Earth's Outer Radiation Research Articles

An Empirical Model of the Equatorial Electron Pitch Angle Distributions in Earth's Outer Radiation Belt

AbstractIn this study, we present an empirical model of the equatorial electron pitch angle distributions (PADs) in the outer radiation belt based on the full data set collected by the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes in 2012–2019. The PADs are fitted with a combination of the first, third and fifth sine harmonics. The resulting equation resolves all PAD types found in the outer radiation belt (pancake, flat‐top, butterfly and cap PADs) and can be analytically integrated to derive omnidirectional flux. We introduce a two‐step modeling procedure that for the first time ensures a continuous dependence on L, magnetic local time and activity, parametrized by the solar wind dynamic pressure. We propose two methods to reconstruct equatorial electron flux using the model. The first approach requires two uni‐directional flux observations and is applicable to low‐PA data. The second method can be used to reconstruct the full equatorial PADs from a single uni‐ or omnidirectional measurement at off‐equatorial latitudes. The model can be used for converting the long‐term data sets of electron fluxes to phase space density in terms of adiabatic invariants, for physics‐based modeling in the form of boundary conditions, and for data assimilation purposes.

Medium-term prediction of the fluence of relativistic electrons in geostationary orbit using solar wind streams forecast based on solar observations

The new approaches to the modelling of the Sun-solar wind-magnetosphere-radiation belts chain and their implementation at MSU's Space weather center (Space Monitoring Data Center, SMDC) are presented. The main idea is to use data from medium-term (3–5 days) forecasts of the speed of quasi-stationary solar wind streams based on data obtained from solar images in the UV wavelength range to improve the quality of forecasts of the magnetospheric factors of the space environment. An operational model for predicting the daily fluence of relativistic electrons (>2 MeV) in the Earth's outer radiation belt has been developed, with the extended set of input parameters that include the forecast of the solar wind velocity. To validate the model, the period from October 11, 2016 to February 18, 2017 was considered, during which the Sun made several revolutions and it was possible to observe the evolution of coronal holes and the development of solar sporadic phenomena. The arrival of high-speed solar wind streams from coronal holes to the Earth's magnetosphere, as well as, as impact of coronal mass ejections, provoked geomagnetic disturbances of various intensity, and caused significant variations of the electron fluxes of the Earth's outer radiation belt. SMDC website implements solar wind model based on the data of solar observations by the AIA instrument of the SDO space observatory at wavelengths of 19.3 and 21.1 nm that forecasts the daily fluence of relativistic electrons (E > 2 MeV) in geostationary orbit. It is shown that the use of the results of medium-term forecasting of the solar wind speed as an input parameter in the daily electron fluence forecast model can significantly improve its quality with a forecast horizon of up to 4 days.

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation.

Short and intense lower‐band chorus wave packets are ubiquitous in the Earth's outer radiation belt. In this article, we perform various Vlasov hybrid simulations, with one or two triggering waves, to study the generation of short chorus packets/subpackets inside long rising tone elements. We show that the length of the generated short wave packets is consistent with a criterion of resonance non‐overlap for two independent superposed waves, and that these chorus packets have similar characteristics as in Van Allen Probes observations. We find that short wave packets are mainly formed near the middle/end of long rising tones for moderate linear growth rates, and everywhere for stronger linear growth rates. Finally, we analyze an event characterized by Time History of Events and Macroscale Interactions during Substorms spacecraft measurements of chorus rising tones near the equator and simultaneous measurements by low altitude ELFIN CubeSats of precipitating and trapped electron fluxes in the same sector. The measured precipitating electron fluxes are well recovered by test particle simulations performed using measured plasma and wave properties. We show that short chorus wave packets of moderate amplitudes (160–250 pT) essentially lead to a more diffusive‐like transport of 50–200 keV electrons toward the loss cone than long packets. In contrast, long chorus packets are found to produce important nonlinear effects via anomalous trapping, which significantly reduces electron precipitation below 150 keV, especially for higher wave amplitudes.

The Seed Populations in the Earth's Outer Radiation Belt During the Main Phase of Magnetic Storms: A Statistical Study

AbstractUsing the data from Van Allen Probes, we investigate in detail the seed populations in the outer radiation belt during the storm main phase. Based on the different flux variations of the seed electrons before the storm and around the storm peak, these storm events are divided into “flux decay events,” “flux enhancement events” and “strong flux enhancement events.” For flux decay events, a shorter storm main phase (≤12 hr) and smaller southward interplanetary magnetic field (IMF) and solar wind electric field (−15 nT < Bzmin < −5 nT, 2 mV/m < Eymax < 7 mV/m) are found. For flux enhancement events, large southward Bzmin and Eymax (−29 nT < Bzmin < −10 nT, 5 mV/m < Eymax < 11 mV/m) for the shorter main phase or smaller southward Bzmin and Eymax (−23 nT < Bzmin < −5 nT, 3 mV/m < Eymax < 11 mV/m) for the longer main phase (>12 hr) are observed. For strong flux enhancement events, larger southward Bzmin and Eymax (−32 nT < Bzmin < −13 nT, 5 mV/m < Eymax < 22 mV/m) for the shorter main phase or smaller southward Bzmin and Eymax (−26 nT < Bzmin < −9 nT, 3 mV/m < Eymax < 16 mV/m) for the longer main phase are observed. These suggest that the flux variations of the seed electrons during the storm main phase are highly influenced by the duration of the storm main phase, southward IMF, and solar wind electric field.

Statistical Characteristics of Energetic Electron Pitch Angle Distributions in the Van Allen Probe Era: 1. Butterfly Distributions With Flux Peaks at Preferred Pitch Angles

AbstractWe present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude.

A Statistical Examination of EMIC Wave‐Driven Electron Pitch Angle Scattering Signatures

AbstractElectromagnetic ion cyclotron (EMIC) waves have long been thought to contribute significantly to the variability of Earth's outer radiation belt because of their ability to scatter relativistic (>0.5 MeV) electrons into the loss cone on short timescales. We investigate statistically such pitch angle scattering signatures (“bite‐outs”) and their quantitative agreement with cold plasma theory. Our analysis is based on EMIC wave events and local plasma parameters observed simultaneously with energetic electrons by the Van Allen Probes between February and May 2017. We determine the energy and pitch angle ranges in which relativistic electron scattering is likely and find that, on average, EMIC wave‐driven scattering of core (E ≤ 2 MeV) electrons is possible in 28% of events. By comparing the expected ranges for scattering to electron flux measurements co‐located with the wave activity, we determine that scattering signatures are found in 46% of those. Additionally, we find that the scattering signatures are more likely to occur under increased geomagnetic activity and for waves with greater wave power. These results provide observational evidence that EMIC wave‐driven scattering of core relativistic electrons is common in the duskside outer radiation belt.

Radiation Belt Model Including Semi‐Annual Variation and Solar Driving (Sentinel)

AbstractThe Earth's outer radiation belt response to geospace disturbances is extremely variable spanning from a few hours to several months. In addition, the numerous physical mechanisms, which control this response depend on the electron energy, the timescale, and the various types of geospace disturbances. As a consequence, various models that currently exist are either specialized, orbit‐specific data‐driven models, or sophisticated physics‐based ones. In this paper, we present a new approach for radiation belt modeling using Machine Learning methods driven solely by Solar wind speed and pressure, Solar flux at 10.7 cm, and the angle controlling the Russell‐McPherron effect (θRM). We show that the model can successfully reproduce and predict the electron fluxes of the outer radiation belt in a broad energy (0.033–4.062 MeV) and L‐shell (2.5–5.9) range, and moreover, it can capture the long‐term modulation of the semi‐annual variation. We also show that the model can generalize well and provide successful predictions, even outside of the spatio‐temporal range it has been trained with, using 0.8 MeV electron flux measurements from GOES‐15/EPEAD at a geostationary orbit.

Energetic Electron Precipitation Induced by Oblique Whistler Mode Chorus Emissions

AbstractEnergetic electron acceleration and precipitation in the Earth's outer radiation belt are highly associated with wave‐particle interactions between whistler mode chorus waves and electrons. We perform test particle simulations to investigate electrons interacting with parallel and oblique chorus emissions with maximum amplitude 2.1 nT and 370 pT at L = 4.5. We build up a database of Green's functions, which are treated as results of the input electrons interacting with one chorus emission. The loss process of electrons caused by consecutive chorus emissions in the outer radiation belt are traced by applying the convolution integrals of distribution functions and the Green's functions. Oblique chorus emissions lead to more electron precipitation than parallel chorus emissions in the range 370 pT–2.1 nT. By checking the resonance condition and resonant energies, we find that electrons are difficult to be scattered into the loss cone directly by Landau resonance. We propose a two‐step precipitation process for oblique chorus emissions that contributes to more electron loss: (a) Through Landau resonance interaction with a chorus emission, electrons at high pitch angles are effectively accelerated in the parallel direction, and their pitch angles become lower. (b) The electrons bounce back toward the equator, and they are pushed into loss cone through nonlinear scattering due to cyclotron resonance with another chorus emission. The combination of Landau resonance and cyclotron resonance by oblique chorus emissions results in a higher precipitation rate than the single cyclotron resonance by purely parallel chorus emissions.

Relativistic Electron Model in the Outer Radiation Belt Using a Neural Network Approach

AbstractWe present a machine‐learning‐based model of relativistic electron fluxes >1.8 MeV using a neural network approach in the Earth's outer radiation belt. The Outer RadIation belt Electron Neural net model for Relativistic electrons (ORIENT‐R) uses only solar wind conditions and geomagnetic indices as input. For the first time, we show that the state of the outer radiation belt can be determined using only solar wind conditions and geomagnetic indices, without any initial and boundary conditions. The most important features for determining outer radiation belt dynamics are found to be AL, solar wind flow speed and density, and SYM‐H indices. ORIENT‐R reproduces out‐of‐sample relativistic electron fluxes with a correlation coefficient of 0.95 and an uncertainty factor of ∼2. ORIENT‐R reproduces radiation belt dynamics during an out‐of‐sample geomagnetic storm with good agreement to the observations. In addition, ORIENT‐R was run for a completely out‐of‐sample period between March 2018 and October 2019 when the AL index ended and was replaced with the predicted AL index (lasp.colorado.edu/home/personnel/xinlin.li). It reproduces electron fluxes with a correlation coefficient of 0.92 and an out‐of‐sample uncertainty factor of ∼3. Furthermore, ORIENT‐R captured the trend in the electron fluxes from low‐earth‐orbit (LEO) SAMPEX, which is a completely out‐of‐sample data set both temporally and spatially. In sum, the ORIENT‐R model can reproduce transport, acceleration, decay, and dropouts of the outer radiation belt anywhere from short timescales (i.e., geomagnetic storms) and very long timescales (i.e., solar cycle) variations.

A Study of the Performances of Widely Used External Magnetic Field Models in the Outer Zone of the Earth's Radiation Belts by Comparing the Field Observations From Van Allen Probe‐A and the Model Estimations

AbstractUsing magnetic field observations from Van Allen Probe‐A (VAP‐A), we evaluated the performances of 13 widely used external magnetic field models in the Earth's outer radiation belt region in detail. It is shown that later models tend to match observations better than earlier models except for T03s, which has a top performance similar to the latest TA16_RBF model. Additionally, in its applicable domain from quiet to moderately disturbed geomagnetic conditions, T02 has comparable performance to T03s. In the dusk, dawn and night sectors, highly stretched magnetic fields are seen in both the observations and model output between LmIGRF ∼5.7 and 6.1, which suggests the existence of steep radial gradients in the plasma pressures inside the outer belt region. The contributions from the currents located in the dusk sector during a disturbed period, such as the partial ring current and the substorm current wedge, are not adequately accounted for by the models, and the contribution of field‐aligned currents may also be underestimated. The results of this study will help select reliable external field models for future radiation belt studies and help assess uncertainties caused by the selected models. In addition, detailed analysis of the field components depicted by the empirical models could shed light on understanding the contributions from different current systems in this region.

Role of Ducting in Relativistic Electron Loss by Whistler‐Mode Wave Scattering

AbstractResonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating only keV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.

Can Earth's Magnetotail Plasma Sheet Produce a Source of Relativistic Electrons for the Radiation Belts?

AbstractSimultaneous observations from Van Allen Probes in Earth's outer radiation belt (∼4–6 RE) and Magnetospheric Multiscale (MMS) in the magnetotail plasma sheet at >20 RE geocentric distance are used to compare relative levels of relativistic electron phase space density (PSD) for constant values of the first adiabatic invariant, M. We present new evidence from two events showing: (a) at times, there is sufficient PSD in the central plasma sheet to provide a source of >1 MeV electrons into the outer belt; (b) the most intense levels of relativistic electrons are not accelerated in the solar wind or transported from the inner magnetosphere and thus must be accelerated rapidly (within ∼minutes or less) and efficiently across a broad region of the magnetotail itself; and (c) the highest intensity relativistic electrons observed by MMS were confined within only the central plasma sheet. The answer to the title question here is: yes, it can, however whether Earth's plasma sheet actually does provide a source of several 100s keV to >1 MeV electrons to the outer belt and how often it does so remain important outstanding questions.

Driving Parameters for Multi‐MeV Electrons Flux Variations in Outer Radiation Belt

AbstractThe Earth's outer radiation belt is highly dynamic and the variations of multi‐MeV electrons flux in the region depend on solar wind and interplanetary magnetic field () parameters. This study attempts to characterize the causality and timing of the solar wind and IMF parameter activities on the outer radiation belt environment based on three tools: time shifted cross correlation, mutual information, and transfer entropy statistical analysis. The Van Allen Probes mission measurements of multi‐MeV (3.4–6.3 MeV) electrons flux data in the period from January 01, 2017 to December 31, 2018 have been used. The results indicate that the information transfer from solar wind and IMF parameters into multi‐MeV electrons flux is dependent on the kind of driving parameter, range of L‐shell, and energy of electrons. The result identifies the rank of driving parameters based on their maximum values in the correlation and information transfer analysis. Thus, solar wind velocity is the dominant driving parameter for the variation of multi‐MeV electrons flux in the outer radiation belt. Given three ranges of L‐shell (, and ), with decreasing L‐shell values the maximum information transfer from driving parameters into electron flux decreases and the lag time increases. Such dependency analysis can help to select the model parameters that influence the variation of multi‐MeV electrons flux and improve the existed models of the dynamical system.

Cross‐ Coherence of the Outer Radiation Belt During Storms and the Role of the Plasmapause

AbstractThe high energy electron population in Earth's outer radiation belt is extremely variable, changing by multiple orders of magnitude on timescales that vary from under an hour to several weeks. These changes are typically linked to geomagnetic activity such as storms and substorms. In this study, we seek to understand how coherent changes in the radiation belt are across all radial distances, in order to provide a spatial insight into apparent global variations. We do this by calculating the correlation between fluxes on differentmeasured by the PET instrument aboard the SAMPEX spacecraft for times associated with 15 large storms. Our results show that during these times, variations in the0.63 MeV electron flux are coherent outside the minimum plasmapause location and also coherent inside the minimum plasmapause location, when flux is present. However, variations in the electron fluxes inside the plasmapause show little correlation with those outside the plasmapause. During storm recovery and possibly main phases, flux variations are coherent across allregardless of plasmapause location, due to a rapid decrease, followed by an increase in radiation belt fluxes across all.

Estimation of electron fluxes in the Earth's geostationary orbit with Tikhonov regularization during a magnetically quiet period

Electron energy fluxes in the Earth's outer radiation belt are estimated using an inverse theory of the Tikhonov regularization based upon observations in geostationary orbits. Particle Detector (PD) experiment aboard a geostationary satellite GEO-KOMPSAT-2A (GK2A) at a geographic longitude of 128.2°E provided observations of electrons within a 150–2,400 keV energy range with an unprecedented energy resolution of ΔE/E in the range of 5–25%. Instrument response functions, calculated with Monte-Carlo simulations, are deconvoluted with electron observations. Using regularization parameters determined from Generalized Cross Validation (GCV), the Tikhonov method was applied to observations made during a geomagnetically quiet period. This Tikhonov regularization method, now possible for observations in the Earth's radiation belt for the first time, allows direct inference of electron fluxes without resorting to predetermined functional forms. Comparisons of our results with those from conventional methods indicate differences among the results as large as ∼200%.

PreMevE Update: Forecasting Ultra‐Relativistic Electrons Inside Earth's Outer Radiation Belt

AbstractEnergetic electrons inside Earth's Van Allen belts pose a major radiation threat to space‐borne electronics that often play vital roles in modern society. Ultra‐relativistic electrons with energies greater than or equal to two megaelectron‐volt (MeV) are of particular interest, and thus forecasting these ≥2 MeV electrons has a significant meaning to all space sectors. Here, we update the latest development of the predictive model for MeV electrons in the outer radiation belt. The new version, called PREdictive MEV Electron (PreMevE)‐2E, forecasts ultra‐relativistic electron flux distributions across the outer belt, with no need for in situ measurements of the trapped MeV electron population except at the geosynchronous orbit (GEO). Model inputs include precipitating electrons observed in low‐Earth‐orbits by NOAA satellites, upstream solar wind speeds and densities from solar wind monitors, as well as ultra‐relativistic electrons measured by one Los Alamos GEO satellite. We evaluated 32 supervised machine learning models that fall into four different classes of linear and neural network architectures, and successfully tested ensemble forecasting by using groups of top‐performing models. All models are individually trained, validated, and tested by in situ electron data from NASA's Van Allen Probes mission. It is shown that the final ensemble model outperforms individual models at most L‐shells, and this PreMevE‐2E model can provide 25‐h (∼1‐day) and 50‐h (∼2‐day) forecasts with high mean performance efficiency and correlation values. Our results also suggest that this new model is dominated by nonlinear components at L‐shells <∼4 for ultra‐relativistic electrons, different from the dominance of linear components for 1 MeV electrons as previously discovered.

Measurability of the Nonlinear Response of Electron Distribution Function to Chorus Emissions in the Earth's Radiation Belt

AbstractWe conduct test particle simulations to study the perturbations in a hot electron velocity distribution caused by a rising chorus element propagating parallel to the ambient magnetic field in the Earth's outer radiation belt. The wavefield is constructed from the nonlinear growth theory of chorus emissions of Omura (2021, https://doi.org/10.1186/s40623-021-01380-w), with additional considerations about saturation and propagation of the transverse resonant current being applied to model the subpacket structure. Using Liouville's theorem, we trace electrons back in time to reconstruct the evolution of electron velocity distribution at the magnetic equator. The electromagnetic hole created by nonlinear trapping and transport effects appears as a depression in the velocity distribution, aligned with the resonance velocity curve. We analyze the decrease of particle flux in this depression and estimate the energy resolution, pitch angle resolution, time resolution and geometric factor of particle analyzers needed to observe the perturbation. We conclude that particle detectors on current or recently operating spacecraft are always lacking in at least one of these parameters, which explains the missing direct observations of sharp phase space density depressions during chorus‐electron nonlinear resonant interaction. However, with a dedicated experiment and appropriate measurement strategy, such observations are within the possibilities of the current technology. Similarity of the simulated density perturbation and a step function mathematical model is used to draw an analogy between the backward wave oscillator regime of chorus generation and the nonlinear growth theory.

Van Allen Probes Observations of Multi‐MeV Electron Drift‐Periodic Flux Oscillations in Earth's Outer Radiation Belt During the March 2017 Event

AbstractRadiation belt electrons undergo frequent acceleration, transport, and loss processes under various physical mechanisms. One of the most prevalent mechanisms is radial diffusion, caused by the resonant interactions between energetic electrons and ULF waves in the Pc4‐5 band. An indication of this resonant interaction is believed to be the appearance of periodic flux oscillations. In this study, we report long‐lasting, drift‐periodic flux oscillations of relativistic and ultrarelativistic electrons with energies up to ∼7.7 MeV in the outer radiation belt, observed by the Van Allen Probes mission. During this March 2017 event, multi‐MeV electron flux oscillations at the electron drift frequency appeared coincidently with enhanced Pc5 ULF wave activity and lasted for over 10 h in the center of the outer belt. The amplitude of such flux oscillations is well correlated with the radial gradient of electron phase space density (PSD), with almost no oscillation observed near the PSD peak. The temporal evolution of the PSD radial profile also suggests the dominant role of radial diffusion in multi‐MeV electron dynamics during this event. By combining these observations, we conclude that these multi‐MeV electron flux oscillations are caused by the resonant interactions between electrons and broadband Pc5 ULF waves and are an indicator of the ongoing radial diffusion process during this event. They contain essential information of radial diffusion and have the potential to be further used to quantify the radial diffusion effects and aid in a better understanding of this prevailing mechanism.

Investigating the Link Between Outer Radiation Belt Losses and Energetic Electron Escape at the Magnetopause: A Case Study Using Multi‐Mission Observations and Simulations

AbstractRadiation belt flux dropout events are sudden and often significant reductions in high‐energy electrons from Earth's outer radiation belts. These losses are theorized to be due to interactions with the dayside magnetopause and possibly connected to observations of escaping magnetospheric particles. This study focuses on radiation belt losses during a moderate‐strength, nonstorm dropout event on November 21, 2016. The potential loss mechanisms and the linkage to dayside escape are investigated using combined energetic electron observations throughout the dayside magnetosphere from the Magnetospheric Multiscale and Van Allen Probes spacecraft along with global magnetohydrodynamic and test particle simulations. In particular, this nonstorm‐time event simplifies the magnetospheric conditions and removes ambiguity in the interpretation of results, allowing focus on subsequent losses from enhanced outward radial transport that can occur after initial compression and relaxation of the magnetopause boundary. The evolution of measured phase space density profiles suggest a total loss of approximately 60% of the initial radiation belt content during the event. Together the in situ observations and high‐resolution simulations help to characterize the loss by bounding the following parameters: (a) the duration of the loss, (b) the relative distribution of losses and surface area of the magnetopause over which loss occurs, and (c) the escaping flux (i.e., loss) rate across the magnetopause. In particular, this study is able to estimate the surface area of loss to less than 2.9 × 106 RE2 and the duration of loss to greater than 6 h, while also demonstrating the magnetic local time‐dependence of the escaping flux and energy spectrum.

Constraining the Location of the Outer Boundary of Earth's Outer Radiation Belt

AbstractCharacterizing the location of the outer boundary of the outer radiation belt is a key aspect of improving radiation belt models and helps to constrain our understanding of the mechanisms by which the source and seed electron populations are transported into the radiation belts. In this paper, we hypothesize that there are statistical differences in the electron distribution function across the radiation belt outer boundary, and thus analyze electron flux data from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) satellites to identify this location. We validate our hypothesis by using modeled electron L* values to approximately characterize the differences between electron distribution functions inside and outside of the radiation belts. Initially, we perform a simple statistical analysis by studying the radial evolution of the electron distribution functions. This approach does not yield a clear discontinuity, thus highlighting the need for more complex statistical treatment of the data. Subsequently, we employ machine learning (with no dependence on radial position or L*) to test a range of candidate outer boundary locations. By analyzing the performance of the models at each candidate location, we identify a statistical boundary at ≈8 RE, with results suggesting some variability. This statistical boundary is typically further out than those used in current radiation belt models.