacecraft floating potential is initially estimated using a current balance model, and relies on the use of completely independent measurements of the electron density obtained from the fit of the spectrum of quasi-thermal noise around the electron plasma frequency measured by the  Wind/Waves electric field antennas.  This “quasi thermal noise” (QTN) technique is immune to spacecraft potential and therefore offer an independent and highly accurate measure of electron density and temperature, which we use as reference.  Final “fit” values of the spacecraft potential are obtained by comparing the total density of the eVDF fit to the QTN density; the algorithm requires that they are equal.

We discuss the properties and variability of this “measured” spacecraft potential, in comparison to the potential determined as a solution of a comprehensive current balance model between photoelectrons, and solar wind electrons and ions, taking into account both their thermal and bulk motions. We use this current balance model to derive an estimate of the unknown photoelectron current and we discuss its properties and variability.