2D He+pickup ion velocity distribution functions: STEREO PLASTIC observations

Abstract

Context. He+ pickup ions are either born from the ionization of interstellar neutral helium inside our heliosphere, the so-called interstellar pickup ions, or through the interaction of solar wind ions with small dust particles, the so-called inner source of pickup ions. Until now, most observations of pickup ions were limited to reduced 1D velocity spectra, which are insufficient to study certain characteristics of the He+ velocity distribution function (VDF).Aims. It is generally assumed that rapid pitch-angle scattering of freshly created pickup ions quickly leads to a fully isotropic He+ VDF. In light of recent observations, this assumption has found to be oversimplified and needs to be reinvestigated.Methods. Using He+ pickup ion data from the PLASTIC instrument on board the STEREO A spacecraft, we reconstruct a reduced form of the He+ VDF in two dimensions. This allows us to study relative changes of the 2D He+ VDF as a function of the configuration of the heliospheric magnetic field.Results. Our observations show that the He+ VDF is highly anisotropic and even indicates that, at least for certain configurations of B , it is not fully gyrotropic. Our results further suggest, that the observed velocity and pitch angle of He+ depends strongly on the local solar magnetic field vector, B , the ecliptic longitude, λ , the solar wind speed, v sw , and the global distribution of B . Conclusions. We found two distinct signatures that systematically change as a function of the alignment of B : (1) a ring beam distribution that is most pronounced at w sw > 0.5 and likely attributed to interstellar He+ ; (2) a beam signature aligned parallel to B that is most pronounced at w sw . The strong anisotropy and the aforementioned dependencies of the He+ VDF also imply that observations of 1D velocity spectra of He+ pickup ions are potentially deceiving.