Mykhaylo Shumko

We present a case study of a moderate geomagnetic storm, which exhibits an abrupt relativistic enhancement in the heart of the radiation belt accompanied by low‐latitude purple auroral emission. The relativistic enhancement occurs on a timescale of tens of minutes and reaches MeV energies deep within the inner magnetosphere. The enhancement is concurrent with mesoscale purple patches which protrude below the pre‐existing auroral oval. We analyze the event using a novel combination of observational data sets and physics‐based modeling. This includes multispectral auroral imaging, dosimeter data from the Global Positioning System constellation, auroral transport modeling, and global geospace modeling. Our analysis reveals the cause of both the radiation belt enhancement and purple emission to be direct, mesoscale injections of electrons reaching relativistic energies. Finally, we show that these injections are most likely caused by near‐Earth reconnection which produces deeply depleted entropy bubbles capable of trapping and transporting energetic particles into the inner magnetosphere.

During substorms, Earth's magnetotail undergoes rapid dipolarization, driving Earthward plasma flows that decelerate and dissipate energy upon encountering the dipole magnetic field in the nightside transition region. This region mediates the interaction between the magnetotail, inner magnetosphere, and the ionospheric auroral zone, though significant mapping uncertainties obscure the precise link and particle acceleration processes. Using data from THEMIS, TREx, and ELFIN, we analyze a storm‐time substorm on 4 September 2022, establishing a relationship, that is, not a causation, between magnetospheric and ionospheric dynamics. Following a dipolarization, the auroral bulge overlapped with the footprints of the electron isotropy boundary (IB) and the outer radiation belt. Notably, the precipitating electron energies reached at least 2 MeV in the bulge, exceeding previous reports. By comparing the latitudes of the electron IB with respect to the auroral bulge, we deduce that the sources of both auroral and relativistic precipitation were confined in the dipolarized region.

As we move towards proliferated space and smaller, more cost-effective satellites, miniaturization of quality science instrumentation is becoming necessary and possible. This paper describes the Relativistic Electron Magnetic Spectrometer (REMS), a miniaturization of NASA’s Van Allen Probes’ MagEIS-Medium instrument, which was designed to measure radiation belt electrons. REMS was designed, built, and delivered for NASA’s GTOSat mission (Blum et al. in Proc. SPIE 11389, micro- and nanotechnology sensors, systems, and applications XII, 113892E, 2020), and was integrated onto the spacecraft bus in 2021. REMS’s electron energy range is one decade and can be set between ∼30–1500 keV. The GTOSat REMS main rate electron energy centroids range from 136–1088 keV. REMS has 9 electron main rate energy channels and 32 electron histogram channels per detector pixel, allowing for more precise energy measurements that enable background subtraction as was accomplished with MagEIS. GTOSat REMS measures ∼400–9000 keV protons. The angular resolution ranges from ∼3°–20° depending on the REMS orientation and the detector pixel in question. On the spinning GTOSat, REMS provides a pitch angle resolution down to 5.63° (32 sectors per spin) for the main rate electron data product, 30° for the proton data product, and 22.5° for the histogram data product. A different sectoring choice would alter the pitch angle resolution. At the time of writing, the mission is awaiting a launch. In the meantime, REMS has been included in additional mission concepts and proposals. The REMS instrument provides high quality energy and pitch angle resolved measurements of electrons and protons, enabling future exploration and monitoring of Earth’s radiation belts with small satellites.

Relativistic electron microbursts are correlated with resonant scattering by whistler‐mode chorus waves. Here, we use chorus wave properties obtained from Van Allen Probe A to calculate the duration of relativistic microbursts. A detailed quantitative comparison between observed and calculated microburst durations shows consistent ranges and similar correlations with shell and magnetic local time (MLT). The most apparent trend is in MLT: microburst duration increases from midnight through the dawn sector but decreases after noon. These results highlight the primary role of chorus waves in driving microbursts. Furthermore, we build a constraint on the upper magnetic latitude boundary of chorus waves that can induce 1 MeV microbursts. The constraint is dependent on MLT. On the dayside, the optimal typically exceeds , which is higher than on the nightside, suggesting ducted chorus propagation dominates. Chorus waves with higher are generally more effective in triggering relativistic microbursts.

During substorms, Earth's magnetotail undergoes rapid dipolarization, driving Earthward plasma flows that decelerate and dissipate energy upon encountering the dipole magnetic field in the nightside transition region. This region mediates the interaction between the magnetotail, inner magnetosphere, and the ionospheric auroral zone, though significant mapping uncertainties obscure the precise link and particle acceleration processes. Using data from THEMIS, TREx, and ELFIN, we analyze a storm-time substorm on 4 September 2022, establishing a direct connection between magnetospheric and ionospheric dynamics. Following a dipolarization, the auroral bulge overlapped with the footprints of the electron isotropy boundary and the outer radiation belt. Notably, the precipitating electron energies reached at least 2 MeV in the bulge, exceeding previous reports. By comparing the latitudes of the electron isotropy boundary with respect to the auroral bulge, we deduce that the sources of both auroral and relativistic precipitation originated within the dipolarized region.

Shock-generated transients, such as hot flow anomalies (HFAs), upstream of planetary bow shocks, play a critical role in electron acceleration. Using multimission data from NASA’s Magnetospheric Multiscale and ESA’s Cluster missions, we demonstrate the transmission of HFAs through Earth’s quasi-parallel bow shock, accelerating electrons to relativistic energies in the process. Energetic electrons initially accelerated upstream are shown to remain broadly confined within the transmitted transient structures downstream, where they get further energized due to the elevated compression levels potentially by betatron acceleration. Additionally, high-speed jets form at the compressive edges of HFAs, exhibiting a significant increase in dynamic pressure and potentially contributing to further localized compression. Our findings emphasize the efficiency of quasi-parallel shocks in driving particle acceleration far beyond the immediate shock transition region, expanding the acceleration region to a larger spatial domain. Finally, this study underscores the importance of a multiscale observational approach in understanding the convoluted processes behind collisionless shock physics and their broader implications.

Earth's magnetotail, a night‐side region characterized by stretched magnetic field lines and strong plasma currents, is the primary site for the release of magnetic field energy and its transformation into plasma heating and kinetic energy plus charged particle acceleration during magnetic reconnection. In this study, we demonstrate that the efficiency of this acceleration can be sufficiently high to produce populations of relativistic and ultra‐relativistic electrons, with energies up to several MeV, which exceeds all previous theoretical and simulation estimates. Using data from the low‐altitude ELFIN and CIRBE CubeSats, we show multiple events of relativistic electron bursts within the magnetotail, far poleward of the outer radiation belt. These bursts are characterized by power‐law energy spectra and can be detected during even moderate substorms.

Precipitation into the atmosphere is one of the main processes by which high energy electrons trapped in Earth's inner magnetosphere are lost from the system. Precipitating electrons can affect the chemical composition of the atmosphere and provide insight into the complex dynamics of the Van Allen radiation belts. This study compares energetic electron precipitation measurements at low‐Earth‐orbit by the Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics (FIREBIRD‐II) CubeSats with NOAA Polar‐orbiting Operational Environmental Satellite (POES) and ESA Meteorological Operational satellite (MetOp) satellites, which are equipped with the Medium‐Energy Proton Electron Detector (MEPED). The analysis considers 51 high quality conjunction events at >300 keV during times of low to moderate geomagnetic activity. The spacecraft capture similar electron flux variability, and FIREBIRD‐II observations fall between POES/MetOp and telescopes, likely a result of FIREBIRD‐II sampling both precipitating and mirrored electrons due to uncertainties in pointing direction. Results demonstrate the value of high‐resolution differential energy observations of electron precipitation by low‐cost CubeSats such as FIREBIRD‐II, especially during periods of low flux.

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.