Characterization of Solar Wind Strahl Electron Scattering Observed by PSP

We aim to investigate the mechanisms that control solar wind acceleration and energy balance in the near-Sun environment through Parker Solar Probe (PSP) in-situ observations. These processes are theorized to be strongly influenced by the solar wind heat flux and ambipolar electric field, both of which are dependent on electron velocity distribution functions. While the electron core and halo components of the distribution are nearly isotropic, the beam population strahl is more anisotropic and typically magnetic field aligned. Due to this asymmetry and non-thermal nature, the electron strahl contributes a major component of the overall heat flux in the near-Sun environment. The population's beam-like shape can be modified from adiabatic focusing by the magnetic field and electron pitch-angle scattering due to Coulomb collisions and/or plasma instabilities. As PSP is the closest spacecraft to ever orbit the Sun, we can study the evolution of the electron strahl over a large range of heliocentric distances under varying solar wind conditions. The onboard Solar Probe ANalyzer Electron (SPAN-E) instruments have measured electron energy distributions at heliocentric distances between 13 and 80 solar radii in the inner heliosphere. We perform fits on these distributions using SPAN-E and FIELDS data to obtain the strahl direction and angular width for energies between 100 and 1200 eV and orbital encounters 1-9. It is important to note that the strahl direction is determined independently of the magnetic field, which can allow an alternative method for inter calibration with the magnetic field instrument. To characterize the causes of strahl scattering, we also obtain measurements of the electron density, electron temperature, and solar wind velocity from distribution fits to compare with strahl angular widths. We find the strahl beam to narrow with increasing electron energy. Additionally, the strahl width is dependent on heliocentric distance, heliospheric current sheet distance, SW velocity, electron density, and electron collisional age. From these results, an empirical model of the strahl is developed, primarily involving effects from Coulomb collisions and wave scattering.