On the Anisotropy of the He+, C+, O+, and Ne+ Pickup Ion Velocity Distribution Function: STEREO PLASTIC Observations

Abstract:

PickUp Ions (PUIs) are typically characterized by (1) their almost exclusively single charge state, (2) a highly non-thermal and anisotropic Velocity Distribution Function (VDF) [Drews et al., 2015], and (3) an extended source population of neutral atoms somewhere between the observer and the Sun. The origin of pickup ions ranges from sources only several solar radii away from the Sun, the so-called inner-source of pickup ions, up to a distance of several hundreds of astronomical units, the local interstellar medium. Their continuous production inside the heliosphere and complex interactions with the magnetized solar wind plasma leads to the development of non-thermal, anisotropic features of both the solar wind and pickup ion velocity distribution functions.

In this study, we present observations of the VDF of He⁺, C⁺, N⁺, O⁺ and Ne⁺ pickup ions with PLASTIC on STEREO A. We have found a PUI flux increase during perpendicular configurations of the local magnetic field that is generally linked to the existence of a so-called torus-distribution [Drews et al., 2015] which is attributed to the production of PUIs close to the observer. A comparison of the PUI VDF between radial and perpendicular configurations of the local magnetic field vector is used to quantify the anisotropy of the PUI VDF and thereby enables us to estimate the mean free path for pitch-angle scattering of He, C, N, O and Ne pickup ions without the necessity of an over-simplified heliospheric model to describe the PUI phase space transport. Our results show a clear signature of a C⁺ torus signature at 1 AU as well as significant differences between the anisotropies of the He+ and O⁺ VDF. We will discuss our results in the light of recent studies about the nature of the inner-source of PUIs [Berger et al., 2015] and observations of the 2D VDF of He⁺[Drews et al., 2015].

Figure Caption: Velocity space diagrams of a pickup ion torus distribution as a (v_x-v_y)-projection (top left panel) and in the v_z = 0 km/s plane (top right) are shown for magnetic configuration in which B is almost perpendicular. The bottom two panels show the torus distribution under the influence of pitch-angle scattering (right) and adiabatic cooling (left). To illustrate the torus character of the distribution the (v_x-v_y)-plane is slightly tilted in this diagram.