Robyn M. Millan

The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission was designed to study the loss of radiation belt electrons to the upper atmosphere. Previous analysis of data from balloon pairs showed that electron precipitation events manifest coherent quasi‐periodic variations (with periodicity on the order of 10–60 min) often lacking an obvious trigger internal to the magnetosphere. This suggested that the loss mechanism was related to solar wind driving involving quasi‐static magnetospheric field modulation and/or triggering of ultra low frequency waves by periodic density structures (PDSs). In this study, we performed a statistical investigation of the occurrence of quasi‐periodic X‐ray (QPX) variations in the ≈0.5–3.4 mHz (≈4.9–36.2 min) range from the seven BARREL campaigns and their relation to solar wind PDSs. QPX variations occur around 43%–73% of the campaign time of which 26%–30% involve PDSs. The frequency distribution of the QPX variations showed preferential occurrence at ≈0.6–1.0 mHz, ≈1.6–2.3 mHz, and tentatively ≈1.3 mHz. The QPX events map to different regions in the space depending on the frequency of the QPX variations and the type of solar wind driving. Simultaneous observations of the event by multiple balloons show QPX events extending up to 7.3 and 6.5. Finally, PDSs are more likely related to the X‐ray modulation during quiet to moderate geomagnetic conditions following substorm activity and possibly magnetospheric compressions.

We investigate the association between wave‐driven relativistic (>700 keV) electron precipitation (REP) and the two main geomagnetically effective solar wind (SW) drivers: coronal mass ejections (CMEs) and high‐speed streams (HSSs). We combine SW observations from OMNI and a data set of clear wave‐driven REP events (covering 2012–2023, from low Earth orbit electron measurements from the Polar Operational Environmental Satellites and Meteorological Operational satellites) to determine when and under which conditions CMEs and HSSs lead to REP. Out of 91 CMEs and 309 HSSs, 76%, and 70% are associated with REP, respectively, indicating that both drivers are effective at triggering relativistic electron loss into the atmosphere. However, only ∼25% of all REP events over 2012–2023 are associated with these major SW structures, suggesting that large‐scale drivers are not required in most cases. We also found that the strongest controlling factor for REP occurrence is not the driver type or storm intensity alone, but the preconditioning of the outer belt: drivers associated with REP typically impact a magnetosphere with elevated fluxes of trapped >700 keV electrons. REP occurs primarily during the main phase, although HSS‐driven REP is more often sustained into the recovery phase. In terms of properties, HSS‐driven REP appears broader in radial extent and is associated with higher electron flux, whereas CME‐driven REP shows larger precipitation‐to‐trapped flux ratios, indicating more efficient wave scattering. These results support a scenario in which SW forcing, storm timing, and radiation belt preconditioning jointly control the occurrence and properties of wave‐driven REP.

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.

Electron microbursts are among the most important loss mechanisms for energetic electrons in the Earth's radiation belts and are often driven by wave‐particle interactions with discrete chorus wave elements. Observations from the Balloon Array for Radiation‐belt Relativistic Electron Losses (BARREL) mission have revealed microburst events with simultaneous burst and smooth precipitation components. We conduct numerical experiments for the reported events and identify the driving mechanisms for these dual‐component structures. A realistic atmospheric backscattering module based on Monte Carlo simulations is incorporated into test particle simulations of wave‐particle interactions to more accurately model electron precipitation. Using parameters constrained by BARREL observations, our simulations reproduce the observed dual‐component microbursts with similar burst‐to‐smooth ratios. Comparative runs show that energy dispersion in precipitation, driven by intense and discrete chorus elements, can naturally produce a smooth component alongside bursty precipitation. This indicates that the smooth component is an intrinsic feature of intense microbursts and contributes significantly to the total electron loss. We show that electron loss rates from microbursts are likely underestimated due to the neglect of these smooth components in observations with insufficient energy resolution. Although the backscattering module does not strongly affect the formation of the smooth component, it does reduce the peak precipitation rate and extends the overall duration of precipitation. These findings highlight the importance of incorporating realistic atmospheric backscattering into wave‐driven precipitation models to improve the accuracy of estimating electron precipitation.

On 15 February 2018 a co‐rotating interaction region (CIR) from an equatorial coronal hole reached the Earth. The CIR initiated a moderate and slowly intensifying geomagnetic storm, which began with a large and strong substorm injection. The substorm injection was exceptionally well‐observed by an array of spacecraft including LANL‐GEO satellites, Van Allen Probes (RBSP), Arase (ERG), and MetOp/POES, as well as ground‐based instruments. These observations enable the unambiguous identification of several important features that have been impossible to measure directly in other events. The substorm injection extended well inside the geosynchronous orbit. A fortuitous conjunction of RBSP‐A (moving inbound) and Arase (simultaneously moving outbound at the same magnetic local time) allows us to establish, very precisely, the location of the inner edge of the injection region at L = 3.8−3.9. In supporting observations, North American riometers saw precipitation extending down to L ≈ 4 but not lower. Arase and RBSP‐A also observed whistler‐mode hiss waves inside the plasmasphere. Analysis of the resonance conditions shows, conclusively, and for the first time, that they were produced by the drifting injected electrons. RBSP‐A observations also show the injection (or transport) of electrons into or through the slot region within hours of the substorm injection onset. Previous studies were not able to clearly connect or separate substorm injections and slot‐filling processes. These new observations clearly identify slot‐filling as a spatially and temporally separate process that is not a direct result of substorm injection.

On 21 June 2022, during the annual Geospace Environment Modeling (GEM) workshop, a panel discussion titled “Radiation Belt Loss: The Long‐Standing Debate Part II” was organized by the focus group “System Understanding of Radiation Belt Particle Dynamics.” The panel focused on unresolved questions regarding the mechanisms driving electron loss in Earth's radiation belts, discussing topics including magnetopause shadowing, outward radial transport, and wave‐particle interactions driving particle precipitation. In this commentary, we provide an overview of the outcomes of this discussion and highlight future needs to better resolve outstanding questions.

Low-altitude measurements provide a unique vantage point for studying processes occurring in the magnetosphere, taking advantage of the fact that energetic particles move quickly along magnetic field lines. Low-Earth-orbiting (LEO) polar satellites can sample a vast volume of space as they rapidly traverse magnetic field lines, obtaining a radial snapshot of the entire magnetotail in minutes. CINEMA (Cross-scale INvestigation of Earth's Magnetotail and Aurora) is a NASA Small Explorer mission concept with the overarching goal to understand the role of plasma sheet structure and evolution in Earth’s multiscale magnetospheric convection cycle. How the magnetotail maintains steady convection, and when and how it decides to explosively release stored energy, are major unsolved mysteries of space physics. CINEMA’s nine satellites in LEO polar orbits each carry an on-board imager, particle sensors, and magnetometers, and quickly traverse the low-altitude footprint of the magnetotail, capturing its evolution at different scales. CINEMA obtains information about the structure of the magnetotail remotely through its imprint on particle pitch-angle distributions, providing an unprecedented view of particle isotropy boundaries. Mesoscale aurora and bursty energetic particle precipitation serve as tracers of specific mesoscale and kinetic-scale dynamics. Field-aligned currents (FACs) that connect the magnetotail to the ionosphere are sensed by measuring magnetic field variations at each satellite. Together, these observations reveal the physics underlying multiscale magnetotail convection.

Microbursts are impulsive injections of energetic (few keV to >MeV) electrons into the atmosphere, primarily caused by nonlinear scattering driven by whistler mode chorus waves. While the relative importance of microburst precipitation as a loss process has not been fully quantified, many studies have shown microbursts may play a significant role in the loss of outer radiation belt electrons. We present a multi‐platform statistical analysis of chorus and energetic electron precipitation in an attempt to constrain the azimuthal spatial extent (MLT) of the microburst precipitation region and determine how this extent varies with geomagnetic activity. Statistical upper bounds of this azimuthal extent are determined with observations of general energetic electron precipitation that can include direct microburst detections, while statistical lower bounds determination requires direct microburst detections. The resulting distributions of both upper and lower bounds azimuthal extent suggest that microbursts may frequently constitute an important source of electron loss from the outer radiation belt. We find that 36% of upper bound events in the dawn sector span more than 5 hr in MLT. This azimuthal extent increases with geomagnetic activity, particularly in the dawn and noon MLT sectors.