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Following the retirement of the community supported Van Allen Probes mission, the quest for high-quality energetic particle measurements in the radiation belts is likely to be taken on by smaller spacecraft like CubeSats in the foreseeable future. Here we introduce the Relativistic Electron Proton Telescope integrated little experiment-2 (REPTile-2), a miniaturized (similar to 1.5 U) solid-state charged particle telescope that aims to undertake this challenging task. It incorporates detailed pulse-height analysis to enable 60 electron channels and 60 proton channels and includes anticoincidence detectors to minimize unwanted background contamination. This paper presents a description of the REPTile-2 design and emphasizes the importance of extensive Geant4-based analysis to inform the design of a new energetic particle detector and characterize the instrument response. Our analysis shows that REPTile-2 can measure similar to 0.3-similar to 4 MeV electrons and similar to 6.7-35 MeV protons with energy resolution ( increment E/E) of 7%-38% for electrons and 1.5%-5% for protons. Results from a Sr-90/Y-90 radioactive source test have verified the instrument performance and the validity of the Geant4 simulations. These energetic particle measurements will enable a new scientific understanding of the inner radiation belt, where unwanted contamination from the unforgiving penetration of highly energetic protons (tens of MeV to GeV) is common, and provide detailed quantification of the inner belt electrons and protons in the low-Earth orbit that is crucial for space weather modeling.

The Relativistic Electron-Proton Telescope (REPT) instruments were designed to measure ∼2 to >18 MeV electrons and ∼18 to > 115 MeV protons as part of the science payloads onboard the dual Radiation Belt Storm Probes (RBSP) spacecraft. The REPT instruments were turned on and configured in their science acquisition modes about 2 days after the RBSP launch on 30 August 2012. The REPT-A and REPT-B instruments both operated flawlessly until mission cessation in 2019. This paper reviews briefly the REPT instrument designs, their operational performance, relevant mode changes and trending over the course of the mission, as well as pertinent background effects (and recommended corrections). A substantial part of this paper highlights discoveries and significant advancement of our understanding of physical-processes obtained using REPT data. We do this for energetic electrons primarily in the outer Van Allen belt and for energetic protons in the inner Van Allen zone. The review also describes several ways in which REPT data were employed for important space weather applications. The paper concludes with assessments of ways that REPT data might further be exploited to continue to advance radiation belt studies. The paper also discusses the pressing and critical need for the operational continuation of REPT-like measurements both for science and for space situational awareness.

International audience; Energetic electrons have occasionally been observed with high intensity in the low-latitude quasi-trapping region, below the inner radiation belt, where their intensity is normally low. During magnetic storms in November 2004 and July 2006, electrons reached magnetic drift shells with L

An empirical model of the proton radiation belt is constructed from data taken during 2013–2017 by the Relativistic Electron-Proton Telescopes on the Van Allen Probes satellites. The model intensity is a function of time, kinetic energy in the range 18–600 MeV, equatorial pitch angle, and L-shell of proton guiding centers. Data are selected, on the basis of energy deposits in each of the 9 silicon detectors, to reduce background caused by hard proton energy spectra at low L. Instrument response functions are computed by Monte Carlo integration, using simulated proton paths through a simplified structural model, to account for energy loss in shielding material for protons outside the nominal field-of-view. Overlap of energy channels, their wide angular response, and changing satellite orientation require the model dependencies on all three independent variables be determined simultaneously. This is done by least-squares minimization with a customized steepest-descent algorithm. Model uncertainty accounts for statistical data error and systematic error in the simulated instrument response. A proton energy spectrum is also computed from data taken during the Jan 8, 2014 solar event, to illustrate methods for the simpler case of an isotropic and homogeneous model distribution. Radiation belt and solar proton results are compared to intensities computed with a simplified, on-axis response that can provide a good approximation under limited circumstances.

Compact environmental anomaly sensor risk reduction (CEASE-RR) is a new sensor designed for anomaly attribution due to the space radiation environment. It does this using two solid-state particle telescopes that have been designed to measure proton and electron fluxes that are the drivers for three of the four primary space environment effects (event total dose, deep-dielectric charging, and single event effects). These telescopes are integrated into a compact package along with space reserved for a planned electrostatic analyzer being developed for the final CEASE 3 design (covering the fourth primary space environment effect—surface charging). The sensors themselves will measure a wider dynamic range in particle flux, provide higher energy resolution, have better out-of-band contamination rejection, and improved diagnostic capability compared to previous CEASE instruments. The CEASE-RR instrument is planned to be launched in 2018 to geostationary orbit as part of an Air Force Research Laboratory flight experiment. The sensor design, calibration, and planned flight experiment objectives are described in this paper.

Straggling functions for protons in thick silicon radiation detectors are computed by Monte Carlo simulation. Mean energy loss is constrained by the silicon stopping power, providing higher straggling at low energy and probabilities for stopping within the detector volume. By matching the first four moments of simulated energy-loss distributions, straggling functions are approximated by a log-normal distribution that is accurate for Vavilov κ ≳ 0.3 . They are verified by comparison to experimental proton data from a charged particle telescope.

International audience; Cosmic Ray Albedo Neutron Decay (CRAND) has been recently confirmed as a source of energetic electrons at the inner edge of the inner belt by the Colorado Student Space Weather Experiment (CSSWE) mission. Here we use observations from the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) mission, to investigate the CRAND contribution to inner belt electrons quantitatively over a broad energy range ( 100-800 keV). Spectral fitting analysis supports the conclusion that CRAND is the most important electron source at the inner edge of the inner belt. For the first time, we show that CRAND is the dominant source of >250-keV quasitrapped electrons throughout the inner belt and slot region during quiet times. We suggest that additional sources for

A 2-dimensional bounce-averaged test particle code was developed to examine trapped electron trajectories during geomagnetic storms with the assumption of conservation of the first and second adiabatic invariants. The March 2013 storm was selected as an example because the geomagnetic activity KP index sharply increased from 2+ to 7- at 6:00UT on March 17. Electron measurements with energies between 37 and 460 keV from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard Van Allen Probes (VAP) are used as initial conditions prior to the storm onset, and served to validate test particle simulations during the storm. Simulation results help to interpret the observed electron injection as non-diffusive radial transport over a short distance in the inner belt and slot region based on various electric field models, although the quantitative comparisons are not precise. We show that electron drift trajectories are sensitive to the selection of electric field models. Moreover, our simulation results suggest that the actual field strength of penetration electric fields during this storm is stronger than any existing electric field model, particularly for L ≤ 2.

We studied the geomagnetic, plasmaspheric, and solar wind context of relativistic electron precipitation (REP) events seen with the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), Proton Electron Telescope (PET) to derive an exponential folding energy E0 for each event. Events with E0 750 keV near dusk (1400 500 keV (“hard REP”), we estimate that roughly 45% of the whole population has the distributions of geomagnetic and solar wind parameters associated with EMIC waves, while 55% does not. We hypothesize that the latter events may be caused by current-sheet scattering (CSS), which can be mistaken for EMIC-wave scattering in that both simultaneously precipitate MeV electrons and keV protons. Since a large number of MeV electrons are lost in the near-midnight hard REP events, and in the large number of E0

Radiation belt protons in the kinetic energy range 24 to 76 MeV are being measured by the Relativistic Electron Proton Telescope on each of the two Van Allen Probes. Data have been processed for the purpose of studying variability in the trapped proton intensity during October 2013 to August 2015. For the lower energies (≲32 MeV), equatorial proton intensity near L = 2 showed a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. It is postulated that these protons were trapped with enhanced efficiency during the 7 March 2012 solar proton event. A model that includes radial diffusion, along with known trapped proton source and loss processes, shows that the observed average rate of increase near L = 2 is predicted by the same model diffusion coefficient that is required to form the entire proton radiation belt, down to low L, over an extended (∼103 year) interval. A slower intensity decrease for lower energies near L = 1.5 may also be caused by inward diffusion, though it is faster than predicted by the model. Higher-energy (≳40 MeV) protons near the L = 1.5 intensity maximum are from cosmic ray albedo neutron decay. Their observed intensity is lower than expected by a factor ∼2, but the discrepancy is resolved by adding an unspecified loss process to the model with a mean lifetime ∼120 years.

The loss of protons in the outer part of the inner radiation belt (L = 2 to 3) during the 6 April 2000 solar energetic particles event has been investigated using test particle simulations that follow full Lorentz trajectories with both magnetic and electric fields calculated from an empirical model. The electric fields are calculated as inductive fields generated by the time-changing magnetic field, which is achieved by time stepping analytic magnetic fields. The simulation results are compared with proton measurements from the highly elliptical orbit satellite for three different energy ranges (8.5–35 MeV, 16–40 MeV, and 27–45 MeV) as well as previous modeling work done. In previous work, inner zone radiation belt loss during geomagnetic storms has been modeled by simulating field line curvature scattering in static magnetic field snapshots with no electric field. The inclusion of the inductive electric field causes an increase in loss to lower L shells, improving the agreement with the satellite data.

The Galaxy is filled with cosmic-ray particles, mostly protons with kinetic energies greater than hundreds of MeV. Around Earth, trapped energetic protons, electrons and other particles circulate at altitudes from about 500 to 40,000 kilometers in the Van Allen radiation belts. Soon after these radiation belts were discovered six decades ago, it was recognized that the main source of inner-belt protons (with kinetic energies at and greater than tens to hundreds of MeV) is cosmic-ray albedo neutron decay (CRAND). In this process, cosmic rays that reach the upper atmosphere interact with neutral atoms to produce albedo neutrons, which, being prone to s-decay, are a possible source of geomagnetically trapped protons and electrons. These protons would retain most of the kinetic energy of the neutrons, while the electrons would have lower energies, mostly less than one MeV. The viability of CRAND as an electron source has, however, been uncertain, because measurements have shown that the electron intensity in the inner Van Allen belt can vary greatly, while the neutron-decay rate should be almost constant. Here we show direct measurements of relativistic electrons near the inner edge of the inner radiation belt, where only quasi-trapped population exist, made by Colorado Student Space Weather Experiment (CSSWE)/Relativistic Electron and Proton Telescope integrated little experiment (REPTile). We demonstrate that the main source of these electrons is indeed CRAND, and that this process also contributes to electrons elsewhere [1].

Access of solar and galactic cosmic rays to the Earth's magnetosphere is quantified in terms of geomagnetic cutoff rigidity. Numerically computed grids of cutoff rigidities are used to model cosmic ray flux in Earth's atmosphere and in low Earth orbit. In recent years, the development of more accurate dynamic geomagnetic field models and an increase in computer power have made a real-time data-driven geomagnetic cutoff computation extending over the inner magnetosphere possible. For computational efficiency, numerically computed cutoffs may be scaled to different altitudes and directions of arrival using the known analytic variation of cutoff in a pure dipole magnetic field. This paper is a presentation of numerical techniques developed to compute effective cutoff rigidities for space weather applications. Numerical tests to determine the error associated with scaling vertical cutoff rigidities with altitude in a realistic geomagnetic field model are included. The tests were performed to guide the development of spatial grids for modeling cosmic ray access to the inner magnetosphere and to gain a better understanding of the accuracy of numerically modeled cutoffs.

No instruments in the inner radiation belt are immune from the unforgiving penetration of the highly energetic protons (tens of MeV to GeV). The inner belt proton flux level, however, is relatively stable; thus, for any given instrument, the proton contamination often leads to a certain background noise. Measurements from the Relativistic Electron and Proton Telescope integrated little experiment on board Colorado Student Space Weather Experiment CubeSat, in a low Earth orbit, clearly demonstrate that there exist sub-MeV electrons in the inner belt because their flux level is orders of magnitude higher than the background, while higher-energy electron (>1.6 MeV) measurements cannot be distinguished from the background. Detailed analysis of high-quality measurements from the Relativistic Electron and Proton Telescope on board Van Allen Probes, in a geo-transfer-like orbit, provides, for the first time, quantified upper limits on MeV electron fluxes in various energy ranges in the inner belt. These upper limits are rather different from flux levels in the AE8 and AE9 models, which were developed based on older data sources. For 1.7, 2.5, and 3.3 MeV electrons, the upper limits are about 1 order of magnitude lower than predicted model fluxes. The implication of this difference is profound in that unless there are extreme solar wind conditions, which have not happened yet since the launch of Van Allen Probes, significant enhancements of MeV electrons do not occur in the inner belt even though such enhancements are commonly seen in the outer belt. Key Points Quantified upper limit of MeV electrons in the inner belt Actual MeV electron intensity likely much lower than the upper limit More detailed understanding of relativistic electrons in the magnetosphere

Electrons derived from cosmic rays become trapped in the radiation belts that surround Earth, but how the electrons are generated has been uncertain; new measurements confirm the involvement of neutron decay. When cosmic rays hit the upper atmosphere, the collisions with atoms create 'albedo neutrons' which subsequently decay to protons and electrons (and electron antineutrinos) with a half-life of about 10 minutes. This is the main source of the protons trapped in Earth's Van Allen radiation belts. In principle, this process could also be the source of the trapped electrons, but it has been difficult to clarify because the electron intensity varies greatly whereas the neutron-decay rate should be fairly constant. Xinlin Li and collaborators report measurements of the electron density near the inner edge of the inner radiation belt and demonstrate that this population of electrons indeed comes from neutron decay. The Galaxy is filled with cosmic-ray particles, mostly protons with kinetic energies greater than hundreds of megaelectronvolts. Around Earth, trapped energetic protons, electrons and other particles circulate at altitudes from about 500 to 40,000 kilometres in the Van Allen radiation belts. Soon after these radiation belts were discovered six decades ago, it was recognized that the main source of inner-belt protons (with kinetic energies of tens to hundreds of megaelectronvolts) is cosmic-ray albedo neutron decay (CRAND)1. In this process, cosmic rays that reach the upper atmosphere interact with neutral atoms to produce albedo neutrons, which, being prone to β-decay, are a possible source of geomagnetically trapped protons and electrons. These protons would retain most of the kinetic energy of the neutrons, while the electrons would have lower energies, mostly less than one megaelectronvolt. The viability of CRAND as an electron source has, however, been uncertain, because measurements have shown that the electron intensity in the inner Van Allen belt can vary greatly, while the neutron-decay rate should be almost constant2,3. Here we report measurements of relativistic electrons near the inner edge of the inner radiation belt. We demonstrate that the main source of these electrons is indeed CRAND, and that this process also contributes to electrons in the inner belt elsewhere. Furthermore, measurement of the intensity of electrons generated by CRAND provides an experimental determination of the neutron density in near-Earth space—2 × 10−9 per cubic centimetre—confirming theoretical estimates4.

Measurements of inner radiation belt protons have been made by the Van Allen Probes Relativistic Electron-Proton Telescopes as a function of kinetic energy (24 to 76 MeV), equatorial pitch angle, and magnetic L shell, during late 2013 and early 2014. A probabilistic data analysis method reduces background from contamination by higher-energy protons. Resulting proton intensities are compared to predictions of a theoretical radiation belt model. Then trapped protons originating both from cosmic ray albedo neutron decay (CRAND) and from trapping of solar protons are evident in the measured distributions. An observed double-peaked distribution in L is attributed, based on the model comparison, to a gap in the occurrence of solar proton events during the 2007 to 2011 solar minimum. Equatorial pitch angle distributions show that trapped solar protons are confined near the magnetic equator but that CRAND protons can reach low altitudes. Narrow pitch angle distributions near the outer edge of the inner belt are characteristic of proton trapping limits.

[1] Observations of geomagnetically trapped 27–45 MeV protons following the November 2003 magnetic storm show a gradual intensity rise that is interpreted as a direct measurement of the cosmic ray albedo neutron decay (CRAND) source strength. The intensity rise is simulated by combining the detector response function with a model CRAND source, obtained by drift-averaging neutron intensity from Monte Carlo simulation of cosmic ray interactions in the atmosphere. The simulation, for 2.4

[1] Evidence for duskside relativistic electron precipitation (DREP) within the Earth's outer radiation belt has historically been seen in a few sets of high altitude balloon data (MAXIS, MINIS, INTERBOA), and in satellite data. We present statistical evidence that the relativistic electron precipitation events from the outer radiation belt with e-folding energies > 0.5 MeV are concentrated in the dusk-to-midnight sector, based on a survey of data collected by the SAMPEX satellite from 1992 to 2004. A correlation between spectral hardness and duskside MLT is observed in our sample, the largest studied to date. Out of 9380 precipitation events within the bounce loss cone, 1048 are observed to have exponentially falling spectra with e-folding energies above 0.5 MeV (“hard events”) and 1648 events below 0.2 MeV. Of the hard events, 81% occur within 12 h to 24 h MLT, compared to only 37% of events having e-folding energies below 0.2 MeV. With microbursts removed from this softer population the percentage of duskside events rises to 46%. The hard events occur at slightly elevated levels of geomagnetic activity (Ap and Dst) relative to softer nonmicroburst events, but these correlations are much weaker than for microbursts. The hard events are observed to peak in occurrence at L ~ 5.5, significantly higher than nonmicroburst softer events, even though the opposite might be expected from compression of the magnetosphere due to the more negative average Dst of the hard events. The hard events are most prevalent during the declining phase of the 11 year solar cycle.

[1] Observed signatures of electron precipitation from the inner radiation belt are shown to be consistent with the theory of resonant scattering by whistler-mode plasma waves, assuming the waves originate in VLF radio transmissions from the ground station NWC. The conclusion is based on a stochastic model of electron transport that includes pitch angle diffusion, radial diffusion, energy loss, and azimuthal drift. The wave scattering causes an increase in quasi-trapped electron intensity, forming the “wisp” signature, and a corresponding decrease in stably trapped intensity at low altitude. A smaller decrease at high altitude is expected to be obscured by inward radial diffusion. If NWC were shut down, the resulting increase in stably trapped electron intensity would be minimal.

The Earths inner and outer radiation belts, comprising energetic electrons and protons, pose a hazard to DoD spacecraft. Air ForceResearch Laboratory (AFRL) has an ongoing research effort to model and forecast the configurations of the belts, and to develop protective technologies for spacecraft. This final report summarizes these developments and research performed at AFRL during FY 20132015 to address them, funded by AFOSR grant 13RV08COR, Radiation Belt Dynamics.

In the paper we confirm earlier work on the dependence of the L-position (Lmax) of the peak intensity of storm-injected relativistic electrons upon the magnetic storm amplitude |Dst|max. Early results for |Dst|max=30–140 nT were reported by Williams et al. (1968). Later studies gave the following formula: |Dst|max= 2.75·104/Lmax4 for |Dst| up to 400 nT (Tverskaya, 1986). This sort of relationship also can be applied to the lowest position of the westward electrojet center during the storm, and the boundaries of both the trapped radiation zone and discrete auroral forms. Data from high-altitude (Polar and 1997-068) satellites, and a low-altitude satellite (SAMPEX) were analyzed and compared to the predictions of the above formula. The congruence between formula and observations is excellent: the discrepancy is not larger than 0.2 Re. There are several reasons why the discrepancies might be expected including: fast radial transport due to sudden impulses; the overlapping in time of multiple storms; and the additional injections onto L>Lmax in the storm recovery phase. There could be difficulty relating high and low altitude measurements due to problems with magnetic field models, but we believe this is not a major factor in our analysis.

The middle magnetosphere of Jupiter has not been well studied prior to the Galileo mission. This region (10–30 R_J) has been traversed by Galileo numerous times allowing spatial and temporal features to be differentiated. We present observations of energetic ions (E ∼4.5–18.4 MeV nucleon^(−1)) in the middle Jovian magnetosphere obtained with the Heavy Ion Counter (HIC) on Galileo. The radial dependence of the intensity, spectrum, and composition of these particles indicates that solar material dominates the 15 to 30 R_J region and diffuses without loss toward Jupiter. Near 10 R_J the composition is more Iogenic and shows evidence of strong losses. These observations can be understood in terms of a common energy per charge spectrum for all heavy ions. Due to the substantially different charge to mass ratios of the Iogenic and solar ions, the O/C and S/C abundance ratios vary with magnetic moment and therefore radial distance when observed over the fixed energy interval of the HIC instrument. Near 25 R_J there is evidence of acceleration of ions to MeV nucleon^(−1) energies. The location of this acceleration has been suggested previously but not directly observed before the Galileo mission.

Flux maps of energetic ions in Jupiter's outer radiation belt, calculated with the aid of a model of the current sheet magnetic field, are consistent with a spatial distribution that is uniform along field lines and monotonically decreasing with radial distance. This result is supported by numerical ion trajectory calculations that predict uniform filling of drift shells at high L values. Adiabatic compression by radial transport then provides predictions of radial energy dependence and flux gradients. At lower L values, conservation of the second adiabatic invariant predicts some anisotropy with higher equatorial fluxes.

The acceleration of magnetospheric electrons to multi-MeV energies has been a topic of much interest and study over the last several years. To further investigate this acceleration process, Polar and SAMPEX data from 1997 and 1998 were used to examine the general, repeating characteristics of the relativistic electron injection events that occurred over the 2-year period. Around the intensity peak, usually in the L range from 3.2 to 4.5, it was found that (1) during the main phase of the storm, there is a dropout of the relativistic electron fluxes; (2) fluxes increase as recovery begins; (3) the electrons are most sharply peaked in pitch angle at the beginning of the event and flatten with time; (4) the rate at which the flux increases slows with increasing energy; and (5) strong pitch angle scattering is intimately associated with relativistic electron injection events. A recirculation or resonant heating process coupled with rapid radial diffusion or a melding of both would be consistent with the observations.

A systematic study of energetic ion trajectories in Jupiter's plasma sheet region predicts a significant role for both regular and stochastic types of motion and reveals several aspects of the ion dynamics that aid in the interpretation of Galileo observations. The motion is generally confined to the vicinity of the current sheet in a way that is proportional to the variable current sheet thickness as seen in the particle and field data. The radial extent of the trajectories increases with rigidity and initial radial distance from the planet, explaining the corresponding lack of high-rigidity ions and decreasing radial gradients at lower rigidity. Ion intensity increases associated with changes in current sheet thickness suggest an acceleration region at ∼25 to 30 R_J. Energy dispersion in ion events at larger radial distances can be explained by such a source combined with elastic ion scattering.

A numerical simulation of the anomalous cosmic ray (ACR) radiation belt, including production by electron stripping and decay by energy loss in a model atmosphere, predicts altitude and time variations over the last solar cycle. Satellite ACR data are used to estimate the unknown trapping cross sections. Energy loss to free electrons is significant for the high altitude trapped ACR which have not yet been observed.

Solar cycle variations in the intensity of geomagnetically trapped anomalous cosmic ray (ACR) oxygen are demonstrated by ∼7 years of data from the SAMPEX satellite. They correspond to similar variations in the interplanetary ACR source. Trapped particle losses during the approach to solar maximum are not associated with geomagnetic activity and may be consistent with atmospheric energy degradation. The average trapped particle pitch angle distribution and energy spectra support this interpretation in which both source and loss rates are proportional to atmospheric density.

Observations from the High Sensitivity Telescope (HIST) on Polar made around January and May 1998 are used to constrain the source location of outer radiation belt relativistic electrons. Phase space densities calculated as a function of the three adiabatic invariants show positive radial gradients for L 4, peaks in the radial dependence of the phase space density are suggestive of a local electron source that may be nonadiabatic acceleration or pitch angle scattering. However, discrepancies in the results obtained with different magnetic field models and at different local times make this a tentative conclusion.

One of the Brussels Radiation Belt Workshop recommendations was the establishment of a near-real-time data driven model of the inner magnetospheric energetic particle population (L < 8). Although the “ideal” missions and data sets for such a model do not exist at present, more spacecraft than ever before are currently sampling the inner magnetosphere. We attempt here in a case study of the January 10, 1997 magnetic cloud event to construct such a model with the energetic electron data available from 5 geosynchronous and 5 elliptically orbiting satellites. We examine the constraints and difficulties of putting together a large number of datasets which are measured near-simultaneously at very different locations in the inner magnetosphere. First results indicate that we can achieve a time resolution of about 3 hours for a given “snapshot” of the inner magnetosphere, and that large azimuthal asymmetries of the energetic electron population can be observed during large storms.