Three-dimensional He⁺ pick-up ion velocity distribution functions observed with STEREO-A PLASTIC

Context. Freshly injected interstellar pick-up ion (PUIs) are expected to exhibit a simple, torus-shaped velocity distribution function (VDF). The PUI velocity in the solar wind (SW) frame depends on the velocity of the interstellar neutral (ISN) population at the pick-up position. Aims. We directly compare PUI VDFs measured by the plasma and suprathermal ion composition (PLASTIC) instrument over the full orbit of solar terestrial relations observatory-ahead (STEREO-A). Methods. The STEREO-PLASTIC-A PUI observations were re-analysed, and instrumental effects of the limited field of view (FOV) were accounted for. We then defined a new position-independent velocity measure for PUIs that takes the local direction of the ISN inflow into account. The resulting new PUI velocity measure corrects thereby for the position-dependent contribution of the ISN velocity. Each position in the orbit of STEREO can be reached by ISNs following one of two trajectories, which we call primary and secondary trajectories. Therein, ISNs following the primary trajectory have a higher probability to reach the location before they are ionised than particles following the secondary trajectory. Our new PUI velocity measure can be applied based on the assumption that all particles followed the primary trajectory or that all particles followed the secondary trajectory. Pitch-angle distributions were then analysed depending on the magnetic field azimuthal angle for different orbital positions and different values of the PUI velocity measure. Results. The new velocity measure, ϖ inj, p , shows an approximately constant cut-off over the complete orbit of STEREO-A. A torus signature is visible everywhere. Therein, a broadening of the torus signature outside the focusing cone and crescent regions and for lower ϖ inj, p is observed. In addition, we illustrate the symmetry between the primary and secondary ISN trajectory in the vicinity of the focusing cone. Conclusions. A torus signature associated with freshly injected PUIs is visible over the complete orbit of STEREO-A with increased density in the focusing cone. At least remnants of a torus signature remain for lower values of the PUI velocity measure. The new velocity measure also prepares for PUI studies with Solar Orbiter.