Empirical Characterization of Low‐Altitude Ion Flux Derived from TWINS. Issue 5 (16th May 2018)

• Record Type: Journal Article
• Authors: Goldstein, J.; LLera, K.; McComas, D. J.; Redfern, J.; Valek, P. W.
• Is Part Of: Journal of geophysical research. Volume 123:Issue 5(2018)
• Page Start: 3672
• Page End: 3691
• Publication Date: 2018-05-16
• Subjects: ring current -- ion -- precipitation -- global imaging -- energetic neutral atom -- TWINS; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2017JA024957
• Languages: English
• ISSNs: 2169-9380

Abstract

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 D s t index. Statistical analysis is restricted to pixels within 10% of the LAE emissivity peak. For weak D s t conditions we find a premidnight peak in the average ion precipitation, whose flux and location are relatively insensitive to energy. For moderate D s t, 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 D s t, 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 D s t ), MLT widening and energy‐versus‐MLT dependence (moderate D s t ), and postmidnight low‐energy ion enhancement (strong D s t ) 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. Key Points: As D s t increases from weak to strong, average precipitating flux increases and covers wider magnetic local time extent For stronger disturbances, at lowest energies, average ion precipitation peak shifts from premidnight to postmidnight Strong D s t : average flux increases across nightside at all energies but most intensely at low energies in postmidnight sector