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The MMS satellites encountered a Kelvin-Helmholtz instability (KHI) period in the early non-linear phase at the post-noon flank magnetopause on 8 Sep 2015. The adjacent magnetosheath was characterized by a pre-dominantly northward Bz > 0 magnetic field with weakly positive in-plane components in a GSM coordinate system. Ion velocity distribution functions indicate at least 17 KH vortex intervals with two typically D-shaped ion beam distributions, commonly associated with reconnection exhausts, that stream in both directions along a mostly northward magnetic field at 350–775 km/s with a median 525 km/s ion beam speed. The counter-streaming ion beams are superposed on a core population of slowly drifting magnetosheath ions with a field-aligned 50–200 km/s speed. Each interval lasted no more than 5.25 s with a median duration of 1.95 s corresponding to in-plane spatial scales 3 < ΔS < 22 di assuming a constant 1 di = 61 km ion inertial scale and a tailward VKH∼258 km/s KH vortex propagation speed along the MMS trajectory. The counter-streaming ions are predominantly observed in the warm KH vortex region between the cold magnetosheath proper and the hot isotropic ion temperature of a low-latitude boundary layer as the MMS constellation traverses a KH vortex. The in-plane spatial scales and the locations of the observed counter-streaming ion beams generally agree with the predictions of twice-reconnected magnetic fields at two mid-latitude reconnection (MLR) regions in a two-fluid three-dimensional numerical simulation previously reported for this KH event. MMS typically recorded a higher phase space density of the fast parallel ion beam that we associate with a tailward reconnection exhaust from the southern MLR (SMLR) and a lower phase space density of the fast anti-parallel ion beam that we associate with a tailward reconnection exhaust from the northern MLR (NMLR) of similar speed. This is either consistent with MMS being closer to the SMLR region than the NMLR region, or that the KHI conditions may have favored reconnection in the SMLR region for the observed in-plane magnetosheath magnetic field as predicted by a two-fluid three-dimensional numerical simulation.

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

The Fast Plasma Investigation onboard the Magnetospheric Multiscale mission uses a discrete wavelet transform and bit plane encoder for data compression. This is the first plasma spectrometer suite to use this method of compression and thus serves as a benchmark for future plasma spectrometers. Flight data from MMS confirm viability of this compression algorithm in large areas of the magnetosphere. Because much of the FPI data were compressed losslessly on orbit, this flight data can be used as seed data to investigate performance of the DWT/BPE‐based compression at increased rates of compression. In this study, data from several representative regions of the magnetosphere have been compressed to increasingly small sizes and the resulting error was analyzed. Wavelet‐based compression is shown to be effective in all regions of the magnetosphere and solar wind for plasma count data, with performance varying with local environment. Specifically, plasma distributions that are characterized by low temperature and/or low density are compressed better leading to excellent performance in plasma regions such as solar wind and the magnetosheath. In general, ion data are compressed better than electron data, primarily due to a higher drift velocity of ions relative to their thermal speed and lower count rates. DWT/BPE compression can therefore be recommended for future instruments measuring count data in Earth's magnetosphere and in solar wind.

During the September 2015 to March 2016 duration (sometimes referred to as Phase 1A) of the Magnetospheric Multiscale Mission, the Dual Electron Spectrometers (DES) were configured to generously utilize lossy compression. While this maximized the number of velocity distribution functions downlinked, it came at the expense of lost information content for a fraction of the frames. Following this period of lossy compression, the DES was reconfigured in a way that allowed for 95% of the frames to arrive to the ground without loss. Using this high‐quality set of frames from on‐orbit observations, we compressed and decompressed the frames on the ground to create a side‐by‐side record of the compression effect. This record was used to drive an optimization method that (a) derived basis functions capable of approximating the lossless sample space and with nonnegative coefficients and (b) fitted a function which maps the lossy frames to basis weights that recreate the frame without compression artifacts. This method is introduced and evaluated in this paper. Data users should expect a higher level of confidence in the absolute scale of density/temperature measurements and notice less sinusoidal bias in the velocity X and Y components (GSE), Key Points 57% of the burst resolution frames in the September 2015 to March 2016 duration of the MMS mission were affected by some form of lossy compression errorThis data can be corrected by optimizing a model with side‐by‐side examples of the before/after compression effectData users should expect a higher level of confidence in the absolute scale of density/temperature measurements and notice less sinusoidal bias in the velocity X and Y components (GSE)

Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event‐specific, and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultrarelativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher‐frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave‐driven diffusion. We examine this fast diffusion rate along with derived radial diffusion coefficients using particle and fields instruments on the Van Allen Probes spacecraft mission., Key Points Fast radial diffusion of ultrarelativistic electrons is observed days after storm main phaseEvent‐specific radial diffusion can be orders of magnitude higher than statistical valuesULF‐wave driven acceleration can account for intense particle enhancement observed in inner magnetosphere

Ultralow frequency (ULF) waves are ubiquitous in the magnetosphere and can have a large direct and indirect impact on the radiation belts. Although their relative contribution to radiation belt dynamics are still enthusiastically debated today within the scientific community, the fact that they impact radiation belt dynamics cannot be denied. In this chapter, we will not try to convince the reader of their relative importance, but instead we will lay out the different ways in which ULF waves impact the evolution of the particle populations in the inner magnetosphere. First, we will discuss the mechanisms through which ULF waves can directly cause acceleration and loss of particles. Then we will discuss their indirect effects through modulation of other loss and acceleration mechanisms. Along with outlining the theory we will also show both model and observational evidence for the importance of ULF waves and their impact on radiation belt dynamics.

The relation between radiation belt electrons and solar wind/magnetospheric processes is of particular interest due to both scientific and practical needs. Though many studies have focused on this topic, electron data from Van Allen Probes with wide L shell coverage and fine energy resolution, for the first time, enabled this statistical study on the relation between radiation belt electrons and solar wind parameters/geomagnetic indices as a function of first adiabatic invariant μ and L*. Good correlations between electron phase space density (PSD) and solar wind speed, southward IMF Bz, SYM-H and AL indices are found over wide μ and L* ranges, with higher correlation coefficients and shorter time lags for low-μ electrons than high-μ electrons; the anti-correlation between electron PSD and solar wind proton density is limited to high-μ electrons at high L*. The solar wind dynamic pressure has dominantly positive correlation with low-μ electrons and negative correlation with high-μ electrons at different L*. In addition, electron PSD enhancements also correlate well with various solar wind/geomagnetic parameters, and for most parameters this correlation is even better than that of electron PSD while the time lag is also much shorter. Among all parameters investigated, AL index is shown to correlate the best with electron PSD enhancements, with correlation coefficients up to ~0.8 for low-μ electrons (time lag ~ 0 day) and ~0.7 for high-μ electrons (time lag ~ 1-2 days), suggesting the importance of seed and source populations provided by substorms in radiation belt electron PSD enhancements.

As electromagnetic ion cyclotron (EMIC) waves may play an important role in radiation belt dynamics, there has been a push to better include them into global simulations. How to best include EMIC wave effects is still an open question. Recently many studies have attempted to parameterize EMIC waves and their characteristics by geomagnetic indices. However, this does not fully take into account important physics related to the phase of a geomagnetic storm. In this paper we first consider how EMIC wave occurrence varies with the phase of a geomagnetic storm and the SYM-H, AE, and Kp indices. We show that the storm phase plays an important role in the occurrence probability of EMIC waves. The occurrence rates for a given value of a geomagnetic index change based on the geomagnetic condition. In this study we also describe the typical plasma and wave parameters observed in L and magnetic local time for quiet, storm, and storm phase. These results are given in a tabular format in the supporting information so that more accurate statistics of EMIC wave parameters can be incorporated into modeling efforts.

The relativistic electron dynamics in the radiation belts falls into two categories: diffusive processes, in which particles are scattered randomly, and advective processes, in which particles are collectively driven in one direction in energy or space. In this work, we focus on advection which can occur on a timescale faster than diffusion for CME shock compressions of the dayside magnetosphere. Test-particle simulations are done in the electric and magnetic fields generated by the LFM global MHD model, and the advection coefficient is calculated. Then, the advection coefficient is applied to the transport equation with advection term included and solved for phase space density time evolution. The calculation is done for 17 March 2015 and 6–7 September 2017 CME-shock events, using realistic initial conditions measured by Van Allen Probes, all showing inward motion of electrons on the time scale of minutes. The result is compared with the phase space density calculated based on full test-particle simulations in MHD fields. The inward shift in phase space density is found to be comparable for both methods, while solving the transport equation is much faster once advection coefficients are modeled.

Prompt creation of new multi-MeV electron and proton radiation belts in the normally benign slot region between inner and outer Van Allen zones was observed by the short-lived Combined Release and Radiation Effects Satellite (CRRES) mission almost 30 years ago. Flux oscillations at lower energies coincident with ULF wave measurements and consistent with drift resonance were also a feature of the CRRES observations. The next mission designed to study the radiation belts in similar detail from a near equatorial plane orbit was the Van Allen Probes launched in August 2012. These twin spacecraft in similar orbits observed many milder events which produce drift echoes indicative of injection over a limited range of longitude on less than an electron drift time. Related substorm injection drift echoes were also observed by Van Allen Probes with much higher energy resolution than previously. Along with other flux oscillations associated with Ultra Low Frequency (ULF) waves, these events form a class which contributes to the overall radial transport and energization of radiation belt electrons as well as protons. This paper focuses on the greatly improved measurements from the Van Allen Probes relative to the CRRES era. These measurements illuminate our understanding of the importance of flux oscillations as a signature of radial transport relative to other energization and loss processes affecting the radiation belts.

During early November 2013, the magnetosphere experienced concurrent driving by a coronal mass ejection (CME) during an ongoing high-speed stream (HSS) event. The relativistic electron response to these two kinds of drivers, i.e., HSS and CME, is typically different, with the former often leading to a slower buildup of electrons at larger radial distances, while the latter energizing electrons rapidly with flux enhancements occurring closer to the Earth. We present a detailed analysis of the relativistic electron response including radial profiles of phase space density as observed by both Magnetic Electron and Ion Sensor (MagEIS) and Relativistic Electron Proton Telescope instruments on the Van Allen Probes mission. Data from the MagEIS instrument establish the behavior of lower energy (

© 2015. American Geophysical Union. All Rights Reserved. Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August-September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13-22 September, initiated by a short-lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long-lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize.

A 3-D model for solving the radiation belt diffusion equation in adiabatic invariant coordinates has been developed and tested. The model, named REM (for Radbelt Electron Model), obtains a probabilistic solution by solving a set of Itô stochastic differential equations that are mathematically equivalent to the diffusion equation. This method is capable of solving diffusion equations with a full 3-D diffusion tensor, including the radial-local cross diffusion components. The correct form of the boundary condition at equatorial pitch-angle α0 = 90° is also derived. The model is applied to a simulation of the October 2002 storm event. At α0 near 90°, our results are quantitatively consistent with GPS observations of phase-space density (PSD) increases, suggesting dominance of radial diffusion; at smaller α0, the observed PSD increases are overestimated by the model, possibly due to the α0-independent radial diffusion coefficients, or to insufficientelectron loss in the model, or both. Statistical analysis of the stochastic processes provides further insights into the diffusion processes, showing distinctive electron source distributions with and without local acceleration.

International audience; As spacecraft does not have an independent method to conduct charge to ground, it naturally accumulates charge due to interactions with the ambient plasma and surface emission. This charge produces an electric field surrounding the spacecraft, which takes the form of a plasma sheath. Charged particles traveling through this sheath are altered in both energy and direction, thus affecting derived scientific quantities. While this effect has been known since the advent of space based particle instruments, this work represents the first time that an in situ characterization study of this effect has been possible. The Fast Plasma Investigation, of the Magnetospheric Multiscale mission, obtains near simultaneous measurements of phase space, via particle counts from 512 look directions and 32 energies. These new data allow the relative effects of the plasma sheath to be explored at a high time and spatial resolution. This work presents a method by which these measurements are used to study a ground model that traces the migration of particles through the sheath and estimates the error in measured velocity. This approach, performed statistically, leads to an estimation of uncertainty in particle count distributions for a given location in phase space and characterization of the redistribution of counts within a skymap due to sheath effects.

Since their discovery more than 50 years ago, Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

Particle acceleration and loss in the million electron Volt (MeV) energy range (and above) is the least understood aspect of radiation belt science. In order to measure cleanly and separately both the energetic electron and energetic proton components, there is a need for a carefully designed detector system. The Relativistic Electron-Proton Telescope (REPT) on board the Radiation Belt Storm Probe (RBSP) pair of spacecraft consists of a stack of high-performance silicon solid-state detectors in a telescope configuration, a collimation aperture, and a thick case surrounding the detector stack to shield the sensors from penetrating radiation and bremsstrahlung. The instrument points perpendicular to the spin axis of the spacecraft and measures high-energy electrons (up to ∼20 MeV) with excellent sensitivity and also measures magnetospheric and solar protons to energies well above E=100 MeV. The instrument has a large geometric factor (g=0.2 cm2 sr) to get reasonable count rates (above background) at the higher energies and yet will not saturate at the lower energy ranges. There must be fast enough electronics to avert undue dead-time limitations and chance coincidence effects. The key goal for the REPT design is to measure the directional electron intensities (in the range 10−2–106 particles/cm2 s sr MeV) and energy spectra (ΔE/E∼25 %) throughout the slot and outer radiation belt region. Present simulations and detailed laboratory calibrations show that an excellent design has been attained for the RBSP needs. We describe the engineering design, operational approaches, science objectives, and planned data products for REPT.

As part of the International Heliospheric Year, the Whole Heliosphere Interval, Carrington Rotation 2068, from March 20 to April 16, 2008 was chosen as an internationally coordinated observing and modeling campaign. A pair of solar wind structures identified as Corotating Interaction Regions (CIR), characteristic of the declining phase of the solar cycle and solar minimum, was identified in solar wind plasma measurements from the ACE satellite. Such structures have previously been determined to be geoeffective in producing enhanced outer zone radiation belt electron fluxes, on average greater than at solar maximum. MHD fields from the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) model driven by ACE solar wind measurements at L1 have been used to drive both 2D and 3D weighted test particle simulations of electron dynamics for the CIR subset of the month-long CMIT fields. Dropout in electron flux at geosynchronous orbit and enhancement during recovery phase, characteristic of CIR-driven storms, is seen in these moderate (Dstmin=−56, −33 nT) events, while the two CIRs were characterized by increased solar wind velocity in the 650–750 km/s range. The first beginning March 26 produced a greater enhancement in IMF Bz southward and stronger magnetospheric convection, leading to a greater radiation belt electron response at GOES. This study provides the first comparison of 2D and 3D particle dynamics in MHD simulation fields, incorporating the additional diffusive feature of Shebansky orbit trapping of electrons in the magnetic minima on the dayside above and below the equatorial plane. Overall loss occurs during the main phase for 2D and 3D simulations, while incorporation of plasmasheet injection in 2D runs produces a moderate enhancement for the March 26–30 storm, less than observed at GOES, and recovery to initial flux levels as seen for the April 4–7 storm.

Energetic O+ ions have important dynamic effects on the ring current. Insights into the effects of O+ on ring current dynamics have come primarily from models, not observations. Here, we discuss observations of O+ populations escaping from the ionosphere and their access to the plasma sheet and ring current. We review data establishing that a significant flux of O+ escapes the ionosphere during geomagnetically quiet intervals. We then estimate the relative magnitude of the O+ population in transit between the ionosphere and ring current during quiet intervals before geomagnetic storms. Our analysis suggests that dynamic reconfigurations of the magnetosphere during geomagnetic storms significantly alter the O+ transport paths from the ionosphere to the ring current. During these reconfigurations some of the pre-existing, quiet time, in-transit O+ populations are captured on magnetic field lines leading to the ring current. The prompt appearance of this O+ population in the ring current could modify the evolution of the ring current in the storm growth phase. Our analysis suggests that the consequences of an activity-dependent O+ transport path to the ring current should be systematically investigated.

Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection‐driven event occurred with a Dst (storm time ring current index) value reaching −223 nT. On 22 June 2015 another strong storm (Dst reaching −204 nT) was recorded. These two storms each produced almost total loss of radiation belt high‐energy (E ≳ 1 MeV) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10 MeV in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong “butterfly” distributions with deep minima in flux at α = 90°. However, despite strong driving of outer zone earthward radial diffusion in these storms, the previously reported “impenetrable barrier” at L ≈ 2.8 was pushed inward, but not significantly breached, and no E ≳ 2.0 MeV electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall, these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis., Key Points March and June 2015 storms offer insight into ultrarelativistic electron dynamicsImpenetrable barrier can be pushed inward, still isolates high‐E electrons from inner zoneMulti‐MeV electrons take 1‐2 days to appear following these strong driving events.

Several research groups are presently modeling key portions of the solar–terrestrial environment. Large-scale models of the solar corona, of the interplanetary medium, and of the coupled magnetosphere–ionosphere system are providing important insights into the dynamics and temporal evolution of these regions during normal, as well as disturbed, conditions. Some collaborative groups are now working on even more grand syntheses of models and are striving for a true end-to-end modeling capability. The challenges of such integrated modeling are immense: vastly different spatial and temporal scales must be addressed and codes must be synchronized to a high degree. Because much of the underlying physics at micro- and meso-scales remains unknown (or only poorly known), many key interface regions must be treated using empirical or semi-empirical methods. Validating the results from such global-scale models is, in itself, a major undertaking. And last, but by no means least, the question of how available (sparse) data are assimilated into end-to-end models remains a major challenge to the community. This paper provides an overview of the present status and future challenges in end-to-end modeling of the Sun–Earth system.

A goal of predictive models of the space radiation environment is to provide advanced knowledge of significant variations in the highly energetic particle populations that form the Earth's radiation belts. The global geomagnetic field models that result from the Center for Integrated Space-weather Modeling (CISM) effort will provide a necessary input, a real-time description of the dynamic variation of the electromagnetic fields in the magnetosphere, for conducting detailed simulations of the radiation belts. In this work, we describe the issues and techniques that CISM will use to provide a physical space radiation model. An analysis of the global field configurations typical of the magneto hydrodynamic (MHD) models used in CISM suggest that much of the radiation belt modeling can proceed under a guiding center particle approximation, whereby individual, non-interacting test particles are used to track the aggregate dynamics of the radiation belts. This technique provides a relatively simple means of both conducting the necessary simulations, and coupling the relevant codes with other elements of the CISM project. The guiding center equations used to track the particles are based on a phase-space conserving approximation that conserves energy in regions of high curvature. Examples of test particle simulations with the MHD fields are given, both in the initial trapped particle population, and among the energetic particles that form the plasma sheet in the tail. These simulations suggest that multiple radiation belt models running simultaneously in the framework provided by CISM can be combined to provide an overall picture of the energetic particle environment. Finally, analysis of the spectral properties of the fields suggests how an alternate approach to modeling the global radiation belts, solution of the appropriate transport equations, might be advanced through the CISM effort.

The response of the magnetosphere to interplanetary shocks or pressure pulses can result in sudden injections of energetic particles into the inner magnetosphere. On August 26, 1998, an interplanetary shock caused two injections of energetic particles in close succession: one directly from the dayside and the other indirectly from the nightside associated with a sudden magnetic field enhancement induced by the shock's effect on the magnetotail. The latter injection was different from a typical substorm injection in that the nightside magnetic field at geosynchronous orbit enhanced almost simultaneously over a wide range of local times within 10 min after the arrival of the shock. Available observations and our simulations show that like the dayside, the nightside magnetosphere can also inject energetic particles into the inner magnetosphere from a wide local time region in response to a shock impact. The nightside particle injection was due to changes in magnetic and electric fields over a large region of space and thus shows that the magnetic and electric fields in the magnetotail can respond globally to the shock impact.

Acceleration of electrons due to ULF oscillations is considered to be an explanation for the radiation belt electron flux enhmacement during the storm. In this study, we examine the relationship between the ULF wave activity observed by the Equator-S satellite and the relativistic electron flux variation observed by POLAR satellite during a magnetic storm which commenced on March 10, 1998. The storm was associated with a 5-day high '~peed solar wind interval. Rapid enhancement of MeV electrons was observed in the outer radiation belt within the first half day after the storm commencement with strong pancake distribution in equatorial pitch angle, followed by a more gradual flux increase for the next several days with a more isotropic distribution. During this interval, when both rapid and gradual acceleration of electrons took place, enhanced ULF wave activity was observed in the dawnside magnetosphere extending down to L < 6 region. We compare the observations with different models considering the ULF waves. The observation generally supports, the mechanisms of the electron acceleration due to interaction with ULF waves, although different mechanisms are possibly involved for acceleration during the early recovery phase and the later recovery phase. © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

Particle fluxes in the outer radiation belts can show substantial variation in time, over scales ranging from a few minutes, such as during the sudden commencement phase of geomagnetic storms, to the years-long variations associated with the progression of the solar cycle. As the energetic particles comprising these belts can pose a hazard to human activity in space, considerable effort has gone into understanding both the source of these particles and the physics governing their dynamical behavior. Computationally tracking individual test particles in a model magnetosphere represents a very direct, physically-based approach to modeling storm-time radiation belt dynamics. Using global magnetohydrodynamic models of the Earth–Sun system coupled with test particle simulations of the radiation belts, we show through two examples that such simulations are capable of capturing the outer zone radiation belt configuration at a variety of time scales and through all phases of a geomagnetic storm. Such simulations provide a physically-based method of investigating the dynamics of the outer radiation zone, and hold promise as a viable method of providing global nowcasts of the radiation environment during geomagnetically active periods.

Pc 5 ULF waves are seen concurrently with the rise in radiation belt fluxes associated with CME magnetic cloud events. A 3D global MHD simulation of the 10–11 January, 1997 event has been analyzed for mode structure and shown to contain field line resonance components, both toroidal and poloidal, with peak power on the nightside during southward IMF conditions. A mechanism for inward radial transport and first-invariant conserving acceleration of relativistic electrons is assessed in the context of ULF mode structure analysis, and compared with groundbased and satellite observations.

MHD fields from a global three-dimensional simulation of the great March 24, 1991, storm sudden commencement (SSC) are used to follow the trajectories of particles in a guiding center test particle simulation of radiation belt formation during this event. Modeling of less intense events during the lifetime of the CRRES satellite, with similar morphology but less radial transport and energization, is also presented. In all cases analyzed, a solar proton event was followed by an SSC, leading to the formation of a new proton belt earthward of solar proton penetration. The effect on particle energization of varying solar wind and model pulse parameters is investigated. Both a seed population of solar protons and the SSC shock-induced compression of the magnetosphere are necessary conditions for the formation of a new proton belt. The outer boundary of the inner zone protons can be affected by an SSC and a newly formed belt can be affected by the ensuing or a subsequent storm, which may occur in rapid succession, as was the case in June and July 1991. The acceleration process is effective for both northward and southward IMF, with more energization and inward radial transport for the southward case for otherwise comparable solar wind parameters, because of the initially more compressed magnetopause in the southward case. The inner boundary and stability of the newly formed belt depends on the magnitude of radial transport at the time of formation and subsequent ring current perturbation of adiabatic trapping.

Pc 5 ULF waves are seen concurrently with the rise in radiation belt fluxes associated with CME-magnetic cloud events. Four such geomagnetic storm periods in 1997 and 1998, 10-11 January and 14-15 May, 1997; 1-4 May and 26-28 August, 1998, have been simulated with a 3D global MHD code driven by L1-measured solar wind parameters. The field output has been used to advance electron guiding center trajectories in the equatorial plane. The time series has also been analyzed for ULF wave mode structure. Toroidal field line resonances with low azimuthal mode number and frequencies commensurate with the electron drift period are identified in the radial electric field component, along with enhanced power in the poloidal (azimuthal) electric field component. The simulated time scale for inward radial transport of electrons in the several hundred keV to MeV energy range, from geosynchronous orbit to L=3-4, varies with the level of ULF wave power and overall energy input to the magnetosphere. Of the four events studied, the May 1998 storm period was most geoeffective and the January 1997 least so, in terms of simulated radial transport and energization of electrons. This transport rate is consistent with the level of ULF waves excited in the simulations, and the proposed drift resonant acceleration mechanism.

The Lyon-Fedder-Mobarry global magnetohydrodynamic simulation has been used to model the magnetosphere during a variety of solar wind conditions. Early work with the simulation was concentrated on modeling isolated substorms and the global characteristics of magnetic storms. In this paper we will compare the results from simulations of isolated substorms to those occurring during magnetic storms. The substorm which occurred on August 27, 2001 was a fairly typical isolated substorm which had extensive set of satellite and ground based observations. The LFM simulation of the interval agrees with some of these observations. The simulation results also show that thin current sheets develop during the growth phase of substorms and are disrupted, in part, by flows originating the in mid-tail region prior to ionospheric signatures of substorm onset. Furthermore, detailed analysis of the energy partitioning in the magnetotail shows the transfer of energy from the lobes into the plasma sheet occurs a few minutes before substorm onset. The magnetic storm which began on March 31, 2001 was one of the largest magnetic storms during this solar cycle. In this storm-time interval a very large substorm occurred with an onset at 0630 UT. Results from this simulation agree with some observations made by the constellation of spacecraft present during this substorm. Comparison between the simulation results for these two events indicates that storm-time and isolated substorms share the same essential features, development of intense thin current sheets during growth phase, disruption of this current sheet linked to activity in the mid-tail region, and rapid recovery phases.

The geomagnetic storm beginning on March 31, 2001 was characterized by high solar wind speeds and long intervals of strongly-southward interplanetary magnetic field. These conditions led to high levels of magnetospheric convection and significant distortion in the inner magnetosphere. During a period of particularly intense driving by the solar wind, an ∼0630 UT substorm onset was observed to inject energetic particles into relatively low L-values in the premidnight regions of the inner magnetosphere. In this work we present the results of MHD/particle simulations of the storm, focusing on the period immediately surrounding the 0630 substorm. We analyze the characteristics of the global field prior to onset, and the evolution and effect of the subsequent dipolarization on the keV protons injected from the plasma sheet into the inner magnetosphere. We find good qualitative agreement between global observations of the injection event and the results of the MHD/particle simulations, and suggest that a weak field region in the near-Earth tail may have served as a source region for the more energetic particles injected in the premidnight regions of the magnetosphere.

Particle acceleration is an important consequence of eruptive or catastrophic configuration changes in space plasmas. It is also a feature associated with more persistent structures, such as thin boundary layers, which exhibit strong gradients, instabilities, and turbulence. The most important acceleration regions in or near the magnetosphere are the bow shock, the magnetotail, the magnetopause, and the inner magnetosphere. The spatial and temporal properties of accelerated particles are the consequences of motion within the extant magnetic and electric fields. The characteristics of accelerated particles can provide insight into the underlying processes that shape the geomagnetic fields (for instance, magnetic reconnection). On the other hand, identifying the properties of the electric field combined with the magnetic field is a crucial complement to the study of the energetic particles themselves. The electric fields may be quasi-static, induced by magnetic field collapse, or the result of fluctuations related to plasma instabilities. The acceleration may produce suprathermal as well as very energetic particle populations. Highly energetic particles can be detrimental to satellite instrumentation and subsystems. The identification and modeling of acceleration mechanisms is therefore a crucial element in the prediction of energetic particle flux increases and the potential prevention of detrimental space weather effects. This review focuses on inner magnetospheric acceleration processes. We address several fundamental questions; (1) What are the main acceleration mechanisms?; (2) What are the particle source populations?; and (3) What determines the spatial and temporal characteristics of the energetic particles?