Found 23 entries in the Bibliography.
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Abstract Radiation 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 hours 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. This article is protected by copyright. All rights reserved.
Zhao, Hong; Sarris, Theodore; Li, Xinlin; Weiner, Max; Huckabee, Isabela; Baker, Daniel; Jaynes, Allison; Kanekal, Shrikanth; Elkington, Scot; Barani, Mohammad; Tu, Weichao; Liu, Wenlong; Zhang, Dianjun; Hartinger, Michael;
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021JA029284
AbstractRadial diffusion is one of the dominant physical mechanisms driving acceleration and loss of radiation belt electrons. A number of parameterizations for radial diffusion coefficients have been developed, each differing in the dataset used. Here, we investigate the performance of different parameterizations by Brautigam and Albert (2000), Brautigam et al. (2005), Ozeke et al. (2014), Ali et al. (2015); Ali et al. (2016); Ali (2016), and Liu et al. (2016) on long-term radiation belt modeling using the Versatile Electron Radiation Belt (VERB) code, and compare the results to Van Allen Probes observations. First, 1-D radial diffusion simulations are performed, isolating the contribution of solely radial diffusion. We then take into account effects of local acceleration and loss showing additional 3-D simulations, including diffusion across pitch-angle, energy, and mixed diffusion. For the L* range studied, the difference between simulations with Brautigam and Albert (2000), Ozeke et al. (2014), and Liu et al. (2016) parameterizations is shown to be small, with Brautigam and Albert (2000) offering the smallest averaged (across multiple energies) absolute normalized difference with observations. Using the Ali et al. (2016) parameterization tended to result in a lower flux than both the observations and the VERB simulations using the other coefficients. We find that the 3-D simulations are less sensitive to the radial diffusion coefficient chosen than the 1-D simulations, suggesting that for 3-D radiation belt models, a similar result is likely to be achieved, regardless of whether Brautigam and Albert (2000), Ozeke et al. (2014), and Liu et al. (2016) parameterizations are used.This article is protected by copyright. All rights reserved.
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2020JA028707
In addition to clarifying morphological structures of the Earth\textquoterights radiation belts, it has also been a major achievement of the Van Allen Probes mission to understand more thoroughly how highly relativistic and ultrarelativistic electrons are accelerated deep inside the radiation belts. Prior studies have demonstrated that electrons up to energies of 10 megaelectron volts (MeV) can be produced over broad regions of the outer Van Allen zone on timescales of minutes to a few hours. It often is seen that geomagnetic activity driven by strong solar storms (i.e., coronal mass ejections, or CMEs) almost inexorably leads to relativistic electron production through the intermediary step of intense magnetospheric substorms. In this study, we report observations over the 6-year period 1 September 2012 to 1 September 2018. We focus on data about the relativistic and ultrarelativistic electrons (E>=5 MeV) measured by the Relativistic Electron-Proton Telescope sensors on board the Van Allen Probes spacecraft. This work portrays the radiation belt acceleration, transport, and loss characteristics over a wide range of geomagnetic events. We emphasize features seen repeatedly in the data (three-belt structures, \textquotedblleftimpenetrable\textquotedblright barrier properties, and radial diffusion signatures) in the context of acceleration and loss mechanisms. We especially highlight solar wind forcing of the ultrarelativistic electron populations and extended periods when such electrons were absent. The analysis includes new display tools showing spatial features of the mission-long time variability of the outer Van Allen belt emphasizing the remarkable dynamics of the system.
Published by: Journal of Geophysical Research: Space Physics Published on: 03/2019
YEAR: 2019   DOI: 10.1029/2018JA026259
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. Ultra-relativistic 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.
Published by: Geophysical Research Letters Published on: 09/2018
YEAR: 2018   DOI: 10.1029/2018GL079786
ULF waves are a common occurrence in the inner magnetosphere and they contribute to particle motion, significantly, at times. We used the magnetic and the electric field data from the EMFISIS and the EFW instruments on board the Van Allen Probes to estimate the ULF wave power in the compressional component of the magnetic field and the azimuthal component of the electric field, respectively. Using L*, Kp, and MLT as parameters, we conclude that the noon sector contains higher ULF Pc-5 wave power compared with the other MLT sectors. The dawn, dusk, and midnight sectors have no statistically significant difference between them. The drift-averaged power spectral densities are used to derive the magnetic and the electric component of the radial diffusion coefficient. Both components exhibit little to no energy dependence, resulting in simple analytic models for both components. More importantly, the electric component is larger than the magnetic component by one to two orders of magnitude for almost all L* and Kp; thus, the electric field perturbations are more effective in driving radial diffusion of charged particles in the inner magnetosphere. We also present a comparison of the Van Allen Probes radial diffusion coefficients, including the error estimates, with some of the previous published results. This allows us to gauge the large amount of uncertainty present in such estimates.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016
YEAR: 2016   DOI: 10.1002/2016JA023002
On 2 October 2013, the arrival of an interplanetary shock compressed the Earth\textquoterights magnetosphere and triggered a global ULF (ultra low frequency) oscillation. The Van Allen Probe B spacecraft observed this large-amplitude ULF wave in situ with both magnetic and electric field data. Broadband waves up to approximately 100 Hz were observed in conjunction with, and modulated by, this ULF wave. Detailed analysis of fields and particle data reveals that these broadband waves are Doppler-shifted kinetic Alfv\ en waves. This event suggests that magnetospheric compression by interplanetary shocks can induce abrupt generation of kinetic Alfv\ en waves over large portions of the inner magnetosphere, potentially driving previously unconsidered wave-particle interactions throughout the inner magnetosphere during the initial response of the magnetosphere to shock impacts.
Published by: Geophysical Research Letters Published on: 11/2015
YEAR: 2015   DOI: 10.1002/2015GL065935
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 MagEIS and REPT instruments on the Van Allen Probes mission. Data from the MagEIS instrument establishes the behavior of lower energy (<1MeV) electrons which span both intermediary and seed populations during electron energization. Measurements characterizing the plasma waves and magnetospheric electric and magnetic fields during this period are obtained by the EMFISIS instrument on board Van Allen Probes, SCM and FGM instruments onboard THEMIS, and the low altitude polar orbiting POES satellite. These observations suggest that, during this time period, both radial transport and local in-situ processes are involved in the energization of electrons. The energization attributable to radial diffusion is most clearly evident for the lower energy (<1MeV) electrons, while the effects of in-situ energization by interaction of chorus waves are prominent in the higher energy electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 09/2015
YEAR: 2015   DOI: 10.1002/2015JA021395
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 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.
Jaynes, A.N.; Baker, D.N.; Singer, H.J.; Rodriguez, J.V.; Loto\textquoterightaniu, T.M.; Ali, A.; Elkington, S.R.; Li, X.; Kanekal, S.G.; Fennell, J.F.; Li, W.; Thorne, R.M.; Kletzing, C.A.; Spence, H.E.; Reeves, G.D.;
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2015
YEAR: 2015   DOI: 10.1002/2015JA021234
We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRRES) to estimate the power spectral density (PSD) of the compressional component of the geomagnetic field in the \~1 mHz to \~8 mHz range. We conclude that magnetic wave power is generally higher in the noon sector for quiet times with no significant difference between the dawn, dusk, and the midnight sectors. However, during high Kp activity, the noon sector is not necessarily dominant anymore. The magnetic PSDs have a very distinct dependence on Kp. In addition, the PSDs appear to have a weak dependence on McIlwain parameter L with power slightly increasing as L increases. The magnetic wave PSDs are used along with the Fei et al. (2006) formulation to compute inline image as a function of L and Kp. The L dependence of inline image is systematically studied and is shown to depend on Kp. More significantly, we conclude that inline imageis the dominant term driving radial diffusion, typically exceeding inline image by 1\textendash2 orders of magnitude.
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015
YEAR: 2015   DOI: 10.1002/2014JA020419
A 3-D model for solving the radiation belt diffusion equation in adiabatic invariant coordinates has been developed and tested. The model, named Radbelt Electron Model, obtains a probabilistic solution by solving a set of It\^o 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\textdegree is also derived. The model is applied to a simulation of the October 2002 storm event. At α0 near 90\textdegree, 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 insufficient electron 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.
Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014
YEAR: 2014   DOI: 10.1002/jgra.v119.910.1002/2014JA020127
The Relativistic Electron-Proton Telescope (REPT) Instrument on Board the Radiation Belt Storm Probes (RBSP) Spacecraft: Characterization of Earth\textquoterights Radiation Belt High-Energy Particle Populations
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\textendash106 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.
Baker, D.; Kanekal, S.; Hoxie, V.; Batiste, S.; Bolton, M.; Li, X.; Elkington, S.; Monk, S.; Reukauf, R.; Steg, S.; Westfall, J.; Belting, C.; Bolton, B.; Braun, D.; Cervelli, B.; Hubbell, K.; Kien, M.; Knappmiller, S.; Wade, S.; Lamprecht, B.; Stevens, K.; Wallace, J.; Yehle, A.; Spence, H.; Friedel, R.;
Published by: Space Science Reviews Published on: 11/2013
YEAR: 2013   DOI: 10.1007/s11214-012-9950-9
The Radiation Belt Storm Probes (RBSP)-Energetic Particle, Composition, and Thermal Plasma (ECT) suite contains an innovative complement of particle instruments to ensure the highest quality measurements ever made in the inner magnetosphere and radiation belts. The coordinated RBSP-ECT particle measurements, analyzed in combination with fields and waves observations and state-of-the-art theory and modeling, are necessary for understanding the acceleration, global distribution, and variability of radiation belt electrons and ions, key science objectives of NASA\textquoterights Living With a Star program and the Van Allen Probes mission. The RBSP-ECT suite consists of three highly-coordinated instruments: the Magnetic Electron Ion Spectrometer (MagEIS), the Helium Oxygen Proton Electron (HOPE) sensor, and the Relativistic Electron Proton Telescope (REPT). Collectively they cover, continuously, the full electron and ion spectra from one eV to 10\textquoterights of MeV with sufficient energy resolution, pitch angle coverage and resolution, and with composition measurements in the critical energy range up to 50 keV and also from a few to 50 MeV/nucleon. All three instruments are based on measurement techniques proven in the radiation belts. The instruments use those proven techniques along with innovative new designs, optimized for operation in the most extreme conditions in order to provide unambiguous separation of ions and electrons and clean energy responses even in the presence of extreme penetrating background environments. The design, fabrication and operation of ECT spaceflight instrumentation in the harsh radiation belt environment ensure that particle measurements have the fidelity needed for closure in answering key mission science questions. ECT instrument details are provided in companion papers in this same issue. In this paper, we describe the science objectives of the RBSP-ECT instrument suite on the Van Allen Probe spacecraft within the context of the overall mission objectives, indicate how the characteristics of the instruments satisfy the requirements to achieve these objectives, provide information about science data collection and dissemination, and conclude with a description of some early mission results.
Spence, H.; Reeves, G.; Baker, D.; Blake, J.; Bolton, M.; Bourdarie, S.; Chan, A.; Claudpierre, S.; Clemmons, J.; Cravens, J.; Elkington, S.; Fennell, J.; Friedel, R.; Funsten, H.; Goldstein, J.; Green, J.; Guthrie, A.; Henderson, M.; Horne, R.; Hudson, M.; Jahn, J.-M.; Jordanova, V.; Kanekal, S.; Klatt, B.; Larsen, B.; Li, X.; MacDonald, E.; Mann, I.R.; Niehof, J.; O\textquoterightBrien, T.; Onsager, T.; Salvaggio, D.; Skoug, R.; Smith, S.; Suther, L.; Thomsen, M.; Thorne, R.;
Published by: Space Science Reviews Published on: 11/2013
YEAR: 2013   DOI: DOI: 10.1007/s11214-013-0007-5
Since their discovery more than 50 years ago, Earth\textquoterights 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.
Published by: Science Published on: 04/2013
YEAR: 2013   DOI: 10.1126/science.1233518
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\textendashIonosphere\textendashThermosphere (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\textendash750 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\textendash30 storm, less than observed at GOES, and recovery to initial flux levels as seen for the April 4\textendash7 storm.
Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 07/2012
YEAR: 2012   DOI: 10.1016/j.jastp.2012.03.017
This paper focuses on the modeling of local acceleration and loss processes in the outer radiation belt. We begin by reviewing the statistical properties of waves that violate the first and second adiabatic invariants, leading to the loss and acceleration of high energy electrons in the outer radiation belt. After a brief description of the most commonly accepted methodology for computing quasi-linear diffusion coefficients, we present pitch-angle scattering simulations by (i) plasmaspheric hiss, (ii) a combination of plasmaspheric hiss and electromagnetic ion cyclotron (EMIC) waves, (iii) chorus waves, and (iv) a combination of chorus and EMIC waves. Simulations of the local acceleration and loss processes show that statistically, the net effect of chorus waves is acceleration at MeV energies and loss at hundreds of keV energies. The combination of three-dimensional (3D) simulations of the local processes and radial transport show that the complexity of the behavior of the radiation belts is due to a number of competing processes of acceleration and loss, and depends on the dynamics of the plasmasphere, ring current, and solar wind conditions.
Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 11/2008
YEAR: 2008   DOI: 10.1016/j.jastp.2008.06.014
In this paper, we focus on the modeling of radial transport in the Earth\textquoterights outer radiation belt. A historical overview of the first observations of the radiation belts is presented, followed by a brief description of radial diffusion. We describe how resonant interactions with poloidal and toroidal components of the ULF waves can change the electron\textquoterights energy and provide radial displacements. We also present radial diffusion and guiding center simulations that show the importance of radial transport in redistributing relativistic electron fluxes and also in accelerating and decelerating radiation belt electrons. We conclude by presenting guiding center simulations of the coupled particle tracing and magnetohydrodynamic (MHD) codes and by discussing the origin of relativistic electrons at geosynchronous orbit. Local acceleration and losses and 3D simulations of the dynamics of the radiation belt fluxes are discussed in the companion paper [Shprits, Y.Y., Subbotin, D.A., Meredith, N.P., Elkington, S.R., 2008. Review of modeling of losses and sources of relativistic electrons in the outer radiation belt II: Local acceleration and loss. Journal of Atmospheric and Solar-Terrestrial Physics, this issue. doi:10.1016/j.jastp.2008.06.014].
Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 11/2008
YEAR: 2008   DOI: 10.1016/j.jastp.2008.06.008
In an MHD particle simulation of the September 1998 magnetic storm the evolution of the radiation belt electron radial flux profile appears to be diffusive, and diffusion caused by ULF waves has been invoked as the probable mechanism. In order to separate adiabatic and nonadiabatic effects and to investigate the radial diffusion mechanism during this storm, in this work we solve a radial diffusion equation with ULF wave diffusion coefficients and a time-dependent outer boundary condition, and the results are compared with the phase space density of the MHD particle simulation. The diffusion coefficients include contributions from both symmetric resonance modes (ω ≈ mωd, where ω is the wave frequency, m is the azimuthal wave number, and ωd is the bounce-averaged drift frequency) and asymmetric resonance modes (ω ≈ (m \textpm 1)ωd). ULF wave power spectral densities are obtained from a Fourier analysis of the electric and magnetic fields of the MHD simulation and are used in calculating the radial diffusion coefficients. The asymmetric diffusion coefficients are proportional to the magnetic field asymmetry, which is also calculated from the MHD field. The resulting diffusion coefficients vary with the radial coordinate L (the Roederer L-value) and with time during different phases of the storm. The last closed drift shell defines the location of the outer boundary. Both the location of the outer boundary and the value of the phase space density at the outer boundary are time-varying. The diffusion calculation simulates a 42-hour period during the 24\textendash26 September 1998 magnetic storm, starting just before the storm sudden commencement and ending in the late recovery phase. The differential flux calculated in the MHD particle simulation is converted to phase space density. Phase space densities in both simulations (diffusion and MHD particle) are functions of Roederer L-value for fixed first and second adiabatic invariants. The Roederer L-value is calculated using drift shell tracing in the MHD magnetic field, and particles have zero second invariant. The radial diffusion calculation reproduces the main features of the MHD particle simulation quite well. The symmetric resonance modes dominate the radial diffusion, especially in the inner and middle L region, while the asymmetric resonances are more important in the outer region. Using both symmetric and asymmetric terms gives a better result than using only one or the other and is better than using a simple power law diffusion coefficient. We find that it is important to specify the value of the phase space density on the outer boundary dynamically in order to get better agreement between the radial diffusion simulation and the MHD particle simulation.
Published by: Journal of Geophysical Research Published on: 12/2006
YEAR: 2006   DOI: 10.1029/2005JA011211
Energetic particle fluxes in the outer zone radiation belts can vary over orders of magnitude on a variety of timescales. Power at ULF frequencies, on the order of a few millihertz, have been associated with changes in flux levels among relativis- tic electrons comprising the outer zone of the radiation belts. Power in this part of the spectrum may occur as a result of a number of processes, including internally- generated waves induced by plasma instabilities, and externally generated processes such as shear instabilities at the flanks or compressive variations in the solar wind. Changes in the large-scale convective motion of the magnetosphere are another important class of externally driven variations with power at ULF wavelengths. The mechanism for interaction between ULF variations and the radiation belts may result in (or require) pitch angle scattering, or may conserve the first two adiabatic invariants of particle motion. Of the latter class of interactions, radial diffusion describes the result when ULF variations lead to stochastic motion among the particle populations, and has been studied extensively as a description of radial transport within the belts. Rates of radial diffusion depend strongly on the characteristics of the driving ULF waves. This work is intended as a non- exhaustive review of radiation belt interactions with ULF waves, outlining the cur- rent theories and methods in studying the interaction, and describing pertinent wave properties
Published by: Published on:
YEAR: 2006   DOI: 10.1029/169GM12
The influence of ultralow frequency (ULF) waves in the Pc5 frequency range on radiation belt electrons in a compressed dipole magnetic field is examined. This is the first analysis in three dimensions utilizing model ULF wave electric and magnetic fields on the guiding center trajectories of relativistic electrons. A model is developed, describing magnetic and electric fields associated with poloidal mode Pc5 ULF waves. The frequency and L dependence of the ULF wave power are included in this model by incorporating published ground-based magnetometer data. It is demonstrated here that realistic spectral characteristics play a significant role in the rate of diffusion of relativistic electrons via drift resonance with poloidal mode ULF waves. Radial diffusion rates including bounce motion show a weak pitch angle dependence for αeq >= 50\textdegree (λ <= 20\textdegree) for a power spectral density which is L-independent. The data-based model for greater power at higher L values yields stronger diffusion at αeq = 90\textdegree. The L6 dependence of the diffusion coefficient which is obtained for a power spectral density which is L-independent is amplified by power spectral density which increases with L. During geomagnetic storms when ULF wave power is increased, ULF waves are a significant driver of increased fluxes of relativistic electrons inside geosynchronous orbit. Diffusion timescales obtained here, when frequency and L dependence comparable to observations of ULF wave power are included, support this conclusion.
Published by: Journal of Geophysical Research Published on: 03/2005
YEAR: 2005   DOI: 10.1029/2004JA010760
 The outer zone radiation belt consists of energetic electrons drifting in closed orbits encircling the Earth between \~3 and 7 RE. Electron fluxes in the outer belt show a strong correlation with solar and magnetospheric activity, generally increasing during geomagnetic storms with associated high solar wind speeds, and increasing in the presence of magnetospheric ULF waves in the Pc-5 frequency range. In this paper, we examine the influence of Pc-5 ULF waves on energetic electrons drifting in an asymmetric, compressed dipole and find that such particles may be efficiently accelerated through a drift-resonant interaction with the waves. We find that the efficiency of this acceleration increases with increasing magnetospheric distortion (such as may be attributed to increased solar wind pressure associated with high solar wind speeds) and with increasing ULF wave activity. A preponderance of ULF power in the dawn and dusk flanks is shown to be consistent with the proposed acceleration mechanism. Under a continuum of wave modes and frequencies, we find that the drift resonant acceleration process leads to additional modes of radial diffusion in the outer belts, with timescales that may be appropriate to those observed during geomagnetic storms.
Published by: Journal of Geophysical Research Published on: 03/2003
YEAR: 2003   DOI: 10.1029/2001JA009202
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\textendashSun 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.
Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 04/2002
YEAR: 2002   DOI: 10.1016/S1364-6826(02)00018-4
There has been increasing evidence that Pc-5 ULF oscillations play a fundamental role in the dynamics of outer zone electrons. In this work we examine the adiabatic response of electrons to toroidal-mode Pc-5 field line resonances using a simplified magnetic field model. We find that electrons can be adiabatically accelerated through a drift-resonant interaction with the waves, and present expressions describing the resonance condition and half-width for resonant interaction. The presence of magnetospheric convection electric fields is seen to increase the rate of resonant energization, and allow bulk acceleration of radiation belt electrons. Conditions leading to the greatest rate of acceleration in the proposed mechanism, a nonaxisymmetric magnetic field, superimposed toroidal oscillations, and strong convection electric fields, are likely to prevail during storms associated with high solar wind speeds.
Published by: Geophysical Research Letters Published on: 11/1999
YEAR: 1999   DOI: 10.1029/1999GL003659
The rapid rise of relativistic electron fluxes inside geosynchronous orbit during the January 10-11, 1997, CME-driven magnetic cloud event has been simulated using a relativistic guiding center test particle code driven by out-put from a 3D global MHD simulation of the event. A comparison can be made of this event class, characterized by a moderate solar wind speed (< 600 km/s), and those commonly observed at the last solar maximum with a higher solar wind speed and shock accelerated solar energetic proton component. Relativistic electron flux increase occurred over several hours for the January event, during a period of prolonged southward IMF Bz more rapidly than the 1-2 day delay typical of flux increases driven by solar wind high speed stream interactions. Simulations of the January event captured the flux
Published by: Published on:
YEAR: 1999   DOI: 10.1029/GM10910.1029/GM109p0171