Comparison of TWINS images of low-altitude emission of energetic neutral atoms with DMSP precipitating ion fluxes

The brightest energetic neutral atom (ENA) intensity viewed by spacecraft in high-inclination Earth orbit is from low-altitude emission (LAE). It is a prominent feature in the stereo ENA images obtained by cameras on the NASA TWINS 1/2 Mission of Opportunity. This emission is produced by energetic magnetospheric ions precipitating into the atomic oxygen exosphere at latitudes near the auroral zones at altitudes ∼300 km. The ions undergo multiple atomic collisions including charge exchange of ions and stripping of the resulting neutrals. Consequently, this is a "thick target" process. We introduce a "thick-target" approximation that allows us to extract the shape of the spatially averaged spectra of the precipitating ions from the ENA spectra in the TWINS 1/2 pixels viewing the LAE from near their orbital apogees. These ENA-extracted ion spectra are compared with in situ precipitating ion spectra measured concurrently by DMSP satellites (F15 and F16) at ∼825 km altitude, while they are passing directly over the LAE regions in the TWINS 1/2 images. We obtain good agreement between the shape of the ENA-extracted and in situ ion spectra from three distinct precipitation regions over energies from 2 to 32 keV (assuming precipitating protons). The absolute normalization of the ENA-extracted and in situ spectra depends upon the TWINS viewing geometry because the ENA LAE source is not resolved by the imager. None of the spectral shapes obtained is consistent with a simple thermal intensity spectrum with kT = 5 keV.