Outer Belt Electrons Research Articles

Statistics and Models of the Electron Plasma Density From the Van Allen Probes

AbstractWe use the full NASA Van Allen Probes mission (2012–2019) to extract the electron plasma density from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) and Electric Field and Waves (EFW) instruments and discuss the evolution of the plasmasphere. We generate new statistics including mean and standard deviations of the plasma density with respect to L‐shell, magnetic local time (MLT), and various geomagnetic indices. These statistics are generated to be applied in radiation belt physics and space weather codes (with fits provided). The mean plasmasphere is circular around Earth with respect to MLT for Kp < 1. The mean 100 cm−3 level line is above L = 5 and mean 10 cm−3 level expands above the Van Allen Probes apogee for Kp < 1. The outer electron belt lies within the plasmasphere for 60% of all times. As activity increases (Kp > 2), a gradual MLT asymmetry forms with higher mean density in the afternoon sector due to plumes expanding outward. Conversely, the mean density decreases on the dawn and night sectors. The mean density is between ∼500 and ∼50 cm−3 between L ∼ 4 and L ∼ 6 during quiet and moderately active times (Kp < 3), representing ∼80% of all times. Statistics in regions of high density below L = 2 are underdefined for intense activity. The highest standard deviation of density represents a factor 2.5 to 3 times the mean above L = 5 and for active times. We find the percent difference between the EFW and EMFISIS densities is bounded by ±20% for quiet and moderate activity (Kp < 5) and goes up to ±100% for extreme activity.

Upper Limit of Outer Belt Electron Acceleration and Their Controlling Geomagnetic Conditions: A Comparison of Storm and Non‐Storm Events

AbstractWe perform a comprehensive investigation of the statistical distribution of outer belt electron acceleration events over energies from 300 keV to ∼10 MeV regardless of storm activity using 6‐years of observations from Van Allen Probes. We find that the statistical properties of acceleration events are consistent with the characteristic energies of combined local acceleration by chorus waves and inward radial diffusion. While electron acceleration events frequently occur both at <2 MeV at L < 4.0 and at multi‐MeV at L > 4.5, significant acceleration events are confined to L > ∼4.0. By performing superposed epoch analysis of acceleration events during storm and non/weak storm events and comparing their geomagnetic conditions, we reveal the strong correlation (>0.8) between accumulated impacts of substorms as measured by time‐integrated AL (Int(AL)) and the upper flux limit of electron acceleration. While intense storms can provide favorable conditions for efficient acceleration, they are not necessarily required to produce large maximum fluxes.

Comparative Observations of the Outer Belt Electron Fluxes and Precipitated Relativistic Electrons

AbstractRelativistic electron precipitation (REP) refers to the release of high‐energy electrons initially trapped in the outer radiation belt, which then precipitate into Earth's upper atmosphere, contributing significantly to the rapid depletion of radiation belt electron flux. This study presents a statistical analysis of REP observations collected by the Calorimetric Electron Telescope (CALET) experiment aboard the International Space Station from 2015 to the present day. Specifically, the analysis utilizes count rates acquired from the two top scintillators constituting the top charge detector, each sensitive to electrons with energies above 1.5 and 3.4 MeV, respectively. Analysis of CALET data reveals a previously unreported semi‐annual variation in the occurrence of REP events. REP periodicities resemble those observed for trapped electron fluxes in the outer belt. Furthermore, their amplitude follows the overall trend of solar wind high‐speed streams and the solar activity.

First Results From REPTile‐2 Measurements Onboard CIRBE

AbstractCIRBE (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.

Design and Instrument Characterization of the High‐Energy Resolution Relativistic Electron Telescope (HERT) Using Geant4 Simulation

AbstractEarth's outer radiation belt is filled with relativistic (∼100s of keV–3 MeV) and ultrarelativistic (∼>3 MeV) electrons. The acceleration of electrons to these high energies are believed to be caused by inward radial transport and local acceleration, but the relative importance of the two mechanisms to electrons with different energies are still under considerate debate. In the post Van Allen Probes era, measurements of radiation belt populations must rely on small space missions such as CubeSats and SmallSats. The Miniaturized High‐Energy‐Resolution relativistic electron Telescope (HERT) is a compact (≤3U) telescope designed for a CubeSat mission in geosynchronous transfer orbit (GTO). HERT's main objective is to provide high‐energy‐resolution measurements of outer belt electrons in an energy range of ∼1–7 MeV to help differentiate the contribution of inward radial transport and local acceleration. Geant4 simulations were conducted to characterize the instrument responses. A novel method of using a spherical cap particle source in Geant4 simulation was developed for more efficient instrument characterization. Combined with Bow tie analysis, it is demonstrated that HERT will have an energy resolution of 5% for ∼1.5–3 MeV electrons and 12% for ∼3–7 MeV with current electronics design. In addition, using the AE9 model, we showed the instrument would have statistically sufficient count rates in the outer belt while not saturating the electronics. With a compact configuration and higher energy resolution in comparison to previous instruments, HERT will significantly contribute to the quantitative understanding of the radiation belt electron dynamics.

Dependence of Electron Flux Dropouts in the Earth's Outer Radiation Belt on Energy and Driving Parameters During Geomagnetic Storms

AbstractUsing 5‐year of measurements from Van Allen Probes, we present a survey of the statistical dependence of the Earth's outer radiation belt electron flux dropouts during geomagnetic storms on electron energy and various driving parameters including interplanetary magnetic field Bz, PSW, SYM‐H, and AE. By systematically investigating the dropouts over energies of 1 keV–10 MeV at L‐shells spanning 4.0–6.5, we find that the dropouts are naturally divided into three regions. The dropouts show much higher occurrence rates at energies below ∼100 keV and above ∼1 MeV compared to much smaller occurrence rate at intermediate energies around hundreds of keV. The flux decays more dramatically at energies above ∼1 MeV compared to the energies below ∼100 keV. The flux dropouts of electrons below ∼100 keV strongly depend on magnetic local time (MLT), which demonstrate high occurrence rates on the nightside (18–06 MLT), with the highest occurrence rate associated with northward Bz, strong PSW and SYM‐H, and weak AE conditions. The strongest flux decay of these dropouts is found on the nightside, which strongly depends on PSW and SYM‐H. However, there is no clear MLT dependence of the occurrence rate of relativistic electron flux dropouts above ∼1 MeV, but the flux decay of these dropouts is more significant on the dayside, with stronger decay associated with southward IMF Bz, strong PSW, SYM‐H, and AE conditions. Our statistical results are crucial for understanding of the fundamental physical mechanisms that control the outer belt electron dynamics and developing future potential radiation belt forecasting capability.

Radiation belt dynamics and the decay time scale of electrons at different energy and altitude

Since the discovery of the Radiation belt, scientists have had strong interests in the magnetized space that protects us from the bombardment of solar storms. Early satellite observations show that the radiation belts consist of energetic particles and exhibit a two-belt structure. The outer belt electrons are accelerated during geomagnetic storms, while the inner-belt is mostly formed by the cosmic ray. Recently, the NASA Van Allen probes discovered a temporary storage ring that locates in the middle of the two belts when solar storm event occurs, and the radiation belts may exhibit a three-belt structure. This study aims to analyze the three-belt structure from short-term variation and long-term variation. We calculated the decay time scale in the heart of the radiation belt which describes the lifetime of the high-energy electrons. The study of the decay time is of great significance in understanding the dynamics of the whole radiation belt.

Reconstruction of electron radiation belts using data assimilation and machine learning

We present a reconstruction of radiation belt electron fluxes using data assimilation with low-Earth-orbiting Polar Orbiting Environmental Satellites (POES) measurements mapped to near equatorial regions. Such mapping is a challenging task and the appropriate methodology should be selected. To map POES measurements, we explore two machine learning methods: multivariate linear regression (MLR) and neural network (NN). The reconstructed flux is included in data assimilation with the Versatile Electron Radiation Belts (VERB) model and compared with Van Allen Probes and GOES observations. We demonstrate that data assimilation using MLR-based mapping provides a reasonably good agreement with observations. Furthermore, the data assimilation with the flux reconstructed by NN provides better performance in comparison to the data assimilation using flux reconstructed by MLR. However, the improvement by adding data assimilation is limited when compared to the purely NN model which by itself already has a high performance of predicting electron fluxes at high altitudes. In the case an optimized machine learning model is not possible, our results suggest that data assimilation can be beneficial for reconstructing outer belt electrons by correcting errors of a machine learning based LEO-to-MEO mapping and by providing physics-based extrapolation to the parameter space portion not included in the LEO-to-MEO mapping, such as at the GEO orbit in this study.

Specifying High Altitude Electrons Using Low‐Altitude LEO Systems: Updates to the SHELLS Model

Specifying High Altitude Electrons Using Low‐Altitude LEO Systems: Updates to the SHELLS Model

Dynamics of the inner electron radiation belt: A review

The Van Allen radiation belts are an extraordinary science discovery in the Earth magnetosphere and consist of two electron belts. The inner Van Allen belt contains electrons of 10s to 100s keV; the outer belt consists mainly of 0.1−10 MeV electrons. Their dynamics have been analyzed for decades. The newly-launched Van Allen Probes provide unprecedented opportunities to investigate the inner belt more thoroughly. Data from this advanced mission have allowed scientists to demonstrate that the inner belt was formed not only through inward transport of outer belt electrons but Cosmic Ray Albedo Neutron Decay (CRAND) has also played an important role. In addition, the inner belt electrons show energy-dependent variations and present “zebra stripe” structures in the energy spectrum. At the same time, scientists have further confirmed that the electrons in the inner radiation belt get lost through coulomb collision and wave-particle interaction. Despite these advances, important questions remain unanswered and require further investigation. The launch of Macau ScienceSatellite-1 mission, with its low inclination angle and low altitude orbit, will provide advanced radiation belt data for better understanding of the structure and dynamics of the inner electron radiation belt.

Unraveling the Critical Geomagnetic Conditions Controlling the Upper Limit of Electron Fluxes in the Earth's Outer Radiation Belt

AbstractWe investigate the statistical distribution of the upper limit of outer radiation belt electron fluxes during active times using 5‐year measurements from Van Allen Probes. The upper flux limit at different energies peaks around L ∼ 4.7, with larger flux variation at higher energies >1 MeV compared to energies at hundreds of keV. Based on the correlation analysis between maximum fluxes and various time‐integrated geomagnetic indices, including SYM‐H, AE, and AL, we reveal that maximum fluxes strongly correlate to cumulative effects of substorms instead of storms, with the strongest dependence on the time‐integrated AL (Int(AL)). We investigate the correlation coefficients (CC) between maximum fluxes and time‐integrated substorm indices at different time lags and identify the time lag corresponding to the best CC. Finally, we develop a prediction model of maximum fluxes using only Int(AL) as input, which shows CC of ∼0.80 and allows future forecasts of extreme cases of outer belt electrons.

Charged Particle Fluxes Associated With CALIPSO Low Laser Energy Shots

The cloud-aerosol lidar and infrared pathfinder satellite observation (CALIPSO) mission was launched on 28 April 2006 into a 705-km Sun-synchronous orbit. At launch, the lidar laser canisters were filled with dry air at 130 kPa. The redundant canister pressure has dropped during the mission, reaching 7.6 kPa by late December 2021. Starting on June 23, 2016, individual laser shots started to exhibit anomalously low energies in regions of enhanced energetic charged particle fluxes. Small at first (< 1 mJ), drops in laser energy up to 61 mJ have been observed. This study determines the energetic charged particle populations associated spatially with low laser energy shots and the degree to which time variations in the low laser energy shots can be explained by time variations in the energetic charged particle fluxes. Particle observations from the NOAA-19 medium energy proton and electron detector (MEPED) are traced down magnetic field lines to estimate the fluxes at CALIPSO altitudes that are associated with individual low-energy shots. Predictions of the stability of particle bounce and drift motions in the geomagnetic field are used to sort low-energy shots and fluxes into different radiation belt regions. While most of the low-energy shots occur in the inner belt, there are clear spatial associations with outer belt electrons and with galactic cosmic rays (GCRs). As canister pressure has decreased, GCRs and electrons have become more effective in causing larger laser energy drops.

Quantifying the Size and Duration of a Microburst-Producing Chorus Region on 5 December 2017.

Microbursts are impulsive (<1 s) injections of electrons into the atmosphere, thought to be caused by nonlinear scattering by chorus waves. Although attempts have been made to quantify their contribution to outer belt electron loss, the uncertainty in the overall size and duration of the microburst region is typically large, so that their contribution to outer belt loss is uncertain. We combine datasets that measure chorus waves (Van Allen Probes [RBSP], Arase, ground‐based VLF stations) and microburst (>30 keV) precipitation (FIREBIRD II and AC6 CubeSats, POES) to determine the size of the microburst‐producing chorus source region beginning on 5 December 2017. We estimate that the long‐lasting (∼30 hr) microburst‐producing chorus region extends from 4 to 8 ΔMLT and 2–5 ΔL. We conclude that microbursts likely represent a major loss source of outer radiation belt electrons for this event.

Phase Space Density Analysis of Outer Radiation Belt Electron Energization and Loss During Geoeffective and Nongeoeffective Sheath Regions

AbstractCoronal mass ejection driven sheath regions are one of the key drivers of drastic outer radiation belt responses. The response can however be significantly different based on the sheath properties and the associated inner magnetospheric wave activity. We performed two case studies on the effects of sheaths on outer belt electrons of various energies using data from the Van Allen Probes. One sheath caused a major geomagnetic disturbance and the other had only a minor impact. We especially investigated the phase space density (PSD) of seed, core, and ultrarelativistic electrons to determine the dominant energization and loss processes taking place during the events. Both sheaths produced substantial variation in the electron fluxes from tens of kiloelectronvolts up to ultrarelativistic energies. The responses were however the opposite: the geoeffective sheath mainly led to enhancement, while the nongeoeffective one caused a depletion throughout most of the outer belt. The case studies highlight that both inward and outward radial transport driven by ultra‐low frequency waves played an important role in both electron energization and loss. Additionally, PSD radial profiles revealed a local peak that indicated significant acceleration to core energies by chorus waves during the geoeffective event. The distinct responses and different mechanisms in action during these events were related to the timing of the peaked solar wind dynamic pressure causing magnetopause compression, and the differing levels of substorm activity. The most remarkable changes in the radiation belt system occurred in key sheath sub‐regions near the shock and the ejecta leading edge.

The Acceleration of Outer-Belt Electrons by Localized Electric Fields

To explain the populations of the outer-belt energetic electrons, including relativistic electrons, that sporadically appear in the magnetosphere, a mechanism was proposed long ago for the acceleration of those electrons by short-term bursts of the electric field, which appear on the night side during substorm disturbances (Kropotkin, 1996). This mechanism can be substantially specified if the modern concepts of bursty bulk flows in the geomagnetic tail, the occurrence of dipolarization fronts, and the excitation of localized field-aligned-resonant poloidal Alfvén oscillations involving a strong component of the electric field in the dawn-dusk direction are taken into account.

Driving of Outer Belt Electron Loss by Solar Wind Dynamic Pressure Structures: Analysis of Balloon and Satellite Data

AbstractWe present observations of ~10–60 min solar wind dynamic pressure structures that drive large‐scale coherent ~20–100 keV electron loss from the outer radiation belt. A combination of simultaneous satellite and Balloon Array for Radiation‐belt Relativistic Electron Losses (BARREL) observations on 11–12 January 2014 shows a close association between the pressure structures and precipitation as inferred from BARREL X‐rays. Specifically, the structures drive radial ExB transport of electrons up to 1 Earth radii, modulating the free electron energy available for low‐frequency plasmaspheric hiss growth, and subsequent hiss‐induced loss cone scattering. The dynamic pressure structures, originating near the Sun and commonly observed advecting with the solar wind, are thus able to switch on scattering loss of electrons by hiss over a large spatial scale. Our results provide a direct link between solar wind pressure fluctuations and modulation of electron loss from the outer radiation belt and may explain long‐period modulations and large‐scale coherence of X‐rays commonly observed in the BARREL data set.

Simultaneous Observations of Localized and Global Drift Resonance

AbstractIn this study, we present Van Allen Probe observations showing that seed (hundreds of keV) and core ( 1 MeV) electrons can resonate with ultra‐low‐frequency (ULF) wave modes with distinctive m values simultaneously. An unusual electron energy spectrogram with double‐banded resonant structure was recorded by energetic particle, composition, and thermal plasma (ECT)‐magnetic electron ion spectrometer (MagEIS) and, meanwhile, boomerang stripes in pitch angle spectrogram appeared at the lower energy band. A localized drift resonance with m = 10 wave component was responsible for the resonant band peaked at ∼200 keV while a global drift resonance with m = 3 component gave rise to the upper band resonance peaked at ∼1 MeV. Time‐Of‐Flight on boomerang stripes suggested that the localized drift resonance with ∼200 keV electrons was confined within the plasmaspheric plume. Electron flux modulations were reproduced by numerical simulations in good consistency with the observations, supporting the scenario that localized and global drift resonance could coexist in the outer belt electron dynamics simultaneously.

Outer radiation belt and inner magnetospheric response to sheath regions of coronal mass ejections: a statistical analysis

Abstract. The energetic electron content in the Van Allen radiation belts surrounding the Earth can vary dramatically at several timescales, and these strong electron fluxes present a hazard for spacecraft traversing the belts. The belt response to solar wind driving is, however, largely unpredictable, and the direct response to specific large-scale heliospheric structures has not been considered previously. We investigate the immediate response of electron fluxes in the outer belt that are driven by sheath regions preceding interplanetary coronal mass ejections and the associated wave activity in the inner magnetosphere. We consider the events recorded from 2012 to 2018 in the Van Allen Probes era to utilise the energy- and radial-distance-resolved electron flux observations of the twin spacecraft mission. We perform a statistical study of the events by using the superposed epoch analysis in which the sheaths are superposed separately from the ejecta and resampled to the same average duration. Our results show that the wave power of ultra-low frequency Pc5 and electromagnetic ion cyclotron waves, as measured by a Geostationary Operational Environmental Satellite (GOES), is higher during the sheath than during the ejecta. However, the level of chorus wave power, as measured by the Van Allen Probes, remains approximately the same due to similar substorm activity during the sheath and ejecta. Electron flux enhancements are common at low energies (&lt;1 MeV) throughout the outer belt (L = 3–6), whereas depletion predominantly occurs at high energies for high radial distances (L&gt;4). It is distinctive that the depletion extends to lower energies at larger distances. We suggest that this L-shell and energy-dependent depletion results from the magnetopause shadowing that dominates the losses at large distances, while the wave–particle interactions dominate closer to the Earth. We also show that non-geoeffective sheaths cause significant changes in the outer belt electron fluxes.

Charging Monitor Aboard the Geostationary Satellite GK2A at 128.2° E Longitude

A simple internal charging current monitor has been developed to monitor the penetration of energetic particles within spacecraft in geostationary orbits. The instrument, known as a charging monitor (CM), is a component of a suite of space weather instruments aboard the geostationary satellite GEO-KOMPSAT-2A (GK2A), that was successfully launched on December 4, 2018, with a final longitude of 128.2° E. The instrument measures the integral fluxes of charged particles through a 1-mm-thick aluminum wall, along with another 1-mm-thick aluminum plate, effectively collecting electron and proton currents with energies approximately above 0.6 and 13.1 MeV, respectively. Great care has been exercised in the design of the instrument to extend the lifetime of the analog electronics of the detector, which may easily become vulnerable to the harsh radiation environment of the outer electron belt in geostationary orbits. Comparisons of measurements from the CM have been made with those from particle detectors with a more sophisticated design from the same suite. Initial results of the CM operation for the first four months are presented, including a period of moderate geomagnetic storms. The comparisons clearly show that the CM is sufficiently capable of monitoring the enhanced levels of electrical currents from the penetration of high-energy charged particles in the context of space weather research with a minimal spacecraft resources cost and design efforts.

Specifying High‐Altitude Electrons Using Low‐Altitude LEO Systems: The SHELLS Model

AbstractWe describe an artificial neural network model of the near‐Earth space radiation environment. The geomagnetic activity index and low‐earth‐orbit (LEO) electron flux measurements from the National Oceanic and Atmospheric Administration Polar‐orbiting Operational Environmental Satellite (POES) operational spacecraft are used as model training inputs. Electron fluxes from National Aeronautics and Space Administration's Van Allen Probe spacecraft form the training outputs. We demonstrate that the model can accurately specify outer radiation belt ( –7) electron fluxes at two energies, 350 keV and 1 MeV. Various performance metrics are calculated using out‐of‐sample data, and we find high correlations and low errors between the model specification and the observed flux. We emphasize that once the model is trained, the Van Allen Probes data are no longer needed at model run time; only the POES fluxes and the index are required to specify the outer electron belt using the model.