Pickup hydrogen distributions in the solar wind at ∼11 AU: Do we understand pickup ions in the outer heliosphere?

Abstract

[1] This study examines the detailed distribution of interstellar pickup hydrogen ions measured by the Surface Wave Process Program instrument on the New Horizons spacecraft en route to Pluto. The observations cover an 8 h interval on 15 October 2008 when the craft was at ∼11.3 AU, constituting the first detailed pickup ion measurements in the outer heliosphere, beyond ∼8 AU. We show the pickup ion cutoff to be roughly twice the solar wind speed, as expected; however, we also measure both a significant suprathermal tail above this "cutoff" and a pickup ion asymmetry between inward and outward ions of ∼4:1, very similar to that observed on Ulysses at <5 AU. This anisotropy could indicate some combination of a scattering mean free path that is much longer than previously realized, significantly more radial magnetic field than expected, some missing effect in the current models, and/or some unknown background in the low-energy measurements. Alternately, these observations could indicate that we need to develop a new understanding of pickup ion transport throughout the heliosphere. Understanding these new observations is especially important because recent Voyager measurements have shown that these pickup ions play a critical role in the interaction at the heliosphere's termination shock. Thus, the observations reported here may be important for understanding the heliosphere's whole interstellar interaction.

OBSERVATIONS OF ISOTROPIC INTERSTELLAR PICK-UP IONS AT 11 AND 17 AU FROMNEW HORIZONS

“…Since anisotropic PUI models predict more PUIs at low E/q, the measured spectrum of PUIs would appear as though they are anisotropic if the standard constant G is used. This is the reason McComas et al (2010) initially reported a large PUI anisotropy, comparable to that found by Gloeckler et al (1995).…”

“…The values of μ and ν E are consistent with predictions of these parameters around solar minimum (Ruciński & Bzowski 1995) and consistent with the fact that the measurements were taken during the last extended solar minimum (McComas et al 2008b). The interstellar gas parameters (N, U, and T) were taken from McComas et al (2010) to allow for a direct comparison with their model. We note that these parameters and their uncertainties have just been revised based on new observations from the Interstellar Boundary Explorer (see McComas 2012 and references therein).…”

“…Although we show the spectra as instrument count rate rather than one-dimensional phase space density as in McComas et al (2010), we also observe the familiar aspects of the PUI distribution. Both spectra display the characteristic high-energy PUI drop-off, with the suprathermal tail just beyond that.…”

“…In the first detailed measurements of PUIs by SWAP, reported by McComas et al (2010), PUIs appeared to be highly and unexpectedly anisotropic. PUIs were first discovered to be anisotropic at high latitudes, where the HMF is quasi-radial (Gloeckler et al 1995).…”

The &quot;step feature&quot; of suprathermal ion distributions: a discriminator between acceleration processes?

“…5, our results from Fig. 4 (here for 11 AU) are (after an arbitrary shift corresponding to a normalization constant) overplotted (as the black dotted line) on the figure published by McComas et al (2010). Evidently, the step feature can clearly be identified in these data.…”