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

Institut für Experimentelle und Angewandte Physik (IEAP), Christian Albrechts-Universität zu Kiel, Leibnizstrasse 11, 24118 Kiel, Germany
e-mail: drews@physik.uni-kiel.de

Received: 4 November 2014
Accepted: 21 January 2015

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 < 0.5 and attributed to inner-source He⁺. 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.