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Empirical Characterization of Low‐Altitude Ion Flux Derived from TWINS
Author(s) -
Goldstein J.,
LLera K.,
McComas D. J.,
Redfern J.,
Valek P. W.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2017ja024957
Subject(s) - flux (metallurgy) , ion , physics , ionosphere , emissivity , energy flux , energetic neutral atom , atomic physics , computational physics , astrophysics , geophysics , chemistry , optics , astronomy , organic chemistry , quantum mechanics
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.