Zheng Xiang

Radial transport plays a fundamental role in controlling the dynamics of Earth's radiation belt electrons during geomagnetic storms. As a major energization mechanism, inward radial transport contributes to the refilling of the slot region, with an efficiency that depends on electron energy but remains insufficiently characterized. We present electron observations from the relativistic electron and proton telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE). Operating in low‐Earth‐orbit (LEO), CIRBE enabled repeated sampling of the trapped region approximately every 1.5 hr, though observations are primarily limited to electrons with small equatorial pitch angles. These observations reveal the inward transport of outer belt electrons to the slot region and inner belt and their energy dependence during storms of varying intensities in April, May, and November of 2023. Notably, the April 2023 storm produced a five‐order‐of‐magnitude enhancement of 0.4–1 MeV electron flux at L ≤ 3. In the phase space density (PSD) radial profile, this is reflected in the rapid inward transport of the inner edge of the outer belt. The PSD increased by around three orders of magnitude between L* ∼ 3.3 and L* ∼ 2.5 over ∼2–3 days for μ = 2 MeV/G, by around two orders of magnitude from L* ∼ 3.5 to L* ∼ 3.0 at μ = 20 MeV/G, and by less than two orders of magnitude and largely constrained above L* ∼ 3 at μ = 30 MeV/G. These observations provide an energy‐resolved LEO characterization of storm‐time deep penetration, showing that the magnitude and radial extent of the response vary strongly with energy and among storms with different intensities and prestorm states.

Following an intense geomagnetic storm (Dst < −150 nT) on 5 November 2023, electron fluxes in the outer radiation belt were reduced by more than three orders of magnitude at energies below ∼700 keV while they were enhanced at ∼1–3 MeV, leading to reversed energy spectra. Using high‐energy resolution measurements from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat, we track the evolution of electron spectra across L ≈ 2.5–4.5 over ∼12 days, clearly presenting the spatial extents and structures of reversed energy spectra. We also perform one‐dimensional pitch angle diffusion simulations induced by hiss and magnetosonic (MS) waves to reproduce the key features of the observed reversed spectra. These results highlight the unique observations of CIRBE and the important roles of hiss and MS waves in controlling electron dynamics in radiation belts after geomagnetic storms.

Electromagnetic ion cyclotron (EMIC) waves are recognized as a key driver of relativistic (>1 MeV) electron loss in Earth's magnetosphere. Growing evidence also suggests that EMIC wave can efficiently scatter sub‐MeV electrons, although the precise energy range and the underlying physical mechanisms remain debated. Here, we report a long‐lasting sub‐MeV electron precipitation event measured by the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. We identify H ⁺ band EMIC waves as the main driver and find that the lower‐energy cutoff of the precipitating electrons is likely determined by the fractional EMIC frequency relative to the equatorial proton gyrofrequency. These findings underscore the role of H ⁺ band EMIC waves in scattering sub‐MeV electrons and provide new insights into the interaction processes between EMIC waves and radiation belt electrons.

During the September 2019 geomagnetic storm, long‐lasting, drift‐periodic flux oscillations of multi‐MeV electrons were observed by the REPT instrument on the Van Allen Probes–A. These flux oscillations occurred across the outer belt during the storm main phase, coinciding with enhanced Pc5 ULF wave activity and elevated electron fluxes. During the recovery phase, the oscillations gradually decayed at the center of the outer belt but persisted for days at its inner edge. Using 2D test particle simulations driven by constructed broadband ULF wave fields, we simulated multi‐MeV electron fluxes during two satellite passes and successfully reproduced observed drift‐periodic flux oscillations. The close agreement between simulation and observation confirms a causal relationship between drift‐periodic flux oscillations and resonant interactions of electrons and broadband ULF waves. We further derived the radial diffusion coefficient from the simulation and compared it with empirical models. The magnitude of the resultant radial diffusion coefficient aligns closely with the model by Liu et al. (2016, https://doi.org/10.1002/2015gl067398 ), though it is lower than those by Brautigam and Albert (2000, https://doi.org/10.1029/1999ja900344 ) and Ozeke et al. (2014, https://doi.org/10.1002/2013ja019204 ). The energy‐ and L‐dependence of the diffusion coefficient is also consistent with the model by Liu et al. (2016, https://doi.org/10.1002/2015gl067398 ). We estimated the uncertainty in the derived radial diffusion coefficient to be approximately half an order of magnitude, primarily limited by the instrument's energy resolution. These results demonstrate the potential of inferring radial diffusion rates from electron flux measurements alone and underscore the importance of high‐energy‐resolution electron measurements for accurately quantifying radiation belt dynamics.

Energetic electron precipitation plays a pivotal role in shaping Earth's radiation belt dynamics and drives significant physical and chemical changes in the upper atmosphere. However, the detailed mechanisms governing the loss of relativistic electrons have remained unclear, largely due to the limited energy coverage and coarse resolution of previous measurements. Here we report high‐resolution observations of bursty electron precipitation across a broad energy range (0.3–2.3 MeV), obtained by the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. REPTile‐2 employs a novel instrument design that minimizes background to enable clean spectral measurements with the highest energy resolution achieved to date in low‐Earth orbit for this energy range. During the conjunction events when CIRBE was close to the same field line with Arase satellite at higher altitudes, our analysis shows that pitch angle diffusion driven by chorus waves can fully account for the observed three bursty precipitation events over the entire energy range. These results provide the definitive evidence for a unified chorus‐driven electron loss process acting across a wide energy range and underscore the critical importance of high‐resolution measurements in resolving long‐standing uncertainties in radiation belt dynamics. Furthermore, they offer new insight into the energy‐dependent atmospheric impacts of electron precipitation, with broad implications for space weather forecasting and upper atmospheric chemistry.

Low‐altitude measurements of energetic electron fluxes offer insight into the dynamics of the radiation belts and plasma sheet. However, distinguishing between key magnetospheric regions–such as inner belt, slot region, outer belt, and plasma sheet–based on low‐altitude data remains challenging, particularly for missions lacking pitch‐angle resolution. A commonly used boundary indicator, the flux anisotropy (i.e., precipitating‐to‐trapped flux ratio), is unavailable in such cases. In this study, we propose an alternative approach to identify the plasma sheet—radiation belt interface, commonly associated with the isotropy boundary, using high energy‐resolution measurements from the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. We demonstrate that this boundary is marked by a distinct spectral change: a power‐law form in the plasma sheet transitions to an exponential form within the outer radiation belt. These findings are supported by comparisons with Polar‐orbiting Operational Environmental Satellites (POES) observations, and we discuss the potential mechanisms responsible for the spectral transition.

Electromagnetic ion cyclotron (EMIC) waves are commonly observed electromagnetic emissions in Earth's magnetosphere and are widely considered to efficiently scatter relativistic electrons into bounce loss cones. However, their precise scattering effects remain highly debated due to limited energy coverage and coarse resolution of previous measurements. Here, we present high‐energy‐resolution measurements of EMIC‐induced relativistic electron precipitation from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. A long duration >1 MeV electron precipitation event was measured by CIRBE/REPTile‐2 in both the northern and southern hemispheres on 25 April 2023. The energy versus L dispersions of these >1 MeV precipitating electrons show good agreement with minimum resonance energies of electrons interacting with He ⁺ band EMIC waves at specific frequencies. These novel observations unveil the detailed scattering effect of EMIC waves and provide important clues regarding wave‐particle interaction processes near the equator.

Solar Energetic Particles (SEPs) are present during increased solar activity, often associated with solar flares and coronal mass ejections (CMEs). Measuring and understanding these particles is important both for fundamental solar physics knowledge as well as the determination of radiation risks in interplanetary space. Solid‐state particle telescopes are a useful tool to measure these particles. The Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) was a solid‐state energetic particle telescope that flew onboard the Colorado Inner Radiation Belt Experiment (CIRBE) and demonstrated a capability to measure electrons from 0.25 to 6 MeV and protons from 7 to 100 MeV with high energy and time resolution. REPTile‐2 operated in a low‐Earth orbit (LEO) and primarily measured radiation belt particles but was also able to measure SEPs during high‐latitude passes. Because of REPTile‐2's solid performance and its CubeSat‐scale size, weight, and power, an opportunity arose to fly a modified REPTile‐2, dubbed REPTile‐3, on the Emirates Mission to the Asteroid Belt (EMA). In this paper, Geometry and tracking 4 (Geant4) Monte Carlo simulations are used to motivate changes to improve REPTile‐3's ability to measure SEPs. Additionally, full instrument response functions and estimated count rates are used to understand the instrument's response to SEP fluxes. REPTile‐3 is shown to be able to measure 1.2–35 MeV protons with ΔE/E < 9%, 35–100 MeV protons with ΔE/E < 50%, 0.1–5 MeV electrons with ΔE/E < 14%, 18–131 MeV helium ions with ΔE/E < 7%, and 131–200 MeV helium ions with ΔE/E < 50% with a 102° field of view (FOV).

During the September 2019 geomagnetic storm, long-lasting, drift-periodic flux oscillations of multi-MeV electrons were observed by the REPT instrument on the Van Allen Probes–A. These flux oscillations occurred across the outer belt during the storm main phase, coinciding with enhanced Pc5 ULF wave activity and elevated electron fluxes. During the recovery phase, the oscillations gradually decayed at the center of the outer belt but persisted for days at its inner edge. Using 2D test particle simulations driven by constructed broadband ULF wave fields, we simulated multi-MeV electron fluxes during two satellite passes and successfully reproduced observed drift-periodic flux oscillations. The close agreement between simulation and observation confirms a causal relationship between drift-periodic flux oscillations and resonant interactions of electrons and broadband ULF waves. We further derived the radial diffusion coefficient from simulation and compared it with empirical models. The magnitude of the resultant radial diffusion coefficient aligns closely with the model by Liu et al. (2016, doi:10.1002/2015GL067398), though it is lower than those from Brautigam and Albert (2000, doi:10.1029/1999JA900344) and Ozeke et al. (2014, doi:10.1002/2013JA019204). The energy- and L-dependence of the diffusion coefficient is also consistent with the model by Liu et al. (2016, doi:10.1002/2015GL067398). We estimated the uncertainty in the derived radial diffusion coefficient to be approximately half an order of magnitude, primarily limited by the instrument’s energy resolution. These results demonstrate the potential of inferring radial diffusion rates from electron flux measurements alone and underscore the importance of high-energy-resolution electron measurements for accurately quantifying radiation belt dynamics.

The Earth's magnetotail, located on the night side of the magnetosphere, is a dynamic region where magnetic field energy is released and converted into plasma heating, particle acceleration, and kinetic energy through magnetic reconnection. Recent low‐altitude observations from the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat reveal that the efficiency of particle acceleration in the magnetotail can be high enough to produce relativistic and ultra‐relativistic electrons with energies reaching several MeV. To investigate the underlying acceleration mechanisms, we used the Rice Convection Model (RCM) to simulate the observed magnetotail electron populations. The simulations successfully reproduced key features of CIRBE observations, including the spectral shape and energy range of accelerated electrons. This agreement between RCM results and CIRBE observations offers crucial insights into the physical processes responsible for extreme electron acceleration events in the magnetotail.