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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., Key Points The magnetospheric source parameters that account for observed auroral electron distributions are derived from a generalized kinetic modelThe degree of nonthermality (kappa index) in auroral primaries is required to correctly prescribe the magnetospheric source parametersWe present the first analytical kinetic model for the relationship between density and acceleration potential along auroral field lines

The LEXI and SMILE missions will provide soft X‐ray images of the Earth's magnetosheath and cusps after their anticipated launch in 2023 and 2024, respectively. The IBEX mission showed the potential of an Energetic Neutral Atom (ENA) instrument to image dayside magnetosheath and cusps, albeit over the long hours required to raster an image with a single pixel imager. Thus, it is timely to discuss the two imaging techniques and relevant science topics. We simulate soft X‐ray and low‐ENA images that might be observed by a virtual spacecraft during two interesting solar wind scenarios: a southward turning of the interplanetary magnetic field and a sudden enhancement of the solar wind dynamic pressure. We employ the OpenGGCM global magnetohydrodynamics model and a simple exospheric neutral density model for these calculations. Both the magnetosheath and the cusps generate strong soft X‐rays and ENA signals that can be used to extract the locations and motions of the bow shock and magnetopause. Magnetopause erosion corresponds closely to the enhancement of dayside reconnection rate obtained from the OpenGGCM model, indicating that images can be used to understand global‐scale magnetopause reconnection. When dayside imagers are installed with high‐ENA inner‐magnetosphere and FUV/UV aurora imagers, we can trace the solar wind energy flow from the bow shock to the magnetosphere and then to the ionosphere in a self‐standing manner without relying upon other observatories. Soft X‐ray and/or ENA imagers can also unveil the dayside exosphere density structure and its response to space weather., Key Points Soft X‐ray and Energetic Neutral Atom (ENA) imaging instruments provide an innovative way to visualize the global solar wind‐magnetosphere interactionHigh‐cadence, wide field‐of‐view soft X‐ray, and ENA images can capture the motion of the bow shock and magnetopauseThe magnetopause motion can reveal the magnetopause reconnection mode on a global scale

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity, but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ≃ 2200 h Saturn local time at 36 R S from Saturn. During several entries into the magnetotail lobe, tailward flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept‐forward configuration which is atypical for this region, and the total magnetic field strength is larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening, and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between (2.95 ± 0.43) × 109 and (1.43 ± 0.21) × 1010 cm−2 s−1, 1 or about 2 orders of magnitude larger than suggested by steady state MHD models, with a mass source between 1.4 ×102 and 1.1 ×103 kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7 ±13.4 and 239.8 ±64.8 kg/s for an average auroral oval, and 10 ±4 and 49 ±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind., Key Points Tailward observation of ionospheric plasma in the magnetotail lobeEvidence for simultaneous reconnection in the plasma sheetFirst observational estimate at Saturn of mass source rate from the ionosphere to the magnetotail