Relating Energetic Ion Spectra to Energetic Neutral Atoms

Abstract

Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.

Probing the Length of the Heliospheric Tail with Energetic Neutral Atoms (ENAs) from 0.52 to 80 keV

“…We assume a Maxwellian distribution for the transmitted and reflected PUIs, which are PUI populations energized to ∼1-3 keV and ∼3-5 keV, respectively, at the termination shock (Zank et al 2010). We also model a PUI population, accelerated at the termination shock via diffusive shock acceleration, using a power-law distribution function (Wang et al 2023):…”

“…Additionally, v is the individual particle velocity, and v l and v m are the minimum and maximum velocities for particle acceleration at the termination shock. The detailed derivation and discussion of Equation (1) can be found in Wang et al (2023).…”

“…The long-tail solution is based on the Moscow model (Izmodenov & Alexashov 2015, where the confinement of the solar wind in the heliosheath resembles that seen in the short-heliotail solution of Opher et al (2015), except that the two heliospheric jets exist within a long tail. In this work, we extend the ENA model used in Kornbleuth et al (2021a), as described in Kornbleuth et al (2023), and use both a hybrid model of the termination shock (Giacalone et al 2021) that captures the heating and acceleration of PUIs to infer PUI distributions from the IBEX-Hi energies to the INCA energies and a model for diffusive shock acceleration of PUIs at the termination shock (Wang et al 2023). It is important to note that here we only calculate the acceleration of PUIs at the termination shock.…”

The Role of Pickup Ions in the Interaction of the Solar Wind with the Local Interstellar Medium. I. Importance of Kinetic Processes at the Heliospheric Termination Shock

“…The energy partition was used to obtain the PUI properties from the plasma mixture instead of solving the equations describing the PUI behavior self-consistently. Recently, Kornbleuth et al (2023) have used the hybrid simulations of Giacalone et al (2021) and the theoretical analysis of Wang et al (2023) to investigate the effect of diffusive shock acceleration on the PUI transmission through the TS in the framework of a global model, which did not include PUIs separately. In addition, rather strong assumptions were made to extrapolate or deduce the properties of PUIs across the entire TS surface and the IHS, with the purpose of fitting the IBEX and Cassini INCA measurements of ENA fluxes in the energy range up to 40 keV.…”

Suprathermal H+ Pickup Ion Tails in the Outer Heliosphere

“…The multiply reflected PUI spectra can be approximated by a power-law distribution (Zank et al 1996;Lipatov et al 1998;Rice et al 2001). Part of the reflected PUI population could be further accelerated by the DSA process (Wang et al 2023) provided that the particle speed is above the injection energy. The injection problem in the theory of shock acceleration is still under debate (Giacalone & Jokipii 1999;Zank et al 2001Zank et al , 2006Giacalone 2012;Giacalone et al 2022;Perri et al 2022).…”