Low-Altitude Emission of Energetic Neutral Atoms: A New Diagnostic of the Energetics of Ion Precipitation

We describe a new theoretical understanding of the emission of energetic neutral atoms (ENAs) generated by the precipitation of energetic magnetospheric ions into the Earth’s monatomic oxygen (O) exosphere (200-800 km). This low altitude emission (LAE) is the brightest ENA source in images obtained from Astrid-1/PIPPI, IMAGE/MENA/HENA, and TWINS1/2. The upward ENA “albedo” from the precipitating protons in the energy range 1-100 keV can approach 50% of the incident proton intensity. Unlike FUV imaging, ENA imaging of the LAE allows us to extract the detailed (not integrated) energy spectrum of the precipitating protons. We have verified this claim by comparing ENA images from TWINS 1/2 with in situ ion spectra measured by DMSP spacecraft (~825 km altitude) flying simultaneously under the ENA LAE regions (Bazell et al., J. Geophys. Res., in press 2010, and also this Conference). Quantitative extraction of proton spectra from the ENA images requires a “thick-target” theory that treats properly the multiple atomic collisions (charge exchange of protons, stripping ENA H-atoms) and associated energy losses (including ionization and excitation). Analytic solutions to the coupled proton/H-atom transport equations have been obtained, and they provide quantitative insight into the strong dependence of the ENA LAE upon the pitch angle and the energy of the precipitating protons. Since global ENA images of LAE can be obtained with exposure times of a minute or so during large geomagnetic storms, the distribution in magnetic latitude and local time of their evolving spectra contain critical diagnostics of the physics of not only the precipitation process, but also of the acceleration of the energetic ions themselves. Simulated 24 keV ENA low altitude emission viewed from TWINS-2 generated by precipitating protons below a DMSP pass (Bazell et al., JGR, in press, 2010).