We formulate the first analytical model for energetic neutral atom (ENA) emissivity that partially corrects for the global viewing geometry dependence of low‐altitude emissions (LAEs) observed by Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS). The emissivity correction requires the pitch angle distribution (PAD) and geophysical location of low‐altitude ENAs. To estimate PAD, we create an energy‐dependent analytical model, based on a Monte Carlo simulation. We account for energy binning by integrating model PAD over each energy bin. We account for finite angular pixels by computing emissivity as an integral over the pitch angle range sampled by the pixel. We investigate location uncertainty in TWINS pixels by performing nine variations of the emissivity calculation. Using TWINS 2 ENA imaging data from 1131 to 1145 UT on 6 April 2010, we derive emissivity‐corrected ion fluxes for two angular pixel sizes: 4° and 1°. To evaluate the method, we compare TWINS‐derived ion fluxes to simultaneous in situ data from the National Oceanic and Atmospheric Administration (NOAA) 17 satellite. The TWINS‐NOAA agreement for emissivity‐corrected flux is improved by up to a factor of 7, compared to uncorrected flux. The highest 1° pixel fluxes are a factor of 2 higher than for 4° pixels, consistent with pixel‐derived fluxes that are artificially low because subpixel structures are smoothed out, and indicating a possible slight advantage to oversampling the instrument‐measured LAE signal. Both TWINS and NOAA ion fluxes decrease westward of 2000 magnetic local time. The TWINS‐NOAA comparison indicates that the global ion precipitation oval comprises multiple smaller‐scale (3–5° of latitude) structures.
[1] In this paper, we analyze Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) stereo observations of energetic neutral atoms (ENAs) produced from the low altitude emission (LAE) region during the interval 1130-1146 UT on 6 April 2010. Geometrical calculations determine the geophysical locations of pixels at or near the LAE limb and the associated uncertainties. For our event, the two TWINS imagers observed a broad (8.4-9.2 h wide in magnetic local time (MLT)) region of LAEs on the opposing limb, possibly containing an ion boundary near dusk. The most intense LAEs were detected in a narrow range of magnetic latitude (67 ı -74 ı ) and pitch angle (112 ı -116 ı ). We implement a simplified thick-target approximation (TTA) to obtain ion spectra from TWINS LAEs and perform a validation study using a conjunction of the TWINSobserved LAE crescents with a simultaneous NOAA 17 polar-orbit pass slightly west of the TWINS LAEs. Since TTA is limited to the brightest portion of LAEs, we apply our analysis for pixels with at least 30% of the peak value. TWINS ion spectra are calculated for individual pixels spanning several hours of MLT. The spectra exhibit a pronounced local time dependence. For more westward MLT (and more equatorward latitude), there is a shift toward spectra that are more energetic and peaked. This spatial dependence is consistent with ion drift theory and previous observations. The peaked LAE-derived ion spectra of 6 April 2010 are notably different than those observed during much weaker disturbances, but are consistent with LAE observations from similar activity levels. These results demonstrate that with proper caution in interpreting the results, TWINS ENA imaging resolves MLT-dependent (and to a limited extent, latitude-dependent) low-altitude ion spectral shape information, simultaneously across a broad range of MLT. This study advances previous results that considered much coarser MLT structure in LAEs and augments previous statistical spectral analysis of in situ data.
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