Analytical estimate for low‐altitude ENA emissivity

Abstract

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.

Magnetosphere dynamics during the 14 November 2012 storm inferred from TWINS, AMPERE, Van Allen Probes, and BATS-R-US–CRCM

“…Although LAEs are known to have a highly directional emissivity, the modulation of the LAE signal during dropout 1 is most likely temporal and not caused by a change in viewing geometry. The LAE emissivity function has a broad peak about 12 h away from the imager and about 6 h wide in magnetic local time (Bazell et al, 2010;Goldstein et al, , 2016. During the 02:20 UT to 04:50 UT period depicted in the entirety of Fig.…”

“…LAEs are useful as an indicator of the global distribution of precipitating ions. Previous studies have attempted to deconvolve the LAE signal and extract quantitative ion fluxes from the ENA intensity (Bazell et al, 2010;Goldstein et al, , 2016. In this paper we employ the LAE index solely as a measure of the timing of possible ion precipitation causally linked to the flux dropouts observed by Van Allen Probes.…”

Cross‐scale observations of the 2015 St. Patrick's day storm: THEMIS, Van Allen Probes, and TWINS

“…However, it must be noted that between 1745 UT (Figure b) and 1859 UT (Figure c) TWINS 1 moved along its orbit, introducing possible viewing‐geometry effects to the LAE intensity [ Goldstein et al , ]. During this interval the spacecraft moved 0.6 R E in radius (from 6.4 R E to 7.0 R E ), 1 h in local time (1800 to 1900 MLT) and 5° in latitude (60° to 65°).…”

“…During this interval the spacecraft moved 0.6 R E in radius (from 6.4 R E to 7.0 R E ), 1 h in local time (1800 to 1900 MLT) and 5° in latitude (60° to 65°). The changes in latitude and MLT are especially important for measuring the highly directional LAE signal because they can (more strongly than radius) affect the range of latitudes at which an LAE is sampled [ Goldstein et al , ]. In each energy‐MLT spectrogram of Figure the abcissa is also labeled by its magnetic latitude values; note that each of these values denotes the latitude of the center of the LAE source location (not the imager location).…”

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

“…In this section we compare our MCCI‐derived PAD with published in situ sounding rocket data (Søraas & Aarsnes, ) and a full transport model (Goldstein et al, ). The in situ PAD provide a low‐altitude comparison with our model, while the simulated PAD is a high‐altitude comparison.…”