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

Abstract

Context. The interstellar flow longitude corresponds to the Sun's direction of movement relative to the local interstellar medium. Thus, it constitutes a fundamental parameter for our understanding of the heliosphere and, in particular, its interaction with its surroundings, which is currently investigated by the Interstellar Boundary EXplorer (IBEX). One possibility to derive this parameter is based on pickup ions (PUIs) that are former neutral ions that have been ionized in the inner heliosphere. The neutrals enter the heliosphere as an interstellar wind from the direction of the Sun's movement against the partially ionized interstellar medium. PUIs carry information about the spatial variation of their neutral parent population (density and flow vector field) in their velocity distribution function. From the symmetry of the longitudinal flow velocity distribution, the interstellar flow longitude can be derived. Aims. The aim of this paper is to identify and eliminate systematic errors that are connected to this approach of measuring the interstellar flow longitude; we want to minimize any systematic influences on the result of this analysis and give a reasonable estimate for the uncertainty. Methods. We use He + data measured by the PLAsma and SupraThermal Ion Composition (PLASTIC) sensor on the Solar TErrestrial RElations Observatory Ahead (STEREO A) spacecraft. We analyze a recent approach, identify sources of systematic errors, and propose solutions to eliminate them. Furthermore, a method is introduced to estimate the error associated with this approach. Additionally, we investigate how the selection of interplanetary magnetic field angles, which is closely connected to the pickup ion velocity distribution function, affects the result for the interstellar flow longitude. Results. We find that the revised analysis used to address part of the expected systematic effects obtains significantly different results than presented in the previous study. In particular, the derived uncertainties are considerably larger. Furthermore, an unexpected systematic trend of the resulting interstellar flow longitude with the selection of interplanetary magnetic field orientation is uncovered.

Effect of Rapid Changes of Solar Wind Conditions on the Pickup Ion Velocity Distribution

“…The bulk of this section contains analyses described in Drews et al (2015), Taut et al (2018), and , while adding additional steps to tailor the data set to the purpose of this investigation. The bulk of this section contains analyses described in Drews et al (2015), Taut et al (2018), and , while adding additional steps to tailor the data set to the purpose of this investigation.…”

“…In particular, acceleration processes lead to the development of suprathermal PUI tails (Chalov & Fahr, 1996;Isenberg, 1987;Gloeckler et al, 2008;Fahr & Fichtner, 2011;Taut et al, 2018). These interactions drive deformations in the PUI velocity distribution function (VDF) and, in turn, increasing uncertainty in the inflow determination.…”

“…In the SW frame (represented as a shift by ⃗ v sw to the center of the shell), the PUI speed is determined by its radial distance from the center of the shell, and thus the injection speed is conserved for all IMF orientations (Taut et al, 2018). In the SW frame (represented as a shift by ⃗ v sw to the center of the shell), the PUI speed is determined by its radial distance from the center of the shell, and thus the injection speed is conserved for all IMF orientations (Taut et al, 2018).…”

“…While this quantity has been used to identify inflow to a high degree of statistical certainty , Taut et al (2018) showed that an intrinsic dependence on v sw is introduced to the radial injection speed by normalizing the velocity measurements. In the SW frame, for any IMF orientation, and neglecting SW interaction, the cutoff speed falls at w ′ = 1, for any PUI injected from rest.…”

“…Here, compiling a distribution function in v r results in a cutoff value directly related to the radial speed of the parent neutral atom at injection Taut et al (2018). Here, compiling a distribution function in v r results in a cutoff value directly related to the radial speed of the parent neutral atom at injection Taut et al (2018).…”