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 ( κ ≲  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 h ≳  6 R E, and κ= 2-6.