Fluid-Kinetic Variations in the Storm-Time Inner Magnetosphere.

Storm-time broadband electromagnetic field variations along the interface between the dipolar field of the Earth's inner-magnetosphere and the stretched fields of the plasma-sheet are decomposed as a superposition of fluid-kinetic modes. Using model eigen-vectors operating on the full set of Van Allen Probes fields measurements it is shown how these variations are composed of a broad spectrum of dispersive Alfvén waves with significant spectral energy densities in the fast and slow modes over scales extending into the kinetic range. These modes occupy volumes in k -space that define the field variations observed at each spacecraft frame frequency ( f s c ). They are in aggregate not necessarily planar and often comprise filamentary structures with no distinct propagation direction in the perpendicular plane. Within these volumes the characteristic parallel phase speeds of the fast and Alfvénic modes coincide over a broad range of f s c suggestive of coupling/conversion between modes.

Small-Scale Dynamic Aurora.

Small-scale dynamic auroras have spatial scales of a few km or less, and temporal scales of a few seconds or less, which visualize the complex interplay among charged particles, Alfvén waves, and plasma instabilities working in the magnetosphere-ionosphere coupled regions. We summarize the observed properties of flickering auroras, vortex motions, and filamentary structures. We also summarize the development of fundamental theories, such as dispersive Alfvén waves (DAWs), plasma instabilities in the auroral acceleration region, ionospheric feedback instabilities (IFI), and the ionospheric Alfvén resonator (IAR). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11214-021-00796-w.

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.