Electron Phase Space Density Research Articles

Long-term dynamics driven by resonant wave–particle interactions: from Hamiltonian resonance theory to phase space mapping

In this study we consider the Hamiltonian approach for the construction of a map for a system with nonlinear resonant interaction, including phase trapping and phase bunching effects. We derive basic equations for a single resonant trajectory analysis and then generalize them into a map in the energy/pitch-angle space. The main advances of this approach are the possibility of considering effects of many resonances and to simulate the evolution of the resonant particle ensemble on long time ranges. For illustrative purposes we consider the system with resonant relativistic electrons and field-aligned whistler-mode waves. The simulation results show that the electron phase space density within the resonant region is flattened with reduction of gradients. This evolution is much faster than the predictions of quasi-linear theory. We discuss further applications of the proposed approach and possible ways for its generalization.

Energetic Electron Enhancement and Dropout Echoes Induced by Solar Wind Dynamic Pressure Decrease: The Effect of Phase Space Density Profile

AbstractThe interplanetary (IP) shock with the sudden increase of solar wind dynamic pressure can cause the acceleration and transportation of energetic electrons in the inner magnetosphere. In this study, we further examine the influence of the sudden drop of solar wind dynamic pressure on magnetospheric energetic particles, through case study of energetic electron drift echoes and statistics with 25 cases from 2012 to 2017. We found that there were both dropout and enhancement drift echoes of electrons in some dynamic pressure decrease events, while other events only show electron dropout drift echoes. In addition, we analyzed the electron phase space density (PSD) and found that the radial profile of electron PSD were different for the two types of events, suggesting that the electron PSD plays an important role in the variations of energy electron fluxes induced by the dynamics pressure decrease. The results of this study, combined with previous studies of energetic electron dynamics caused by IP shocks, can provide a more general picture of both solar wind dynamic pressure decrease and increase effects on the energetic electrons.

Non-storm erosion of MeV electron outer radiation belt down to <em>L</em>* < 4.0 associated with successive enhancements of solar wind density

We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements (SWDEs) on November 27−30, 2015. Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions (PAD) first developed at <italic>L</italic>* = 5.5 and then moved down to <italic>L</italic>* &lt; 4. According to the evolution of the electron phase space density (PSD) profile, losses of electrons with small pitch angles at <italic>L</italic>* &gt; 4 during SWDE1 are mainly due to outward radial diffusion. However during SWDE2&amp;3, scattering loss due to EMIC waves is dominant at 4 &lt; <italic>L</italic>* &lt; 5. As for electrons with large pitch angles, outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell (LCDS). The inner edge of EMIC wave activity moved from <italic>L</italic>* ~5 to <italic>L</italic>* ~4 and from <italic>L</italic> ~6.4 to <italic>L</italic> ~4.2 from SWDE1 to SWDE2&amp;3, respectively, observed by Van Allen Probes and by ground stations. This is consistent with the inward penetration of anisotropic energetic protons from <italic>L</italic>* = 4.5 to <italic>L</italic>* = 3.5, suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.

Interplay of source/seed electrons and wave-particle interactions in producing relativistic electron PSD enhancements in the outer Van Allen belt

We perform a superposed epoch analysis on two groups of selected geospace disturbance events, emerging from single and isolated interplanetary drivers, and resulting in either the enhancement or the depletion of the average relativistic electron Phase Space Density (PSD). We investigate the occurrence of behavioural/temporal patterns of solar wind and geomagnetic parameters, chorus and ULF Pc5 wave activity, and the source/seed electron PSD in the outer Van Allen radiation belt. Our results indicate that the source electron population of μ = 10 MeV/G exhibits a similar behaviour during both event groups, while the μ = 1 MeV/G population can be considered as negligible for the whole process of wave excitation and electron energisation. Moreover, events that result in relativistic electron enhancement are characterised by statistically stronger and prolonged storm, substorm and wave activity, combined with an abundance of seed electrons of μ = 100 MeV/G, mostly at L* = 4–5, the nominal heart of the outer radiation belt.

Accounting for Variability in ULF Wave Radial Diffusion Models

AbstractMany modern outer radiation belt models simulate the long‐time behavior of high‐energy electrons by solving a three‐dimensional Fokker‐Planck equation for the drift‐ and bounce‐averaged electron phase space density that includes radial, pitch‐angle, and energy diffusion. Radial diffusion is an important process, often characterized by a deterministic diffusion coefficient. One widely used parameterization is based on the median of statistical ultralow frequency (ULF) wave power for a particular geomagnetic index Kp. We perform idealized numerical ensemble experiments on radial diffusion, introducing temporal and spatial variability to the diffusion coefficient through stochastic parameterization, constrained by statistical properties of its underlying observations. Our results demonstrate the sensitivity of radial diffusion over a long time period to the full distribution of the radial diffusion coefficient, highlighting that information is lost when only using median ULF wave power. When temporal variability is included, ensembles exhibit greater diffusion with more rapidly varying diffusion coefficients, larger variance of the diffusion coefficients and for distributions with heavier tails. When we introduce spatial variability, the variance in the set of all ensemble solutions increases with larger spatial scales of variability. Our results demonstrate that the variability of diffusion affects the temporal evolution of phase space density in the outer radiation belt. We discuss the need to identify important temporal and length scales to constrain variability in diffusion models. We suggest that the application of stochastic parameterization techniques in the diffusion equation may allow the inclusion of natural variability and uncertainty in modeling of wave‐particle interactions in the inner magnetosphere.

Energetic Electron Scattering due to Whistler Mode Chorus Waves Using Realistic Magnetic Field and Density Models in Jupiter's Magnetosphere

AbstractWe evaluate energetic electron scattering in pitch angle and energy using realistic magnetic field and density models due to whistler mode chorus waves in Jupiter's magnetosphere and study their dependences on various wave and background parameters. We calculate the bounce‐averaged diffusion coefficients by considering the latitudinal variation of total electron density and ambient magnetic field intensity, using the VIP4 internal magnetic field and CAN current sheet model. The electron phase space density evolution due to chorus waves is simulated at M shell of 10, using the central wave frequency at 0.1fce and wave amplitude of 30 pT. Under the typical values of the ratio between the plasma frequency and electron cyclotron frequency, chorus waves could cause fast pitch angle scattering loss of energetic electrons from tens to several hundred keV in several hours, and gradual acceleration of relativistic electrons at several MeV in several days. The electron pitch angle scattering at ~500 keV and the acceleration at several MeV are both enhanced using the latitudinally varying density and VIP4 + CAN magnetic field model compared to the electron evolution using the constant density and dipole magnetic field model. Our sensitivity study indicates that the electron scattering at higher energy is caused by waves at lower frequencies or in a lower‐density background plasma, and the scattering is faster for waves at smaller wave normal angles. The electron diffusion is mainly caused by waves at lower latitudes, but the waves at higher latitudes (&gt;30°) contribute to the electron loss at higher energies (&gt;2 MeV).

Whistler Mode Waves in the Compressional Boundary of Foreshock Transients

AbstractEarth's foreshock is filled with backstreaming particles that can generate a variety of waves and foreshock transients. According to recent studies, these particles can be further accelerated while being scattered by field fluctuations, including waves, inside foreshock transients, contributing to particle acceleration at the parent bow shock. The properties of these waves and how they interact with particles and affect particle acceleration inside foreshock transients are still unclear, however. Here we take the first step to study one important type of these waves, whistler waves. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations and employ multiple case studies to investigate the properties of whistler waves in the compressional boundaries of foreshock transients where THEMIS wave burst mode is triggered. We show that the whistler waves are quasi parallel propagating with bidirectional Poynting vectors, suggesting that they are locally generated. We focus on how they interact with electrons. We show that the diffusion surfaces for these waves in the electron velocity space match the observed electron phase space density distribution contours better when the modeled pitch angle diffusion coefficients from these waves are higher. We also demonstrate that higher‐energy electrons are more likely to be scattered by whistler waves. Our results suggest that whistler waves are important for scattering tens to hundreds of electronvolt electrons inside foreshock transients and elucidate electron dynamics and whistler wave properties in such environments.

Localization of the Source of Quasiperiodic VLF Emissions in the Magnetosphere by Using Simultaneous Ground and Space Observations: A Case Study

AbstractWe study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

Two-step Dropouts of Radiation Belt Electron Phase Space Density Induced by a Magnetic Cloud Event

Abstract We report a two-step dropout event of radiation belt electron phase space density (PSD) induced by a typical magnetic cloud (MC) that drove an intense geomagnetic storm. The first and second steps of PSD dropout occurred, respectively, in the initial and main phases of the storm with a short-time partial recovery between the two dropouts. In this event, the initial phase after the sudden commencement lasted for near 21 hr, which gives an ideal opportunity to investigate the nature of the radiation belt electron dropout by isolating the main phase from any losses occurring during the initial phase. Detailed analysis shows that the first step of the dropout in the initial phase is likely associated with the magnetopause shadowing effect in combination with ultra-low frequency wave-induced outward transport caused by sustaining enhanced dynamic pressure activity before the MC. Comparably, the prolonged strong southward interplanetary magnetic field inside the MC that resulted in the storm main phase is supposed to play an important role in the second step of significant electron losses to the interplanetary space. Additionally, the partial recovery of electron PSD between the two steps of the dropout is possibly due to the acceleration processes via wave-particle interactions with whistler-mode chorus waves. Our study demonstrated that persistently enhanced solar wind dynamic pressure, which is frequently observed inside interplanetary coronal mass ejections and corotating interaction regions, can play an important role in modulating the radiation belt electron dynamics before the storm main phase driven by these solar wind disturbances.

Distribution of energetic electrons in the near earth space: New observations from the BeiDa Imaging Electron Spectrometer and the Van Allen Probes

We study the distribution of magnetospheric electrons using data from the BeiDa Imaging Electron Spectrometer (BD-IES) onboard a Chinese navigation spacecraft and the Magnetic Electron Ion Spectrometer (MagEIS) onboard the Van Allen Probes (VAP). While the MagEIS instrument monitors electron flux in the inner magnetosphere (L∗<6), the BD-IES instrument extends the range of measurement beyond the geosynchronous distance (6<L∗<9). Utilizing one year of the multi-spacecraft data, the distribution of electron phase space density (PSD) of wide spatial and energy coverage is statistically derived for the first time. Evident peaks are identified in the radial PSD profiles of energetic electrons (μ>50MeV/G) at L∗~5.5, with the inner and outer sides of the PSD peaks unambiguously observed by MagEIS and BD-IES respectively. These peaked distributions confirm that local acceleration plays an important role in the electron dynamics. Besides, the electron spectra in the vicinity of the PSD peak are harder, with spectral indexes of ~−2.5. The spectral index drops to ~−4 inside L∗~3. This softer spectrum indicates less acceleration and/or more loss in the most inner part of the magnetosphere. The new observational results from the BD-IES, incorporated with VAP data, provide us with a more comprehensive view of the energetic electrons in the near Earth space.

On Phase Space Density and Its Radial Gradient of Outer Radiation Belt Seed Electrons: MMS/FEEPS Observations

AbstractElectrons of ∼102 keV in the inner magnetosphere are suggested potentially to be the “seeds” of outer radiation belt electrons. Investigations into them could shed light on the origination of the outer radiation belt. Here we conduct statistics on seed electron phase space density (PSD) and its radial gradient, using 2‐year data obtained by Fly's Eye Energetic Particle Spectrometer (FEEPS) onboard Magnetospheric Multiscale (MMS). The radial distributions of the PSD are μ and K dependent, and azimuthally asymmetric. In the nightside, as L* increases, PSD at lower μ always increases, while PSD at higher μ first increases and then decreases. In contrast, in the noonside, PSD first increases and then decreases at all covered μ. Crossover μ, defined as μ at which PSD radial gradient is equal to 0, is identified from the statistics. It relatively stays a constant of ∼300 MeV/G inside L*∼6, but decreases as L* increases outside L*∼6. Crossover μ is larger in the nightside than in the noonside and is larger at smaller K. The radial structures of seed electron PSD should be taken into account when considering the dynamics of the outer radiation belt.

Persistent EMIC Wave Activity Across the Nightside Inner Magnetosphere

AbstractElectromagnetic ion cyclotron (EMIC) waves can act as a loss process for both ring current ions and radiation belt electrons, and the spatial and temporal characteristics of these waves are important for quantifying their effects on energetic particles. Here we utilize observations from multiple spacecraft to constrain the azimuthal and radial dimensions as well as the duration of an EMIC wave event occurring on the nightside of the inner magnetosphere on 7 July 2013. These combined observations reveal waves limited to a narrow radial extent but persisting ~10+ hr and spanning ~12 hr in local time. The solar wind conditions, geomagnetic activity, and plasma environment are also examined to better understand the conditions under which persistent nightside EMIC waves can occur. Relativistic electron phase space density profiles during this event reveal local minima concurrent with the wave activity, consistent with EMIC‐driven scattering and loss of radiation belt electrons.

On the Confinement of Ultrarelativistic Electron Remnant Belts to Low Shells

AbstractUltrarelativistic electron remnant belts are frequently observed at low shells between the inner belt and a re‐forming outer belt following geomagnetic disturbances that led to a dropout of electron fluxes at higher radial distances from the Earth. Using wave, particle, and plasma measurements from the Van Allen Probes and Pc3‐Pc5 ultra low frequency (ULF) wave data from ground magnetometers from September 2012 to November 2017, we find significant correlations between the upper edge of the remnant belts and the minimum plasmapause and last closed drift shell locations. The maximum 2‐hr‐averaged radial diffusion rate based on ULF wave power recorded during the dropouts is correlated with the upper edge of the remnant belts and last closed drift shell position. Frequently, ULF wave power is sufficiently strong down to the upper edge of the remnant belts to allow a fast outward radial diffusion of electrons up to the last closed drift shell and to account for the observed confinement of remnant belts to low shells. The electron phase space density often exhibits the needed negative or oscillating outward gradients in the region of flux loss. Accordingly, fast outward radial diffusion by ULF waves turns out to be a crucial contributor to the depletion of the outer belt that leads to the formation of remnant belts of ultrarelativistic electrons, although we show that multi‐MeV electron precipitation through combined scattering by contemporaneous electromagnetic ion cyclotron (EMIC) and lower‐band chorus waves probably contributes in a limited number of cases.

Rapid Outer Radiation Belt Flux Dropouts and Fast Acceleration During the March 2015 and 2013 Storms: The Role of Ultra‐Low Frequency Wave Transport From a Dynamic Outer Boundary

AbstractWe present simulations of the outer radiation belt electron flux during the March 2015 and 2013 storms using a radial diffusion model. Despite differences in disturbance short‐time intensity between the two storms, the response of the ultra‐relativistic electrons in the outer radiation belt was remarkably similar, both showing a sudden drop in the electron flux followed by a rapid enhancement in the outer belt flux to levels over an order of magnitude higher than those observed during the pre‐storm interval. Simulations of the ultra‐relativistic electron flux during the March 2015 storm show that outward radial diffusion can explain the flux dropout down to L*~4. However, in order to reproduce, the observed flux dropout at L* &lt; 4 requires the addition of a loss process characterized by an electron lifetime of around 1 hr operating below L*~3.5 during the flux dropout interval. Nonetheless, during the pre‐storm and recovery phase of both storms, the radial diffusion simulation reproduces the observed flux dynamics. For the March 2013 storm, the flux dropout across all L‐shells is reproduced by outward radial diffusion activity alone. However, during the flux enhancement interval at relativistic energies, there is evidence of a growing local peak in the electron phase space density at L*~3.8, consistent with local acceleration such as by very low frequency chorus waves. Overall, the simulation results for both storms can accurately reproduce the observed electron flux only when event specific radial diffusion coefficients are used, instead of the empirical diffusion coefficients derived from ultra‐low frequency wave statistics.

Nonlinear Wave Growth Analysis of Whistler‐Mode Chorus Generation Regions Based on Coupled MHD and Advection Simulation of the Inner Magnetosphere

AbstractWe show the regions where nonlinear growth of whistler‐mode chorus waves is preferred to occur in the inner magnetosphere. A global magnetohydrodynamics (MHD) simulation was used to obtain large‐scale electric and magnetic fields under the southward interplanetary magnetic field condition. With the electric and magnetic fields obtained by the MHD simulation, we ran a comprehensive inner magnetosphere‐ionosphere model to solve the evolution of phase space density of electrons. Hot electrons originating from the tail region drift sunward and penetrate deep into the inner region due to a combination of convection and substorm‐associated electric fields. Cold electrons also drift sunward, resulting in a contraction of the plasmasphere. We obtained the following results. (1) The whistler waves can first grow due to the linear mechanism (pitch angle anisotropy) in the premidnight‐prenoon region outside the plasmapause, followed by rapid, nonlinear mechanism accompanied with rising‐tone chorus elements. (2) When the solar wind speed is high, the whistler waves grow more efficiently due to linear and nonlinear mechanisms over a wider area because of deep penetration of hot electrons and the large contraction of the plasmasphere. This is consistent with the observation that the outer belt electrons increase for the fast solar wind. (3) For slow solar wind, the linear growth is mostly suppressed, but the nonlinear growth can still take place when external seed waves are present. This may explain the persistence of dawn chorus and large‐amplitude chorus waves that are often observed in the premidnight‐postdawn region in relatively weak geomagnetic activities.

On the loss mechanisms of radiation belt electron dropouts during the 12 September 2014 geomagnetic storm

Radiation belt electron dropouts indicate electron flux decay to the background level during geomagnetic storms, which is commonly attributed to the effects of wave-induced pitch angle scattering and magnetopause shadowing. To investigate the loss mechanisms of radiation belt electron dropouts triggered by a solar wind dynamic pressure pulse event on 12 September 2014, we comprehensively analyzed the particle and wave measurements from Van Allen Probes. The dropout event was divided into three periods: before the storm, the initial phase of the storm, and the main phase of the storm. The electron pitch angle distributions (PADs) and electron flux dropouts during the initial and main phases of this storm were investigated, and the evolution of the radial profile of electron phase space density (PSD) and the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD dropouts (where <italic>μ</italic>, <italic>K</italic>, and <italic>L</italic>* are the three adiabatic invariants) were analyzed. The energy-independent decay of electrons at <italic>L</italic> &gt; 4.5 was accompanied by butterfly PADs, suggesting that the magnetopause shadowing process may be the major loss mechanism during the initial phase of the storm at <italic>L</italic> &gt; 4.5. The features of electron dropouts and 90°-peaked PADs were observed only for &gt;1 MeV electrons at <italic>L</italic> &lt; 4, indicating that the wave-induced scattering effect may dominate the electron loss processes at the lower <italic>L</italic>-shell during the main phase of the storm. Evaluations of the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD drops and calculations of the minimum electron resonant energies of H⁺-band electromagnetic ion cyclotron (EMIC) waves support the scenario that the observed PSD drop peaks around <italic>L</italic>* = 3.9 may be caused mainly by the scattering of EMIC waves, whereas the drop peaks around <italic>L</italic>* = 4.6 may result from a combination of EMIC wave scattering and outward radial diffusion.

Identifying Radiation Belt Electron Source and Loss Processes by Assimilating Spacecraft Data in a Three‐Dimensional Diffusion Model

AbstractData assimilation aims to blend incomplete and inaccurate data with physics‐based dynamical models. In the Earth's radiation belts, it is used to reconstruct electron phase space density, and it has become an increasingly important tool in validating our current understanding of radiation belt dynamics, identifying new physical processes, and predicting the near‐Earth hazardous radiation environment. In this study, we perform reanalysis of the sparse measurements from four spacecraft using the three‐dimensional Versatile Electron Radiation Belt diffusion model and a split‐operator Kalman filter over a 6‐month period from 1 October 2012 to 1 April 2013. In comparison to previous works, our 3‐D model accounts for more physical processes, namely, mixed pitch angle‐energy diffusion, scattering by Electromagnetic Ion Cyclotron waves, and magnetopause shadowing. We describe how data assimilation, by means of the innovation vector, can be used to account for missing physics in the model. We use this method to identify the radial distances from the Earth and the geomagnetic conditions where our model is inconsistent with the measured phase space density for different values of the invariants and . As a result, the Kalman filter adjusts the predictions in order to match the observations, and we interpret this as evidence of where and when additional source or loss processes are active. The current work demonstrates that 3‐D data assimilation provides a comprehensive picture of the radiation belt electrons and is a crucial step toward performing reanalysis using measurements from ongoing and future missions.

On the Acceleration Mechanism of Ultrarelativistic Electrons in the Center of the Outer Radiation Belt: A Statistical Study

AbstractUsing energetic particle and wave measurements from the Van Allen Probes, Polar Orbiting Environmental Satellites (POES), and Geostationary Operational Environmental Satellite (GOES), the acceleration mechanism of ultrarelativistic electrons (&gt;3 MeV) in the center of the outer radiation belt is investigated statistically. A superposed epoch analysis is conducted using 19 storms, which caused flux enhancements of 1.8–7.7 MeV electrons. The evolution of electron phase space density radial profile suggests an energy‐dependent acceleration of ultrarelativistic electrons in the outer belt. Especially, for electrons with very high energies (~7 MeV), prevalent positive phase space density radial gradients support inward radial diffusion being responsible for electron acceleration in the center of the outer belt (L*~3–5) during most enhancement events in the Van Allen Probes era. We propose a two‐step acceleration process to explain the acceleration of ~7 MeV electrons in the outer belt: intense and sustained chorus waves locally energize core electron populations to ultrarelativistic energies at high L region beyond the Van Allen Probes' apogee, followed by inward radial diffusion which further energizes these populations to even higher energies. Statistical results of chorus wave activity inferred from POES precipitating electron measurements as well as core electron populations observed by the Van Allen Probes and GOES support this hypothesis.

Understanding Electron Dropout Echoes Induced by Interplanetary Shocks: Test Particle Simulations

AbstractRecently, interplanetary shocks have been reported to cause “electron dropout echoes” in the outer radiation belt, which is manifested as repeated dropout and recovery signals in electron fluxes. Both previous case and statistical studies have shown that electron dropout echoes are mostly found for high‐energy (&gt;300 keV) electrons, and the initial dropout region is mainly located at the dusk magnetosphere, regardless of shock parameters such as shock normal. To understand these properties, we model the electron dropout echoes at geosynchronous orbit by tracing electrons in the analytic field model of the shock‐induced propagating pulse. It is shown that the characteristics of shock‐induced electron dropout echo events including energy dependence and localization are well reproduced by our model. By analyzing the trajectories of typical electrons, we find that electrons are inward transported and accelerated through “drift‐resonance‐like” interactions with the magnetosonic pulse. Two causes of the dawn‐dusk asymmetric response are presented: (1) the difference between the interaction time of electrons with the magnetosonic pulse and (2) the opposite radial ∇B drift of the electrons at dawnside and duskside. Further, we calculate the contributions to electron dynamics and phase space density variations from three terms: E×B drift, radial ∇B drift, and gyrobetatron acceleration. The details of electron flux variations could vary with the form of the shock‐induced pulse and the initial electron distribution, thus be different from our results; however, the basic ingredients of the electron interaction with the pulse could provide a general frame for understanding and evaluating electron flux responses.

Highly Relativistic Electron Flux Enhancement During the Weak Geomagnetic Storm of April–May 2017

AbstractWe report observations of energetic electron flux and phase space density to show that a relatively weak magnetic storm with Sym‐Hmin≈−50 nT, resulted in a relativistic and ultrarelativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with Sym‐Hmin≈−235 nT. This enhancement appeared at energies up to ≈10 MeV, lasted for at least 24 days, and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of phase space density radial profiles and Fokker‐Planck simulation, we show that the enhancement of relativistic and ultrarelativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of 21–25 April accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 Ultra‐Low Frequency (ULF) waves further accelerates them to ultrarelativistic energies.