A Ulysses Detection of Secondary Helium Neutrals

Abstract

The Interstellar Boundary EXplorer (IBEX) mission has recently studied the flow of interstellar neutral He atoms through the solar system, and discovered the existence of a secondary He flow likely originating in the outer heliosheath. We find evidence for this secondary component in Ulysses data. By coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise, we identify a weak signal that is credibly associated with the secondary component. Assuming a laminar flow from infinity, we infer the following He flow parameters: V = 12.8 ± 1.9 km s −1 , λ = 74.4 ± 1.8• , β = −10.5 ± 4.1• , and T = 3000 ± 1100 K; where λ and β are the ecliptic longitude and latitude direction in J2000 coordinates. The secondary component has a density that is 4.9 ± 0.9% that of the primary component. These measurements are reasonably consistent with measurements from IBEX, with the exception of temperature, where our temperature is much lower than IBEX's T = 9500 K. Even the higher IBEX temperature is suspiciously low compared to expectactions for the outer heliosheath source region. The implausibly low temperatures are due to the incorrect assumption of a laminar flow instead of a diverging one, given that the flow in the outer heliosheath source region will be deflecting around the heliopause. As for why the IBEX and Ulysses T values are different, difficulties with background subtraction in the Ulysses data are a potential source of concern, but the discrepancy may also be another effect of the improper laminar flow assumption, which could affect the IBEX and Ulysses analyses differently.

Following the interstellar magnetic field from the heliosphere into space with polarized starlight

“…The filament polarizations ( §4.2, Frisch et al 2015b,a) and the warm breeze of deflected interstellar neutrals (Kubiak et al 2014;Bzowski et al 2015;Kubiak et al 2016;Schwadron et al 2016;Wood et al 2017;Bzowski et al 2017) trace the same interstellar upwind direction at the heliosphere nose ( Table 1). The alignment of these two directions suggests that the parent interstellar ions of the secondary neutral atoms, and the charged interstellar dust grains, are trapped in the same interstellar magnetic field lines that are interacting with the heliosphere.…”

“…-27 -A warm breeze of secondary interstellar He • atoms, originating mainly with interstellar helium ions displaced by Lorentz forces, 10 was discovered by IBEX and confirmed by Ulysses (Kubiak et al 2014;Schwadron et al 2015aSchwadron et al , 2016Kubiak et al 2016;Bzowski et al 2017;Wood et al 2017). The spatial offsets between the directions of primary and secondary neutrals are aligned with the B chm -V chm plane (Figure 13, adapted from Schwadron et al 2015b;, The magnetic field direction in Figure 13 (star) is given by MHD models of the IBEX ribbon (Zirnstein et al 2016).…”

Evidence for Asymmetry in the Velocity Distribution of the Interstellar Neutral Helium Flow Observed by IBEX and Ulysses

“…We refer the reader to Wood et al (2015) for details. Before fitting the data, we subtract the secondary He contribution reported by Wood et al (2017). Figure 10(a) shows an example of a Ulysses He beam map, from 2001 February 4, and the best fit to this data from Wood et al (2015) in Figure 10(b), with the residuals in Figure 10(c).…”

Collisional Heating of Interstellar Helium Flux at 1 AU

“…In situ measurements of low‐energy neutral atom fluxes could rely on various experimental techniques (Gruntman, , , ; Moebius, Kucharek, et al, ; Rosenbauer et al, ). The He atom instruments on Ulysses (Rosenbauer et al, ; Witte et al, , ) and IBEX (Fuselier et al, ), based on different physical detection processes, obtained interstellar helium properties that are in general but not complete agreement (Bzowski et al, , ; Moebius et al, , ; Witte, ; Wood et al, , ).…”

“…Gruntman () also pointed out, but not quantified, that elastic collisions with solar wind ions would broaden the core, the main “hump,” of directional intensity distributions, thus increasing derived LISM temperatures. Accounting for this heretofore disregarded effective heating of helium is particularly important because the stated uncertainties of LISM temperatures obtained from the Ulysses and IBEX observations are only a few hundred kelvins (Bzowski et al, ; Wood et al, , ). In addition, the derived interstellar wind velocity vectors and temperatures are interdependent and the velocity vector provided the foundation for the conclusions of temporal and/or spatial evolution of the LISM (e.g., Frisch et al, , ).…”