Inferring Source Properties of Monoenergetic Electron Precipitation From Kappa and Maxwellian Moment‐Voltage Relationships

We present two case studies of FAST electrostatic analyzer measurements of both highly nonthermal ( [Formula: see text] 2.5) and weakly nonthermal/thermal monoenergetic electron precipitation at ∼4,000 km, from which we infer the properties of the magnetospheric source distributions via comparison of experimentally determined number density‐, current density‐, and energy flux‐voltage relationships with corresponding theoretical relationships. We also discuss the properties of the two new theoretical number density‐voltage relationships that we employ. Moment uncertainties, which are calculated analytically via application of the Gershman et al. (2015, https://doi.org/10.1002/2014JA020775) moment uncertainty framework, are used in Monte Carlo simulations to infer ranges of magnetospheric source population densities, temperatures, κ values, and altitudes. We identify the most likely ranges of source parameters by requiring that the range of κ values inferred from fitting experimental moment‐voltage relationships correspond to the range of κ values inferred from directly fitting observed electron distributions with two‐dimensional kappa distribution functions. Observations in the first case study, which are made over ∼78–79° invariant latitude in the Northern Hemisphere and 4.5–5.5 magnetic local time, are consistent with a magnetospheric source population density n (m)= 0.7–0.8 cm(−3), source temperature T (m)≈ 70 eV, source altitude h= 6.4–7.7 R (E), and κ= 2.2–2.8. Observations in the second case study, which are made over 76–79° invariant latitude in the Southern Hemisphere and ∼21 magnetic local time, are consistent with a magnetospheric source population density n (m)= 0.07–0.09 cm(−3), source temperature T (m)≈ 95 eV, source altitude [Formula: see text] 6 R (E), and κ= 2–6.