Observations of electron velocity distribution functions in the solar wind by the WIND Spacecraft: High angular resolution Strahl measurements

The WIND Solar Wind instrument (SWE) includes a sensor especially configured to measure the solar wind strahl. The strahl is the excess electron (≥100 eV) halo component of the solar wind most closely aligned with the magnetic field. Strahl electrons originate in the inner corona and move freely out to 1 A.U., providing information on the state of the corona. Electron data acquired during seven solar rotations show that the electron velocity distributions are most anisotropic and the strahl flux most intense during high speed streams. The strahl angular width is smallest when the solar wind velocity is largest, approximately 5° in width at 600 eV, and becomes much wider when the velocity is low. Coincident with the strahl is a small reverse electron flux which does not form a beam but fills the field of view. This variability of the flux and angular distribution of the strahl and anti‐strahl suggests that a pitch angle scattering process may be acting to broaden the strahl and that the anti‐strahl may be due to backscattering of the strahl.