During active geomagnetic periods both electrons and protons in the outer radiation belt have been frequently observed to penetrate to low
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Abstract L (<4). Previous studies have demonstrated systematic differences in the deep penetration of the two species of particles, most notably that the penetration of protons is observed less frequently than for electrons of the same energies. A recent study by Mei et al. (2023,https://doi.org/10.1029/2022GL101921 ) showed that the time‐varying convection electric field contributes to the deeper penetration of low‐energy electrons and that a radial diffusion‐convection model can be used to reproduce the storm‐time penetration of lower‐energy electrons to lowerL . In this study, we analyze and provide physical explanations for the different behaviors of electrons and protons in terms of their penetration depth to lowL . A radial diffusion‐convection model is applied for the two species with coefficients that are adjusted according to the mass‐dependent relativistic effects on electron and proton drift velocity, and the different loss mechanisms included for each species. Electromagnetic ion cyclotron (EMIC) wave scattering losses for 100s of keV protons during a specific event are modeled and quantified; the results suggest that EMIC waves interacting with protons of lower energies than electrons can contribute to prevent the inward transport of the protons.Free, publicly-accessible full text available August 1, 2025 -
Abstract Deep penetration of outer radiation belt electrons to low
L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lowerL . This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E ‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lowerL to match the observations of storm time values ofE ‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthalE ‐fields contribute preferentially to the deep penetration of lower‐energy electrons. -
Abstract CIRBE (Colorado Inner Radiation Belt Experiment), a 3U CubeSat, was launched on 15 April 2023 into a sun synchronous orbit (97.4° inclination and 509 km altitude). The sole science payload onboard is REPTile‐2 (Relativistic Electron and Proton Telescope integrated little experiment—2), an advanced version of REPTile which operated in space between 2012 and 2014. REPTile‐2 has 60 channels for electrons (0.25–6 MeV) and 60 channels for protons (6.5–100 MeV). It has been working well, capturing detailed dynamics of the radiation belt electrons, including several orders of magnitude enhancements of the outer belt electrons after an intense magnetic storm, multiple “wisps”‐ an electron precipitation phenomenon associated with human‐made very low frequency (VLF) waves in the inner belt, and “drift echoes” of 0.25–1.4 MeV electrons across the entire inner belt and part of the outer belt. These new observations provide opportunities to test the understanding of the physical mechanisms responsible for these features.
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Abstract Understanding local loss processes in Earth’s radiation belts is critical to understanding their overall structure. Electromagnetic ion cyclotron waves can cause rapid loss of multi‐MeV electrons in the radiation belts. These loss effects have been observed at a range of
L * values, recently as low asL * = 3.5. Here, we present a case study of an event where a local minimum develops in multi‐MeV electron phase space density (PSD) nearL * = 3.5 and evaluate the possibility of electromagnetic ion cyclotron (EMIC) waves in contributing to the observed loss feature. Signatures of EMIC waves are shown including rapid local loss and pitch angle bite outs. Analysis of the wave power spectral density during the event shows EMIC wave occurrence at higherL * values. Using representative wave parameters, we calculate minimum resonant energies, diffusion coefficients, and simulate the evolution of electron PSD during this event. From these results, we find that O+ band EMIC waves could be contributing to the local loss feature during this event. O+ band EMIC waves are uncommon, but do occur in theseL * ranges, and therefore may be a significant driver of radiation belt dynamics under certain preconditioning of the radiation belts. -
Abstract Motivated by low‐altitude cusp observations of small‐scale (~1 km) field‐aligned currents (SSFACs) interpreted as ionospheric Alfvén resonator modes, we have investigated the effects of Alfvén wave energy deposition on thermospheric upwelling and the formation of air density enhancements in and near the cusp. Such density enhancements were commonly observed near 400 km altitude by the CHAMP satellite. They are not predicted by empirical thermosphere models, and they are well correlated with the observed SSFACs. A parameterized model for the altitude dependence of the Alfvén wave electric field, constrained by CHAMP data, has been developed and embedded in the Joule heating module of the National Center for Atmospheric Research (NCAR) Coupled Magnetosphere‐Ionosphere‐Thermosphere (CMIT) model. The CMIT model was then used to simulate the geospace response to an interplanetary stream interaction region (SIR) that swept past Earth on 26–27 March 2003. CMIT diagnostics for the thermospheric mass density at 400 km altitude show (1) CMIT without Alfvénic Joule heating usually underestimates CHAMP's
orbit‐average density; inclusion of Alfvénic heating modestly improves CMIT's orbit‐average prediction of the density (by a few %), especially during the more active periods of the SIR event. (2) The improvement in CMIT'sinstantaneous density prediction with Alfvénic heating included is more significant (up to 15%) in the vicinity of the cusp heating region, a feature that the MSIS empirical thermosphere model misses for this event. Thermospheric density changes of 20–30% caused by the cusp‐region Alfvénic heating sporadically populate the polar region through the action of corotation and neutral winds. -
Abstract A variety of dynamic behavior of multi‐MeV electrons near the inner edge of the outer radiation belt has been revealed by detailed analysis of Van Allen Probes data. This study presents multi‐MeV electron flux and phase space density (PSD) using Van Allen Probes data during two strong geomagnetic storms to reveal their energy‐dependent dynamics in the outer belt, with dynamics occurring on timescales of hours to ∼1 week. Enhancements are shown down to
L = 2.6, where ∼3 MeV electron populations are enhanced by an order of magnitude during one storm of study. Study of a second comparable storm shows rapid depletion of electron populations up to ∼7 MeV in the region 2.6 ≤L ≤ 3. We also identify a local electron PSD peak atL ≈ 3 that slowly accumulates during quiet time and is rapidly depleted during an intense storm. Possible contributors to these dynamics are discussed. -
Abstract Hennekam lymphangiectasia‐lymphedema syndrome is an autosomal recessive disorder characterized by congenital lymphedema, intestinal lymphangiectasia, facial dysmorphism, and variable intellectual disability. Known disease genes include
CCBE1 ,FAT4 , andADAMTS3 . In a patient with clinically diagnosed Hennekam syndrome but without mutations or copy‐number changes in the three known disease genes, we identified a homozygous single‐exon deletion affectingFBXL7 . Specifically, exon 3, which encodes the F‐box domain and several leucine‐rich repeats of FBXL7, is eliminated. Our analyses of databases representing >100,000 control individuals failed to identify biallelic loss‐of‐function variants inFBXL7 . Published studies inDrosophila indicate Fbxl7 interacts with Fat, of which human FAT4 is an ortholog, and mutation of either gene yields similar morphological consequences. These data suggest thatFBXL7 may be the fourth gene for Hennekam syndrome, acting via a shared pathway withFAT4 .