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 < 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.

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

“…While traveling outward with the solar wind, the initial anisotropic VDF is gradually transformed into an isotropic VDF by pitch-angle scattering, which until recently was expected to be a rapid process. However, independent observations made by different spacecraft showed that in particular the He + VDF remains anisotropic for much longer than previously assumed (Oka et al 2002;Drews et al 2015;Taut et al 2016).…”

“…We define the ideal torus as the theoretical velocity-space trajectory that an injected PUI with initial speed V n = 0 would follow under a perfectly stable IMF configuration. Former PUI studies (e.g., Drews et al 2015Drews et al , 2016 and also Möbius et al (2015) restrict the IMF data to orientations that lie ±20 • within the ecliptic plane. Then it solely depends on the azimuthal angle of the IMF whether an ideal torus falls into PLASTIC's FoV or not.…”

“…2, transforming the PUI speed into the solar wind frame of reference has the advantage that for a given solar wind speed, the injection speed of freshly injected PUIs is conserved for a given ecliptic longitude under any IMF orientation. Therefore, in Drews et al (2015), the data product w He + = v He + v sw has been introduced. This data product has also been utilized in Möbius et al (2015) to determine the cutoff speed.…”

Anisotropy of the He+, C+, N+, O+, and Ne+pickup ion velocity distribution functions

“…For instance, a ratio of R(w sw ) = 3 implies that only ∼50% of the observed pickup ions are distributed isotropically in phase space. However, this is based on the assumption that (1) the anisotropic state of the pickup ion VDF is solely caused by the initial pickup ion torus distribution, and that the torus distribution behaves as described in Drews et al (2015). This means that (2) the inclination of the torus systematically changes with the orientation of B; (3) it does not change in intensity as a function of its orientation and especially that (4) a torus distribution is a common feature neither confined to special heliospheric conditions nor individual pickup ion species.…”

“…As described in Drews et al (2015), the observation of a torus or ring-beam signature of He + pickup ions is confined to specific configurations of the local magnetic field vector. These configurations depend mainly on the angular acceptance of the observing instrument.…”