Low‐Altitude Emission of Energetic Neutral Atoms: Multiple Interactions and Energy Loss

Abstract

Low‐altitude emissions (LAEs) are the energetic neutral atom (ENA) signature of ring current ions precipitating along the magnetic field to an altitude of 200–800 km. This altitude region is considered to be “optically thick” because ring current ions undergo multiple charge changing interactions (MCCIs) with Earth's dense oxygen exosphere. While each interaction involves an energy loss of ~36 eV, no prior study has determined the accumulated energy lost by 1–100 keV H+ emerging as LAEs. We have developed a 2‐D model with a geomagnetic dipole that captures the net effects in energy loss and pitch angle evolution as a result of MCCIs without the computational requirements of a full Monte Carlo simulation. Dependent on the amount of latitudinal migration, the energy loss is greater than 20% for ions below 60 keV for equatorward moving particles (30 keV for poleward). Since the ENA travels ballistically across a geomagnetic dipole, upon reionization, ion velocity along the local field increases (antiparallel in the northern hemisphere). Redirecting the particle upward through MCCIs is most effective during poleward ENA motion. The net effect is to redirect precipitating ions (below 2,500 km) to eventually emerge from the optically thick region either as an ion or ENA. Precipitation is a joint ion‐neutral process, affecting both the energy and pitch angle distribution through the transverse motion of ENA segments in a converging field. For particles that enter the MCCI regime, the energy loss and evolution of the pitch angle distribution must be considered within a realistic magnetic field.

The Big Picture: Imaging of the Global Geospace Environment by the TWINS Mission

“…Analysis of LAEs has also advanced understanding of low‐altitude ion‐neutral interactions. LLera et al () used a novel analytical approximation to a full transport code to study the energy loss experienced by particles in the thick‐target region where LAEs are generated. Precipitating ions undergo multiple charge‐exchanging interactions, and each such interaction incurs an energy penalty of ∼36 eV (Dalgarno, ; Dalgarno & Griffing, ).…”

“…Models are essential to interpreting how the complexities of ion‐neutral interactions produce the low‐altitude ENA signal that TWINS observes. LLera et al () used an analytical approximation to a full transport code to understand the energy loss experienced by ions and neutrals undergoing multiple charge‐exchanging interactions to produce LAEs. With the help of the model, they found that the divergent dipole magnetic field line geometry plays a crucial role in helping ENAs to escape to high altitudes.…”

Composition of 1–128 keV Magnetospheric ENAs

“…The LAE distributions for H and O tend to have a smaller energy spread than the HAEs. LAEs may have significant energy loss due to the multiple interactions (LLera et al, ), so care must be taken when comparing the LAEs and HAEs.…”

Empirical Characterization of Low‐Altitude Ion Flux Derived from TWINS

“…The ion‐neutral interactions that create LAEs have been studied for two decades using transport codes, analytical models, and numerical inversion methods (Bazell et al, ; Brandt et al, ; Galand et al, ; Galand & Richmond, ; LLera et al, ; Perez et al, ; Roelof, ; Roelof & Skinner, ). The first geospace ENA images contained contributions from LAEs (Roelof, ).…”

“…Goldstein et al () reported that isolated substorm injections during the 2015 St. Patrick's Day storm caused LAE spectral cooling by bringing in less energetic ions from the magnetotail. These multiple LAE observational studies have also fostered the development of new analysis tools: energy corrections that convert LAE spectra to ion flux spectra (Bazell et al, ; LLera et al, ), a geometric correction for pixels larger than the LAE source region (Goldstein et al, ), and an analytical model for LAE emissivity (Goldstein et al, ).…”