Heliosheath Proton Distribution in the Plasma Reference Frame

Abstract

Abstract Properties of the inner heliosheath (IHS) plasma are inferred from energetic neutral atom (ENA) observations by ∼1 au spacecraft. However, the Compton–Getting effect due to the plasma velocity relative to the spacecraft is rarely taken into account, even though the plasma speed is a significant fraction of the ENA speed. In this study, we transform Interstellar Boundary Explorer (IBEX) ENA spectra to the IHS plasma frame using flow profiles from a 3D heliosphere simulation. We find that proton spectra in the plasma frame are steeper by ∼30% to 5% at ∼0.5 to 6 keV, respectively, compared to ENAs in the spacecraft frame. While radial plasma flows contribute most to the Compton–Getting effect, transverse flows at mid/high latitudes and the heliosphere flanks account for up to ∼30% of the frame transformation for IBEX-Hi at ∼0.7 keV and up to ∼60% for IBEX-Lo at ∼0.1 keV. We determine that the majority of IHS proton fluxes derived from IBEX-Hi measurements in 2009–2016 are statistically consistent with power-law distributions, with mean proton index ∼2.1 and standard deviation ∼0.4. We find significantly fewer spectral breaks in IBEX observations compared to early analyses, which we determine were a product of the “ion gun” background prevalent in ∼2009–2012 before corrections made by McComas et al. in subsequent data releases. We recommend that future analyses of the IHS plasma utilizing ENA measurements take into account the Compton–Getting effect including radial and transverse flows, particularly IBEX and Interstellar Mapping and Acceleration Probe measurements below ∼10 keV.