N. A. Stepanov

We report a rare quiet‐time observation by NOAA‐18 spacecraft flying nearly along the electron isotropy boundary (IBe). Complemented by data from four nearby meridional IBe crossings of other POES spacecraft, this case study provides a novel quantitative information concerning the regular energy dependence of the loss‐cone filling ratio (Jprec/Jtrap) variations near the IBe foot. Specific strong energy‐dependent difference of loss‐cone filling rates is manifested at different spacecraft and in various events in the same way, which is consistent with the pitch‐angle scattering in the magnetotail field reversal region as confirmed by the numerical simulation. Therefore it provides a landmark to distinguish the field line curvature scattering (FLCS) mechanism from wave‐induced precipitation. In FLCS case, a strongly fluctuating loss cone filling ratio implies that equatorial magnetic field is inherently structured in the magnetotail in the IBe foot region.

Studying the episodes with the most elevated electron density (N_e) values provides valuable information about spatial distribution and drivers of intense energetic electron (EE) precipitation, which has important space weather implications. By combining EISCAT‐Tromso UHF observations in different modes made in 2001–2021, we survey N_e occurrences and study statistical properties of EE precipitation, producing the highest 10% and 1% of N_e values at the altitudes between 100 and 75 km (corresponding to EE energies of ∼20–∼200 keV). We found that the highest N_e values occur in the nightside‐morning local time sector at the altitudes 90–100 km and in the morning‐noon sector at low altitudes (75–85 km). Although this bimodal pattern could partly be contributed by the suppressed N_e in the dark ionosphere, by analyzing measurements in the twilight conditions at all MLTs together with published patterns of EE precipitation, we argue that a dominance of pre‐noon maximum at low altitudes reflects the enhanced precipitation of >100 keV electrons in this local time sector. At all altitudes (especially below 90 km), the appearance of the highest N_e values favors enhanced auroral activity and shows a strong correlation with fast solar wind (V > 550 km/s) events. These properties mimic the well‐known driving characteristics of energetic electrons. From the correlation of N_e with integral magnetic activity indexes we found that the memory of preceding magnetic activity increases with decreasing altitude, reaching roughly one day at H = 75 km. We discuss the advantages of exploring N_e data at specific altitudes (compared to direct measurements of EE precipitation) to investigate statistically the effects of specific precipitated energy components and the evolution of EE energy spectra.