Using Kappa Functions to Characterize Outer Heliosphere Proton Distributions in the Presence of Charge-Exchange

Abstract

Kappa functions have long been used in the analysis and modeling of suprathermal particles in various space plasmas. In situ observations of the supersonic solar wind show its distribution contains a cold ion core and powerlaw tail, which is well-represented by a kappa function. In situ plasma observations by Voyager, as well as observations of energetic neutral atom (ENA) spectra by the Interstellar Boundary Explorer (IBEX), showed that the compressed and heated inner heliosheath (IHS) plasma beyond the termination shock can also be represented by a kappa function. IBEX exposes the IHS plasma properties through the detection of ENAs generated by chargeexchange in the IHS. However, charge-exchange modifies the plasma as it flows through the IHS, and makes it difficult to ascertain the parent proton distribution. In this paper we investigate the evolution of proton distributions, initially represented by a kappa function, that experience losses due to charge-exchange in the IHS. In the absence of other processes, it is no longer representable by a single kappa function due to the energydependent, charge-exchange process. While one can still fit a kappa function to the evolving proton distribution over limited energy ranges, this yields fitting parameters (pseudo-density, pseudo-temperature, pseudo-kappa index) that depend on the energy range of the fit. We discuss the effects of fitting a kappa function to the IHS proton distribution over limited energy ranges, its dependence on the initial proton distribution properties at the termination shock, and implications for understanding the observations.

Geometry and Characteristics of the Heliosheath Revealed in the First Five Years of Interstellar Boundary Explorer Observations

“…In the flow model, which is derived in Appendix A, the solar wind is represented as flow from a point source, the LISM flow past the Sun is represented as a uniform flow from infinity, and a flow divergence term is included due to the presence of charge exchange (and therefore the flow is slightly compressible, but still irrotational). Similar flow models have been used in recent heliospheric studies by, e.g., Siewert et al (2014), Isenberg et al (2015), Röken et al (2015), Sylla & Fichtner (2015), and Zirnstein & McComas (2015).…”

“…, and charge-exchange frequency ν ex = 7.5 × 10 −9 s −1 ) the timescale for acceleration due to charge exchange in the IHS (e.g., Florinski et al 2004;Zirnstein & McComas 2015),…”

“…We employ the same flow streamline model described in Zirnstein & McComas (2015). For completeness, we briefly describe the flow model equations here (for the full derivation see Zirnstein & McComas 2015).…”

“…Several early models of ENA flux from the heliosphere focused on sources from the IHS (e.g., Gruntman 1992; Hsieh et al 1992;Czechowski et al 2001;Gruntman et al 2001). The IHS has remained an important subject for ENA modeling in recent years (e.g., Opher et al 2013;Desai et al 2014;Siewert et al 2014;Zirnstein et al , 2015bGloeckler & Fisk 2015;Heerikhuisen et al 2015;Zirnstein & McComas 2015).…”

Helium Energetic Neutral Atoms from the Heliosphere: Perspectives for Future Observations

“…Nonetheless, in this analysis it is assumed that b = 0. Zirnstein & McComas (2015) and Zirnstein et al (2016b) in their analyses calculated the flow lines effectively using the potential Φ CG with Φ 0 = Φ P and b = −α cx /(12γ), obtaining the additional term in a spherically symmetric form, but the flow has a sink point at a finite distance. In this analysis, two flow potentials are considered.…”