Proton Distribution in the Heliosheath: From Particle Acceleration at the Termination Shock to Energetic Neutral Atom Observation near Earth

The Interstellar Boundary Explorer (IBEX) measures energetic neutral atom (ENA) emissions from the outer heliosphere in the energy range ~0.5-6 keV. ENAs are produced by charge exchange between energetic pickup ions (PUIs) in the heliosheath and cold neutral atoms from the local interstellar medium. PUIs are preferentially accelerated at the heliospheric termination shock (HTS) before propagating through the heliosheath. Thus, IBEX ENA measurements reveal line-of-sight averaged proton spectra over the sky and their acceleration mechanism.

We present results from two studies that derive the properties and origin of the heliosheath proton distribution. First, we transform IBEX ENA observations to proton fluxes in the heliosheath plasma reference frame using a 3D simulation of the heliosphere, where the Compton-Getting transformation is sensitive to the radial and transverse flows of the heliosheath plasma. We find that most proton spectra derived from IBEX data in 2009-2016 are statistically consistent with power-law distributions with mean spectral index 2.1 and show slowly varying structures across the sky.

The “power-law” distribution of proton fluxes likely reveals the mechanism of PUI acceleration at the HTS. Using a test particle simulation, we show that PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without turbulence at wavenumbers (k) close to the PUI gyroradius (R_g). Voyager 2 observations of the downstream magnetic field reveal a moderate amount of turbulence ((dB/B₀)² ~ 0.01) at kR_g ~ 1 that can produce a PUI tail but not at intensities observed by IBEX. However, motivated by Voyager 2 measurements of stronger turbulence within the shock ramp, we show that a proton distribution can develop a tail consistent with IBEX data if (dB/B₀)² ≧ 0.1 at kR_g ~ 1 near the shock foot. Thus, IBEX observations reveal the global acceleration of PUIs at the HTS via shock drift acceleration.