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

• Published in: Journal of geophysical research. Space physics 2017-10, Vol.122 (10), p.10,203-10,234
• Main Authors: LLera, K., Goldstein, J., McComas, D. J., Valek, P. W.
• Format: Article
• Language: English
• Subjects: Altitude; Charged particles; Computer simulation; Dipoles; Earth; Emissions; energetic neutral atom; Energetic neutral atoms; Energetic particles; Energy; Energy distribution; Energy loss; Energy measurement; Equatorial regions; Evolution; Exosphere; Geomagnetic field; Geomagnetism; H particle; Ion velocity; Ionization; Ions; ion‐neutral interactions; Low altitude; Magnetic fields; Monte Carlo simulation; Neutral atoms; Northern Hemisphere; Oxygen; Pitch (inclination); Precipitation; Ring current ions; Ring currents; Trapped particles; Two dimensional models
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2017JA024016

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. Plain Language Summary The ring current is composed of charged energetic particles, trapped by the Earth's magnetic field (~Earth‐sized bar magnet). About 2–6 Earth radii away, these trapped particles bounce along the magnetic field line they are bound to. As they approach Earth, these energetic particles transition to higher neutral densities and lose energy switching between charged and neutral states. Although the individual energy loss is 1% or less of the original ring current energy, the accumulated energy loss is unknown. During the neutral state, the particle is no longer influenced by the magnetic field but moves in a straight path until it becomes charged again. The new location changes the charged particle's motion along a new magnetic field line. We modeled the particle's path (both charged and neutral states) and found that the energy loss is significant, speci