Relativistic Electron Proton Telescope Research Articles

Simulated Trapping of Solar Energetic Protons for the 8–10 March 2012 Geomagnetic Storm: Impact on Inner Zone Protons as Measured by Van Allen Probes

AbstractSolar energetic protons (SEPs) have been shown to contribute significantly to the inner zone trapped proton population for energies <100 MeV and L > 1.3 (Selesnick et al., 2007, https://doi.org/10.1029/2006sw000275). The Relativistic Electron Proton Telescope (REPT) on the Van Allen Probes launched 30 August 2012 observed a double‐peaked (in L) inner zone population throughout the 7‐year lifetime of the mission. It has been proposed that a strong SEP event accompanied by a CME‐shock in early March 2012 provided the SEP source for the higher L trapped proton population, which then diffused radially inward to be observed by REPT at L ∼ 2. Here, we follow trajectories of SEP protons launched isotropically from a sphere at 7 Re in 15 s cadence fields from an Lyon‐Fedder‐Mobarry coupled to Rice Convection Model global magnetohydrodynamic (MHD) simulation driven by measured upstream solar wind parameters. The timescale of the interplanetary shock arrival is captured, launching a magnetosonic impulse propagating azimuthally along the dawn and dusk flanks inside the magnetosphere, shown previously to produce SEP trapping. The MHD‐test particle simulation uses Geostationary Operational Environmental Satellite (GOES) proton energy spectra to weight the initial radial profile required for the radial diffusion calculation over the following 2 years. GOES proton measurements also provide a dynamic outer boundary condition for radial diffusion. A direct comparison with REPT measurements 20 months following the trapping event in March 2012 supports this novel combination of short‐term and long‐term evolution of the newly trapped protons.

Using machine learning to diagnose relativistic electron distributions in the Van Allen radiation belts

Abstract The behaviour of relativistic electrons in the radiation belt is difficult to diagnose as their dynamics are controlled by simultaneous physical processes, some of which may be still unknown. Signatures of these physical processes are difficult to identify in large amounts of data; therefore, a machine learning approach is developed to classify energetic electron distributions which have been driven by different mechanisms. A series of unsupervised machine learning tools have been applied to 7 yrs of Van Allen Probe Relativistic Electron-Proton Telescope data to identify six different typical types of plasma conditions, each with a distinctly shaped energy-dependent pitch angle distribution (PAD). The PADs at lower energies have shapes as expected from previous studies – either butterfly, pancake, or flattop, providing evidence that machine learning has been able to reliably classify the relativistic electrons in the radiation belts. Further applications of this technique could be applied to other space plasma regions, and data sets from inner heliospheric missions such as Parker Solar Probe and Solar Orbiter, to planetary magnetospheres and the JUICE mission. Understanding PADs across the heliosphere enables researchers to determine the physical mechanisms that drive pitch angle evolution and investigate their spatial and temporal dependence and physical properties.

On the Challenges of Measuring Energetic Particles in the Inner Belt: A Geant4 Simulation of an Energetic Particle Detector Instrument, REPTile‐2

AbstractFollowing 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 (∼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 ∼0.3–∼4 MeV electrons and ∼6.7–35 MeV protons with energy resolution (∆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.

MHD‐Test Particles Simulations of Moderate CME and CIR‐Driven Geomagnetic Storms at Solar Minimum

AbstractAs part of the Whole Heliosphere and Planetary Interactions initiative, contrasting drivers of radiation belt electron response at solar minimum have been investigated with MHD‐test particle simulations for the May 13–14, 2019 Coronal Mass Ejection (CME)‐shock event and the August 30–September 3, 2019 high speed solar wind interval. Both solar wind drivers produced moderate geomagnetic storms characterized by a minimum Dst = −65 nT and −52 nT, respectively, with the August ‐ September event accompanied by prolonged substorm activity. The latter, with characteristic features of a Corotating Interaction Region (CIR)‐driven storm, produced the hardest relativistic electron spectrum observed by Van Allen Probes during the last two years of the mission, ending in October 2019. MHD simulations were performed using both the Lyon‐Fedder‐Mobarry global MHD code and recently developed GAMERA model coupled to the Rice Convection Model, run with measured L1 solar wind input for both events studied, and coupled with test particle simulations, including an initial trapped and injected population. Initial electron Phase Space Density (PSD) profiles used measurements from the Relativistic Electron Proton Telescope and MagEIS energetic particle instruments on Van Allen Probes for test particle weighting and updating of the injected population at apogee. Results were compared directly with measurements and found to reproduce magnetopause loss for the CME‐shock event and increased PSD for the CIR event. The two classes of events are contrasted for their impact on outer zone relativistic electrons near the end of Solar Cycle 24.

Harmonization of RBSP and Arase Energetic Electron Measurements Utilizing ESA Radiation Monitor Data

AbstractAccurate measurements of trapped energetic electron fluxes are of major importance for the studies of the complex nature of radiation belts and the characterization of space radiation environment. The harmonization of measurements between different instruments increases the accuracy of scientific studies and the reliability of data‐driven models that treat the specification of space radiation environment. An intercalibration analysis of the energetic electron flux measurements of the Magnetic Electron Ion Spectrometer (MagEIS) and the Relativistic Electron‐Proton Telescope (REPT) instruments on‐board the Van Allen Probes (VAP) Mission versus the measurements of the Extremely High Energy Electron Experiment (XEP) unit on‐board Arase satellite is presented. The performed analysis demonstrates a remarkable agreement between the majority of MagEIS and XEP measurements and suggests the rescaling of MagEIS HIGH unit and of REPT measurements for the treatment of flux spectra discontinuities. The proposed adjustments were validated successfully using measurements from ESA Environmental Monitoring Unit (EMU) on‐board GSAT0207 and the Standard Radiation Monitor (SREM) on‐board INTEGRAL. The derived results lead to the harmonization of science‐class experiments on‐board VAP (2012–2019) and Arase (2017–) and propose the use of the data sets as reference in a series of space weather and space radiation environment developments.

Solar Energetic Proton Access to the Near‐Equatorial Inner Magnetosphere

AbstractIn this study we examine the ability of protons of solar origin to access the near‐equatorial inner magnetosphere. Here we examine four distinct solar proton events from 20–200 MeV, concurrent with both quiet time and storm time conditions using proton data from the ACE satellite in the solar wind upstream of Earth and data from the Relativistic Electron Proton Telescope (REPT) instrument aboard Van Allen Probes. We examine the direct flux correspondence between interplanetary space and the inner magnetosphere. Small substructures in interplanetary space are observable in the REPT flux profiles, which can penetrate down to L values of ≤4. Furthermore, there are orbit‐to‐orbit variations in the west‐to‐east anisotropic flux ratios. The anisotropic flux ratios are used as a proxy for cutoff energies and display cutoff variations with L shell and energy. The dependence of the anisotropic flux ratio on Dst values is shown. The results paint a picture of highly dynamic spatial and temporal proton cutoff rigidities in the near‐equatorial inner magnetosphere.

Evolution of Pitch Angle‐Distributed Megaelectron Volt Electrons During Each Phase of the Geomagnetic Storm

AbstractUsing Relativistic Electron Proton Telescope measurements onboard Van Allen Probes, the evolution of electron pitch angle distributions (PADs) during the different phases of magnetic storms is studied. Electron fluxes are sorted in terms of storm phase, value, energy, and magnetic local time (MLT) sectors for 55 magnetic storms from October 2012 through May 2017. To understand the potential mechanisms for the evolution of electron PADs, we fit PADs to a sinusoidal function , where is the equatorial pitch angle and n is a real number. The major inferences from our study are (i) at L 5, the prestorm electron PADs are nearly isotropic (n 0), which evolves differently in different MLT sectors during the main phase subsequently recovering back to nearly isotropic distribution type during the storm recovery phase; (ii) for 3.4 MeV, the main phase electron PADs become more pancake like on the dayside with high n values (>3), while it becomes more flattop to butterfly like on the nightside, (iii) at L = 5, magnetic field strength during the storm main phase enhances during the daytime and decreases during the nighttime. (iv) Conversely, at L 3, the electron PADs neither respond significantly to the different phase of the magnetic storm nor reflect any MLT dependence. (v) Main phase, electron fluxes with <4.2 MeV shows a persistent 90° maximum PAD with n ranging between 0 and 2, while for 4.2 MeV the distribution appears flattop and butterfly like. Our study shows that the relativistic electron PADs depend upon the geomagnetic storm phase and possible underlying mechanisms are discussed in this paper.

Simulation of Prompt Acceleration of Radiation Belt Electrons During the 16 July 2017 Storm

AbstractWe investigate the prompt enhancement of radiation belt electron flux observed by the Relativistic Electron Proton Telescope instrument on board Van Allen Probes following the 16 July 2017 CME‐shock compression using MHD‐test particle simulations. The prompt enhancements can be explained by the source population interacting with the azimuthally directed electric field impulses induced by CME‐shock compressions of the dayside magnetopause. Electrons in drift resonance with the electric field impulse were accelerated by ∼ 0.6 MeV on a drift period timescale (in minutes) as the impulse propagated from the dayside to the nightside around the flanks of the magnetosphere. MHD test particle simulation of energization and drift phase bunching, due to the bipolar electric field that accompanies the dayside compression and relaxation, is found to be consistent with Van Allen Probes observations. This study reproduces the energy‐dependent drift echoes integrated over pitch angle and observed change in spectra for the first time.

Characteristic Signatures of Radial or Local Acceleration of Electron inside Earth Radiation Belt

Radiation belt science has several enigmatic issues among which is yet unexplained electron acceleration in the million electron Volt (MeV) energy range. An extensive data set of Relativistic Electron-Proton Tele-scope (REPT) on board the Radiation Belt Storm Probe (RBSP) is studied for the 28 June, 2013 electron acceleration event. Phase space density is first determined for 2.30 MeV particles from measured integral flux and then calculated for the appropriate energy that conserves the first adiabatic invariant. It is shown that the time dependent radial profile of phase space density supports the local acceleration mechanism.

Characteristic Signatures of Radial or Local Acceleration of Electron inside Earth Radiation Belt

Radiation belt science has several enigmatic issues among which is yet unexplained electron acceleration in the million electron Volt (MeV) energy range. An extensive data set of Relativistic Electron-Proton Tele-scope (REPT) on board the Radiation Belt Storm Probe (RBSP) is studied for the 28 June, 2013 electron acceleration event. Phase space density is first determined for 2.30 MeV particles from measured integral flux and then calculated for the appropriate energy that conserves the first adiabatic invariant. It is shown that the time dependent radial profile of phase space density supports the local acceleration mechanism.

The Effects of Geomagnetic Storms and Solar Wind Conditions on the Ultrarelativistic Electron Flux Enhancements

AbstractUsing data from the Relativistic Electron Proton Telescope on the Van Allen Probes, the effects of geomagnetic storms and solar wind conditions on the ultrarelativistic electron (E > ~3 MeV) flux enhancements in the outer radiation belt, especially regarding their energy dependence, are investigated. It is showed that, statistically, more intense geomagnetic storms are indeed more likely to cause flux enhancements of ~1.8‐ to 7.7‐MeV electrons, though large variations exist. As the electron energy gets higher, the probability of flux enhancement gets lower. To shed light on which conditions of the storms are preferred to cause ultrarelativistic electron flux enhancement, detailed superposed epoch analyses of solar wind parameters and geomagnetic indices during moderate and intense storms with/without flux enhancements of different energy electrons are conducted. The results suggest that the storms with higher solar wind speed, sustained southward interplanetary magnetic field Bz, lower solar wind number density, higher solar wind Ey, and elevated and sustained substorm activity are more likely to cause ultrarelativistic electron flux enhancements in the outer belt. Comparing results of different energy electrons, the solar wind speed and AE index are the two parameters mostly correlated with the energy‐dependent acceleration of ultrarelativistic electrons: Storms with higher solar wind speed and elevated and sustained substorm activity are more likely to cause flux enhancement of ultrarelativistic electrons with higher energies. This suggests the important roles of inward radial diffusion as well as the source and seed populations provided by substorms on the energy‐dependent acceleration of ultrarelativistic electrons.

On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study*

Abstract Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron–Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L < 5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

Plasma Waves and Particles during Multi-dip Storms Invoked by Turbulent Solar Wind

Abstract The temporal variations of the disturbance storm time index show the occurrence of multi-dip storms during 2015 April. Specifically, a double-dip storm was observed during April 9–12 and a six-dip storm occurred during April 13–21. This work analyzes the data from the electric field and wave (EFW) instrument, relativistic electron proton telescope (REPT), and electric and magnetic field instrument suite (EMFISIS) on board radiation belt storm probe A (RBSP-A) during the selected multi-dip storms. The ultra low-frequency (ULF) as well as very low-frequency (VLF) electromagnetic oscillations were observed by EFW and EMFISIS simultaneously with the REPT observed fluctuating fluxes of relativistic electrons in the slot and outer Van Allen belt. The timescales, for relativistic electrons flux increase in slot and depletion, refilling of the outer Van Allen belt, were much faster than the temporal variations expected from the radial diffusion driven by drift resonance between ULF waves and relativistic electrons. Therefore, the contribution of VLF interaction with the relativistic electrons is important for understanding rapid flux variations on times of few hours during the considered multi-dip geomagnetic storms. The findings of this study may be helpful for understanding the relativistic electrons dynamics and transport in the Earth’s magnetosphere during multi-dip geomagnetic storms.

The Outer Radiation Belt Response to the Storm Time Development of Seed Electrons and Chorus Wave Activity During CME and CIR Driven Storms

AbstractGyroresonant wave‐particle interactions with very low frequency whistler mode chorus waves can accelerate subrelativistic seed electrons (hundreds of keV) to relativistic energies in the outer radiation belt during geomagnetic storms. In this study, we conduct a superposed epoch analysis of the chorus wave activity, the seed electron development, and the outer radiation belt electron response between L* = 2.5 and 5.5, for 25 coronal mass ejection and 35 corotating interaction region storms using Van Allen Probes observations. Electron data from the Magnetic Electron Ion Spectrometer and Relativistic Electron Proton Telescope instruments are used to monitor the storm‐phase development of the seed and relativistic electrons, and magnetic field measurements from the Electric and Magnetic Field Instrument Suite and Integrated Science instrument are used to identify the chorus wave activity. Our results show a deeper (lower L*), stronger (higher flux), and earlier (epoch time) average seed electron enhancement and a resulting greater average radiation belt electron enhancement in coronal mass ejection storms compared to the corotating interaction region storms despite similar levels and lifetimes of average chorus wave activity for the two storm drivers. The earlier and deeper seed electron enhancement during the coronal mass ejection storms, likely driven by greater convection and substorm activity, provides a higher probability for local acceleration. These results emphasize the importance of the timing and the level of the seed electron enhancements in radiation belt dynamics.

Artificial intelligence unfolding for space radiation monitor data

The reliable and accurate calculation of incident particle radiation fluxes from space radiation monitor measurements, i.e. count-rates, is of great interest and importance. Radiation monitors are relatively simple and easy to implement instruments found on board multiple spacecrafts and can thus provide information about the radiation environment in various regions of space ranging from Low Earth orbit to missions in Lagrangian points and even interplanetary missions. However, the unfolding of fluxes from monitor count-rates, being an ill-posed inverse problem, is not trivial and prone to serious errors due to the inherent difficulties present in such problems. In this work we present a novel unfolding method which uses tools from the fields of Artificial Intelligence and Machine Learning to achieve good unfolding of monitor measurements. The unfolding method combines a Case Based Reasoning approach with a Genetic Algorithm, which are both widely used. We benchmark the method on data from European Space Agency’s (ESA) Standard Radiation Environment Monitor (SREM) on board the INTEGRAL mission by calculating proton fluxes during Solar Energetic Particle Events and electron fluxes from measurements within the outer Radiation Belt. Extensive evaluation studies are made by comparing the unfolded proton fluxes with data from the SEPEM Reference Dataset v2.0 and the unfolded electron fluxes with data from the Van Allen Probes mission instruments Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT).

Simulated Prompt Acceleration of Multi‐MeV Electrons by the 17 March 2015 Interplanetary Shock

AbstractPrompt enhancement of relativistic electron flux at L = 3–5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by ∼1 MeV is inferred on less than a drift timescale as seen in prior shock compression events, which launch a magnetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at >1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression.

Comparison of Van Allen Probes radiation belt proton data with test particle simulation for the 17 March 2015 storm

AbstractThe loss of protons in the outer part of the inner radiation belt (L = 2 to 3) during the 17 March 2015 geomagnetic storm was investigated using test particle simulations that follow full Lorentz trajectories with both magnetic and electric fields calculated from an empirical model. The simulation results presented here are compared with proton pitch angle measurements from the Van Allen Probe satellites Relativistic Electron Proton Telescope (REPT) instrument before and after the coronal mass ejection‐shock‐driven storm of 17–18 March 2015, with minimum Dst =− 223 nT, the strongest storm of Solar Cycle 24, for four different energy ranges with 30, 38, 50, and 66 MeV mean energies. Two simulations have been run, one with an inductive electric field and one without. All four energy channels show good agreement with the Van Allen Probes REPT measurements for low L (L < 2.4) in both simulations but diverge for higher L values. The inclusion of the inductive electric field, calculated from the time‐changing magnetic field, significantly improves the agreement between simulation and REPT measurements at L > 2.4. A previous study using the Highly Elliptical Orbiter 3 spacecraft also showed improved agreement when including the inductive electric field but was unable to compare effects on the pitch angle distributions.

Current energetic particle sensors

AbstractSeveral energetic particle sensors designed to make measurements in the current decade are described and their technology and capabilities discussed and demonstrated. Most of these instruments are already on orbit or approaching launch. These include the Magnetic Electron Ion Spectrometers (MagEIS) and the Relativistic Electron Proton Telescope (REPT) that are flying on the Van Allen Probes, the Fly's Eye Electron Proton Spectrometers (FEEPS) flying on the Magnetospheric Multiscale (MMS) mission, and Dosimeters flying on the AC6 Cubesat mission. We focus mostly on the electron measurement capability of these sensors while providing summary comments of their ion measurement capabilities if they have any.

Occurrence characteristics of outer zone relativistic electron butterfly distribution: A survey of Van Allen Probes REPT measurements

AbstractUsing Van Allen Probes Relativistic Electron Proton Telescope (REPT) pitch angle resolved electron flux data from September 2012 to March 2015, we investigate in detail the global occurrence pattern of equatorial (|λ| ≤ 3°) butterfly distribution of outer zone relativistic electrons and its potential correlation with the solar wind dynamic pressure. The statistical results demonstrate that these butterfly distributions occur with the highest occurrence rate ~ 80% at ~ 20–04 magnetic local time (MLT) and L > ~ 5.5 and with the second peak (> ~ 50%) at ~ 11–15 MLT of lower L shells ~ 4.0. They can also extend to L = 3.5 and to other MLT intervals but with the occurrence rates predominantly < ~25%. It is further shown that outer zone relativistic electron butterfly distributions are likely to peak between 58° and 79° for L = 4.0 and 5.0 and between 37° and 58° for L = 6.0, regardless of the level of solar wind dynamic pressure. Relativistic electron butterfly distributions at L = 4.0 also exhibit a pronounced day‐night asymmetry in response to the Pdyn variations. Compared to the significant L shell and MLT dependence of the global occurrence pattern, outer zone relativistic electron butterfly distributions show much less but still discernable sensitivity to Pdyn, geomagnetic activity level, and electron energy, the full understanding of which requires future attempts of detailed simulations that combine and differentiate underlying physical mechanisms of the geomagnetic field asymmetry and scattering by various magnetospheric waves.

Simulation of energy‐dependent electron diffusion processes in the Earth's outer radiation belt

AbstractThe radial and local diffusion processes induced by various plasma waves govern the highly energetic electron dynamics in the Earth's radiation belts, causing distinct characteristics in electron distributions at various energies. In this study, we present our simulation results of the energetic electron evolution during a geomagnetic storm using the University of California, Los Angeles 3‐D diffusion code. Following the plasma sheet electron injections, the electrons at different energy bands detected by the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT) instruments on board the Van Allen Probes exhibit a rapid enhancement followed by a slow diffusive movement in differential energy fluxes, and the radial extent to which electrons can penetrate into depends on energy with closer penetration toward the Earth at lower energies than higher energies. We incorporate radial diffusion, local acceleration, and loss processes due to whistler mode wave observations to perform a 3‐D diffusion simulation. Our simulation results demonstrate that chorus waves cause electron flux increase by more than 1 order of magnitude during the first 18 h, and the subsequent radial extents of the energetic electrons during the storm recovery phase are determined by the coupled radial diffusion and the pitch angle scattering by EMIC waves and plasmaspheric hiss. The radial diffusion caused by ULF waves and local plasma wave scattering are energy dependent, which lead to the observed electron flux variations with energy dependences. This study suggests that plasma wave distributions in the inner magnetosphere are crucial for the energy‐dependent intrusions of several hundred keV to several MeV electrons.