Plasma Pressures in the Heliosheath From Cassini ENA and Voyager 2 Measurements: Validation by the Voyager 2 Heliopause Crossing

Abstract

We report “ground truth,” 28‐ to 3,500‐keV in situ ion and 5.2‐ to 55‐keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle detector and Cassini/Ion and Neutral Camera, respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ∼0.12 cm−3 and an interstellar magnetic field strength of ∼0.5‐nT upstream of the heliopause in the direction of V2, that is, consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ∼119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2‐keV ion energy spectra play a critical role toward determining the pressure balance and acceleration mechanisms inside the heliosheath.

Combined ∼10 eV to ∼344 MeV Particle Spectra and Pressures in the Heliosheath along the Voyager 2 Trajectory

“…At ∼5.2-55 keV energies, the ENA spectra from INCA become significantly softer, consistent with a power law (e.g., 2009-2012: J∼E −(3.5±0.2) and 2013-2016: J∼E −(4.2±0.2) ), with a possible hardening break at ∼35-55 keV. This high-energy break is shown to be consistent with in situ ion measurements from LECP on V2 (Dialynas et al 2019).…”

“…Assuming that all ∼0.11 to 55 keV ENAs shown in Figure 1 are due to charge-exchange (CE) interactions (Lindsay & Stebbings 2005) between heliosheath ions and an IS neutral H distribution of n H ∼0.12 cm −3 , over a line of sight of L HS ∼35 au (measured HS thickness in the V2 direction), measured ENA intensities are convertible to ion intensities about the V2 pixel and vice versa (e.g., Krimigis et al 2010;Dialynas et al 2019). The assumed n H is consistent with the electron densities observed by the Plasma Wave (PWS) instruments on V1 and V2, at the order of ∼0.04-0.14 cm −3 (Gurnett et al 2013(Gurnett et al , 2015Gurnett & Kurth 2017;Kurth & Gurnett 2020), which are comparable to n H , assuming that the equilibrium ionization fraction is ∼50% for the LISM.…”

“…These crossings led to the discovery of the reservoir of suprathermal particles and weak magnetic fields that constitute the heliosheath (HS). This region is characterized by plasma with β (particle pressure divided by the magnetic field pressure; P particle /P MAG ) always ?1 and mostly >10 (e.g., Decker et al 2015;Dialynas et al 2019), and extends from the TS out to the heliopause (HP), the outer boundary of our solar bubble.…”

“…The INCA images also revealed two prominent "basins," identified as two extended heliosphere lobes, where ENA minima occur (Krimigis et al 2009;Dialynas et al 2013). Remotely sensed >5.2 keV INCA/ ENAs, together with in situ>28 keV ion measurements from the Voyager missions in overlapping energy bands (Decker et al 2005), were combined to estimate the V1 and V2 HP crossing distances (Krimigis et al 2011;Dialynas et al 2019), and the magnitude of the magnetic field upstream of the HP (Krimigis et al 2010;Dialynas et al 2019). These measurements showed that the heliosphere responds promptly, within ∼2-3 years, to outward-propagating SW changes in both the nose and tail directions over the solar cycle and supported the interpretation of a diamagnetic, "bubble-like" heliosphere, with few substantial tail-like features (Dialynas et al 2017a(Dialynas et al , 2017b.…”

Statistical Uncertainties of Space Plasma Properties Described by Kappa Distributions

“…Moreover, the authors of [ 33 , 34 , 35 , 36 , 37 , 38 , 39 ] derive the VDFs of plasma particles within the outer heliosphere and within the inner heliosheath and show that kappa distribution functions fit accurately the observations. However, the study by [ 40 ] shows that a single kappa distribution cannot describe the entire energy spectra of heliosheath particles within a few eVs to MeVs. Possibly, a combination of kappa distributions is a better description of these observations.…”