Characteristics and Evolution of Strahl Electrons in the Inner Heliosphere

The solar wind strahl is a narrow, magnetic field aligned, suprathermal electron beam observed in the energy range from ~50 eV to ~1 keV that carries heat flux away from the solar corona. Because of their small gyro-radius, high gyro frequency, and large mean free path, strahl electrons are excellent tracers of magnetic topology. It is important to understand their behavior in order to better understand the global coronal structure. We present observations of the strahl made by the SPAN-Electron sensors of the SWEAP instrument on Parker Solar Probe (PSP) from 35 to 50 Rsun to show that, in general, the strahl width is narrower at closer heliocentric distances. However, there is considerable variation in strahl width as a function of plasma parameters such as plasma beta, collisional age, solar wind speed and plasma density. Furthermore the beam width tends to get narrower with increasing electron energy. The strahl width is controlled by competing mechanisms: The mirror force of the diverging solar magnetic field tends to narrow the beam whereas scattering processes such as wave particle interactions and Coulomb collisions tend to widen the beam. Using the strahl characteristics measured inside of 50 Rs, we will address the relative importance of these mechanisms.