On the anisotropy of the He+ velocity distribution function

Abstract

Shortly after their ionization, PickUp Ions (PUIs) develop a highly anisotropic toroidal Velocity Distribution Function (VDF) due to their interaction with the magnetized solar wind plasma. In this study we present a method to quantify the anisotropic state of pickup ion velocity distribution functions via in-situ PUI observations. The ratio R(wsw) = f (wsw)⊥/f (wsw)|| is obtained by sorting the observed PUI velocity spectra, f (wsw), into two different magnetic regimes, one magnetic field orientation, Φ⊥, in which the PUI torus distribution lies inside the instrument’s field of view (Φ⊥), and one in which the torus distribution lies exclusively outside the instrument’s aperture (Φ‖). Because R(wsw) is a measure of the relative flux increase induced by PUIs that are distributed anisotropically compared to PUIs distributed isotropically in phase space, we can quantify the anisotropy of the pickup ion VDF via this approach. The method is then applied to the example of He+ observation by STEREO PLASTIC in an energy range between 0.2 < wsw < 2.7