Determination of Interstellar O Parameters Using the First Two Years of Data from the Interstellar Boundary Explorer

by Schwadron et al.

Abstract: The direct measurements of interstellar matter by the Interstellar Boundary Explorer (IBEX) mission have opened a new and important chapter in our study of the interactions that control the boundaries of our heliosphere. Here we derive for the quantitative information about interstellar O flow parameters from IBEX low-energy neutral atom data for the first time. Specifically, we derive a relatively narrow four-dimensional parameter tube along which interstellar O flow parameters must lie. Along the parameter tube, we find a large uncertainty in interstellar O flow longitude, 76.0° ± 3.4° from χ2 analysis and 76.5° ± 6.2° from a maximum likelihood fit, which is statistically consistent with the flow longitude derived for interstellar He, 75.6° ± 1.4°. The best-fit O and He temperatures are almost identical at a reference flow longitude of 76°, which provides a strong indication that the local interstellar plasma near the Sun is relatively unaffected by turbulent heating. However, key differences include an oxygen parameter tube for the interstellar speed (relation between speed and longitude) that has higher speeds than those in the corresponding parameter tube for He, and an upstream flow latitude for oxygen that is southward of the upstream flow latitude for helium. Both of these differences are likely the result of enhanced filtration of interstellar oxygen due to its charge-exchange ionization rate, which is higher than that for helium. Furthermore, we derive an interstellar O density near the termination shock of \( 5.8_{-0.8}^{+0.9} \) x 10-5 cm−3 that, within uncertainties, is consistent with previous estimates. Thus, we use IBEX data to probe the interstellar properties of oxygen.

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Figure 11 of Schwadron et al. 2016: Direction of interstellar neutral O measurements (green) is shown in the context of other interstellar determinations. Panel (a) includes all measurements, and panel (b) is a blow-up focusing on interstellar O, He, and H measurements. Interstellar neutral He atoms, due to their high first ionization potential, predominantly survive the journey from the interstellar medium into 1 au. Therefore, interstellar He represents a particularly good sample of the interstellar flow. In contrast, charge exchange between protons in the outer heliosheath and inflowing interstellar neutral H causes the slowing, heating, and deflection of the neutral H. SOHO/SWAN detects the Lyα resonant backscatter from the inflowing H, providing the average flow direction of interstellar H inside the heliosphere (Lallement et al. 2005, 2010). Deflection of H relative to He measured by IBEX or Ulysses provides a plane (the so-called B–V plane) that is believed to contain the direction of the interstellar magnetic field since it breaks the flow symmetry of the global heliosphere (Lallement et al. 2005; Schwadron et al. 2015c). The blue curve shows the B–V plane containing the flow deflection of interstellar H relative to He based on measurements from SOHO/SWAN and IBEX. Dashed blue curves show the uncertainty limits of the B–V plane. The gray curve is the B–V provided by fitting secondary He in addition to H and primary He (Kubiak et al. 2016). Also shown is the B–V plane (purple curve) based on measurements from SOHO/SWAN and Ulysses. The centers of the IBEX ribbon (Funsten et al. 2013, black points) line up well with the projected B–V plane. The direction of the interstellar magnetic field measured by Voyager 1 (Schwadron et al. 2015c) also lines up well with the B–V plane and the ribbon center when projected out in time based on the observed temporal gradient in the field direction.
Figure 12 of Schwadron et al. 2016: (Top) Schematic of global heliosphere showing compression of heliospheric boundaries southward of the upwind direction. This compression creates conditions that weaken filtration of incident neutral atoms southward of the upwind direction. In contrast, the thickened outer heliosheath to the north of the upwind direction provides for the creation of additional secondaries. (Bottom) Figure adapted from Schwadron et al. (2014) showing the asymmetry in the energetic neutral atom pressure times line of sight. The line of sight integrated pressure maximum is shifted south of the upwind direction, presumably due to compression by the draped local interstellar magnetic field.