Empirical Characterization of Low‐Altitude Ion Flux Derived from TWINS

Abstract

In this study we analyze ion differential flux from 10 events between 2008 and 2015. The ion fluxes are derived from low-altitude emissions (LAEs) in energetic neutral atom (ENA) images obtained by Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS). The data set comprises 119.44 hr of observations, including 4,284 per energy images with 128,277 values of differential ENA flux from pixels near Earth's limb. Limb pixel data are extracted and mapped to a common polar ionospheric grid and associated with values of the Dst index. Statistical analysis is restricted to pixels within 10% of the LAE emissivity peak. For weak Dst conditions we find a premidnight peak in the average ion precipitation, whose flux and location are relatively insensitive to energy. For moderate Dst, elevated flux levels appear over a wider magnetic local time (MLT) range, with a separation of peak locations by energy. Strong disturbances bring a dramatic flux increase across the entire nightside at all energies but strongest for low energies in the postmidnight sector. The arrival of low-energy ions can lower the average energy for strong Dst, even as it raises the total integral number flux. TWINS-derived ion fluxes provide a macroscale measurement of the average precipitating ion distribution and confirm that convection, either quasi-steady or bursty, is an important process controlling the spatial and spectral properties of precipitating ions. The premidnight peak (weak Dst), MLT widening and energy-versus-MLT dependence (moderate Dst), and postmidnight low-energy ion enhancement (strong Dst) are consistent with observations and models of steady or bursty convective transport.Plain Language Summary This paper performs a statistical analysis of data measured by NASA's Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission. The data consist of measurements of energetic neutral atoms that are emitted from the near Earth region, several hundred kilometers in altitude. We find that there is a profound change in the distributions in space and in energy that occurs for large space storms.

Global ENA Imaging and In Situ Observations of Substorm Dipolarization on 10 August 2016

“…For this study we require only relative flux versus time. Estimation of low‐altitude ion flux from limb ENAs. Following previous studies (Goldstein et al, , ), we extracted ENA flux values from pixels near the Earth limb, converted them to ion flux using an analytical energy‐dependent correction factor (Bazell et al, ), and mapped them to polar ionospheric (magnetic) latitude (Λ) and MLT. The mapping assumes the ENA emission region lies within the altitude range 100–700 km ( h =400 km, b h =300 km; cf.…”

“…The latitudes of the mapped limb flux pixels are highest near the global (per image) peak and decrease with MLT away from the peak, as is typical for limb ENA imaging (Goldstein et al, ). Determination of peak low‐altitude ion flux. Previous work has shown that the MLT dependence of limb‐derived flux is strongly controlled by the viewing geometry (Goldstein et al, ; Pollock et al, ; Roelof & Skinner, ), so we limit our quantitative analysis to the peak flux (Goldstein et al, ) that occured in the postmidnight sector for the 10 August 2016 TWINS 2 images. The similarity of the pixel spatial coverage (in both latitude Λ and MLT) in the four polar maps of Figure b illustrates that the TWINS 2 viewing geometry did not change significantly during the times of interest (before and after the dipolarization, about 0900–1100 UT).…”

Average Ring Current Response to Solar Wind Drivers: Statistical Analysis of 61 Days of ENA Images

“…In an earlier study of TWINS low altitude ENA images, Goldstein et al. (2018) observed during strongly disturbed D st a similar preferential enhancement of low‐energy precipitating ions manifested as monotonically decreasing flux spectra.…”

“…(Using low‐altitude ENA images, Goldstein et al. [2018] found a similar MLT dependence to E AVE for precipitating ions.) The overall distribution is cooler for strong E SW (Figure 16a) than for moderate E SW (Figure 16b), consistent with our finding that stronger convection favors increases of lower‐energy ions, as discussed next (Section 4.3).…”

“…(2018), and the average ENA‐derived low altitude ion fluxes versus D st were derived by Goldstein et al. (2018).…”