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Found 1109 entries in the Bibliography.
Showing entries from 1051 through 1100
| 2007 |
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Review of radiation belt relativistic electron losses We present a brief review of radiation belt electron losses which are vitally important for controlling the dynamics of the radiation belts. A historical overview of early observations is presented, followed by a brief description of important known electron loss mechanisms. We describe key theoretical results and observations related to pitch-angle scattering by resonant interaction with plasmaspheric hiss, whistler-mode chorus and electromagnetic ion cyclotron waves, and review recent work on magnetopause losses. In particular, we attempt to organize recent observational data by loss mechanism and their relative importance to the overall rate of loss. We conclude by suggesting future observational and theoretical work that would contribute to our understanding of this important area of radiation belt research. Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 03/2007 YEAR: 2007 DOI: 10.1016/j.jastp.2006.06.019 |
| 2006 |
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The relativistic electron dropout event on 20 November 2003 is studied using data from a number of satellites including SAMPEX, HEO, ACE, POES, and FAST. The observations suggest that the dropout may have been caused by two separate mechanisms that operate at high and low L-shells, respectively, with a separation at L \~ 5. At high L-shells (L > 5), the dropout is approximately independent of energy and consistent with losses to the magnetopause aided by the Dst effect and outward radial diffusion which can deplete relativistic electrons down to lower L-shells. At low L-shells (L < 5), the dropout is strongly energy-dependent, with the higher-energy electrons being affected most. Moreover, large precipitation bands of both relativistic electrons and energetic protons are observed at low L-shells which are consistent with intense pitch angle scattering driven by electromagnetic ion cyclotron (EMIC) waves and may result in a rapid loss of relativistic electrons near the plasmapause in the dusk sector or in plumes of enhanced density. Bortnik, J.; Thorne, R.; O\textquoterightBrien, T.; Green, J.; Strangeway, R.; Shprits, Y; Baker, D.; Published by: Journal of Geophysical Research Published on: 12/2006 YEAR: 2006 DOI: 10.1029/2006JA011802 |
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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. Fei, Yue; Chan, Anthony; Elkington, Scot; Wiltberger, Michael; Published by: Journal of Geophysical Research Published on: 12/2006 YEAR: 2006 DOI: 10.1029/2005JA011211 |
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Outward radial diffusion driven by losses at magnetopause Loss mechanisms responsible for the sudden depletions of the outer electron radiation belt are examined based on observations and radial diffusion modeling, with L*-derived boundary conditions. SAMPEX data for October\textendashDecember 2003 indicate that depletions often occur when the magnetopause is compressed and geomagnetic activity is high, consistent with outward radial diffusion for L* > 4 driven by loss to the magnetopause. Multichannel Highly Elliptical Orbit (HEO) satellite observations show that depletions at higher L occur at energies as low as a few hundred keV, which excludes the possibility of the electromagnetic ion cyclotron (EMIC) wave-driven pitch angle scattering and loss to the atmosphere at L* > 4. We further examine the viability of the outward radial diffusion loss by comparing CRRES observations with radial diffusion model simulations. Model-data comparison shows that nonadiabatic flux dropouts near geosynchronous orbit can be effectively propagated by the outward radial diffusion to L* = 4 and can account for the main phase depletions of outer radiation belt electron fluxes. Shprits, Y; Thorne, R.; Friedel, R.; Reeves, G.; Fennell, J.; Baker, D.; Kanekal, S.; Published by: Journal of Geophysical Research Published on: 11/2006 YEAR: 2006 DOI: 10.1029/2006JA011657 |
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Storm time evolution of the outer radiation belt: Transport and losses During geomagnetic storms the magnetic field of the inner magnetosphere exhibits large-scale variations over timescales from minutes to days. Being mainly controlled by the magnetic field the motion of relativistic electrons of the outer radiation belt can be highly susceptible to its variations. This paper investigates evolution of the outer belt during the 7 September 2002 storm. Evolution of electron phase space density is calculated with the use of a test-particle simulation in storm time magnetic and electric fields. The results show that storm time intensification of the ring current produces a large impact on the belt. In contrast to the conventional Dst effect the dominant effects are nonadiabatic and lead to profound and irreversible transformations of the belt. The diamagnetic influence of the partial ring current leads to expansion of electron drift orbits such that their paths intersect the magnetopause leading to rapid electron losses. About 2.5 hr after the storm onset most of the electrons outside L = 5 are lost. The partial ring current pressure also leads to an electron trap in the dayside magnetosphere where electrons stay on closed dayside drift orbits for as long as 11 hours. These sequestered electrons are reinjected into the outer belt due to partial recovery of the ring current. The third adiabatic invariant of these electrons exhibits rapid jumps and changes sign. These jumps produce localized peaks in the L*-profile of electron phase space density which have previously been considered as an observable indication of local electron acceleration. Ukhorskiy, A; Anderson, B.; Brandt, P.; Tsyganenko, N.; Published by: Journal of Geophysical Research Published on: 11/2006 YEAR: 2006 DOI: 10.1029/2006JA011690 |
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In the present work, a test particle simulation is performed in a model of analytic Ultra Low Frequency, ULF, perturbations in the electric and magnetic fields of the Earth\textquoterights magnetosphere. The goal of this work is to examine if the radial transport of energetic particles in quiet-time ULF magnetospheric perturbations of various azimuthal mode numbers can be described as a diffusive process and be approximated by theoretically derived radial diffusion coefficients. In the model realistic compressional electromagnetic field perturbations are constructed by a superposition of a large number of propagating electric and consistent magnetic pulses. The diffusion rates of the electrons under the effect of the fluctuating fields are calculated numerically through the test-particle simulation as a function of the radial coordinate L in a dipolar magnetosphere; these calculations are then compared to the symmetric, electromagnetic radial diffusion coefficients for compressional, poloidal perturbations in the Earth\textquoterights magnetosphere. In the model the amplitude of the perturbation fields can be adjusted to represent realistic states of magnetospheric activity. Similarly, the azimuthal modulation of the fields can be adjusted to represent different azimuthal modes of fluctuations and the contribution to radial diffusion from each mode can be quantified. Two simulations of quiet-time magnetospheric variability are performed: in the first simulation, diffusion due to poloidal perturbations of mode number m=1 is calculated; in the second, the diffusion rates from multiple-mode (m=0 to m=8) perturbations are calculated. The numerical calculations of the diffusion coefficients derived from the particle orbits are found to agree with the corresponding theoretical estimates of the diffusion coefficient within a factor of two. Sarris, T.; Li, X.; Temerin, M.; Published by: Annales Geophysicae Published on: 10/2006 YEAR: 2006 DOI: 10.5194/angeo-24-2583-2006 |
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Where Are the "Killer Electrons" of the Declining Phase of Solar Cycle 23 \textquotedblleftKiller electrons,\textquotedblright enhanced fluxes of radiation belt electrons in the magnetosphere\textendashespecially those at geosynchronous orbit (GEO)\textendashwere an important space weather phenomenon during the decline to minimum of the last 11-year solar cycle (1993\textendash1995). Indeed, the fluxes of these electrons were reported at the time to have significantly influenced the incidence of anomalies on numerous spacecraft, both commercial and national defense. The incidences of spacecraft anomalies and the \textquotedblleftpumping up\textquotedblright of the GEO electron fluxes gave rise to the picture that solar minimum did not provide a benign environment for space-based technologies as had been assumed by many. The decline to minimum of this current (23th) solar cycle has as yet to produce the same number of reported spacecraft anomalies as the previous cycle. This cycle has also failed to produce the periodic large increases in GEO electron fluxes (insofar as can be ascertained from the fluxes reported from the NOAA GOES spacecraft). Why is this? Is there less reporting by industry and government of anomalies than there was in the past? Or is it that there are not such high fluxes of energetic electrons as there were during the last approach to solar minimum? These are important questions for future space weather modeling and forecasting that need to be addressed by the space weather research and operations communities. A major cause of the enhanced energetic electrons during the declining phase of the 22nd cycle was the Where Are the \textquotedblleftKiller Electrons\textquotedblright of the Declining Phase of Solar Cycle 2... http://onlinelibrary.wiley.com/enhanced/doi/10.1029/2006SW000259/ 1 of 2 8/7/2014 9:30 AM Browse Publications Browse by Subject Resources Help About Us | Advertisers | Agents | Contact Us | Cookies Media | Privacy | Site Map | Terms \& Conditions Copyright \textcopyright 1999-2014 John Wiley \& Sons, Inc. All Rights Reserved. occurrence of quasiperiodic geomagnetic storms that arose throughout the interval of decline. The precise physical mechanism or mechanisms by which these storms produced the fluxes of killer electrons is still a matter of intense theoretical debate in the scientific literature. These storms were caused by high-speed solar wind streams in the interplanetary medium; that is, by the interactions of these streams with Earth\textquoterights magnetosphere. Is the interplanetary structure different during this decline to solar maximum than in the previous cycle? Or are there one or more other physical factors operating to seemingly lessen the effects of the solar wind and interplanetary magnetic field on Earth\textquoterights space environment? These are important questions for future space weather scientific research, research that could make major contributions to eventual practical applications. We encourage the space weather community to address with vigor and creativity in the next months the questions we raise here, prior to the beginning of the 24th solar cycle. Baker, Daniel; Lanzerotti, Louis; Published by: Space Weather Published on: 07/2006 YEAR: 2006 DOI: 10.1029/2006SW000259 |
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Energetic outer zone electron loss timescales during low geomagnetic activity Following enhanced magnetic activity the fluxes of energetic electrons in the Earth\textquoterights outer radiation belt gradually decay to quiet-time levels. We use CRRES observations to estimate the energetic electron loss timescales and to identify the principal loss mechanisms. Gradual loss of energetic electrons in the region 3.0 <= L <= 5.0 occurs during quiet periods (Kp < 3-) following enhanced magnetic activity on timescales ranging from 1.5 to 3.5 days for 214 keV electrons to 5.5 to 6.5 days for 1.09 MeV electrons. The intervals of decay are associated with large average values of the ratio fpe/fce (>7), indicating that the decay takes place in the plasmasphere. We compute loss timescales for pitch-angle scattering by plasmaspheric hiss using the PADIE code with wave properties based on CRRES observations. The resulting timescales suggest that pitch angle scattering by plasmaspheric hiss propagating at small or intermediate wave normal angles is responsible for electron loss over a wide range of energies and L shells. The region where hiss dominates loss is energy-dependent, ranging from 3.5 <= L <= 5.0 at 214 keV to 3.0 <= L <= 4.0 at 1.09 MeV. Plasmaspheric hiss at large wave normal angles does not contribute significantly to the loss rates. At E = 1.09 MeV the loss timescales are overestimated by a factor of \~5 for 4.5 <= L <= 5.0. We suggest that resonant wave-particle interactions with EMIC waves, which become important at MeV energies for larger L (L > \~4.5), may play a significant role in this region. Meredith, Nigel; Horne, Richard; Glauert, Sarah; Thorne, Richard; Summers, D.; Albert, Jay; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 05/2006 YEAR: 2006 DOI: 10.1029/2005JA011516 |
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Phase space density analysis of the outer radiation belt energetic electron dynamics We present an analysis of the electron phase space density in the Earth\textquoterights outer radiation belt during three magnetically disturbed periods to determine the likely roles of inward radial diffusion and local acceleration in the energization of electrons to relativistic energies. During the recovery phase of the 9 October 1990 storm and the period of prolonged substorms between 11 and 16 September 1990, the relativistic electron phase space density increases substantially and peaks in the phase space density occur in the region 4.0 < L* < 5.5 for values of the first adiabatic invariant, M >= 550 MeV/G, corresponding to energies, E > \~0.8 MeV. The peaks in the phase space density are associated with prolonged substorm activity, enhanced chorus amplitudes, and predominantly low values of the ratio between the electron plasma frequency, fpe, and the electron gyrofrequency, fce (fpe/fce < \~4). The data provide further evidence for a local wave acceleration process in addition to radial diffusion operating in the heart of the outer radiation belt. During the recovery phase of the 9 October 1990 storm the peaks are more pronounced at large M (550 MeV/G) and large Kaufmann K (0.11 equation imageRE) than large M (700 MeV/G) and small K (0.025 equation imageRE), which suggests that radial diffusion is more effective below about 0.7 MeV for 5.0 < L* < 5.5 during this period. At low M (M <= 250 MeV/G), corresponding to energies, E < \~0.8 MeV, there is no evidence for a peak in phase space density and the data are more consistent with inward radial diffusion and losses to the atmosphere by pitch angle scattering. During the 26 August 1990 storm there is a net loss in the relativistic electron phase space density for 3.3 < L* < 6.0. At low M (M <= 250 MeV/G) the phase space density decreases by almost a constant factor and the gradient remains positive for all L*, but at high M (M >= 550 MeV/G) the decrease in phase space density is greater at larger L* and the gradient changes from positive to negative. The data show that it is possible to have inward radial diffusion at low energies and outward radial diffusion at higher energies, which would fill the outer radiation belt. Iles, Roger; Meredith, Nigel; Fazakerley, Andrew; Horne, Richard; Published by: Journal of Geophysical Research Published on: 03/2006 YEAR: 2006 DOI: 10.1029/2005JA011206 |
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A review of ULF interactions with radiation belt electrons 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 |
| 2005 |
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Impact of toroidal ULF waves on the outer radiation belt electrons Relativistic electron fluxes in the outer radiation belt exhibit highly variable complex behavior. Previous studies have established a strong correlation of electron fluxes and the inner magnetospheric ULF waves in the Pc 3\textendash5 frequency range. Resonant interaction of ULF waves with the drift motion of radiation belt electrons violates their third adiabatic invariant and consequently leads to their radial transport. If the wave-particle interaction has a stochastic character, then the electron transport is diffusive. The goal of this paper is to analyze the impact of toroidal ULF waves on radiation belt electrons. The study is based on direct measurements of ULF electric fields on the CRRES spacecraft. We show that the electric fields of inner magnetospheric toroidal ULF waves exhibit high asymmetry in magnetic local time and have narrow-band frequency spectra. Such narrow-band waves can induce radial diffusion of energetic electrons, if an extrinsic stochasticity is introduced in the system. The quasi-periodic variations in the solar wind dynamic pressure are identified as a possible source of extrinsic stochasticity. In the asymmetric magnetic field, drifting electrons can interact with both azimuthal and radial electric field components. We derive analytically and then calculate numerically the diffusion rates associated with azimuthal and radial electric field components of the waves. It is shown that even under highly disturbed geomagnetic conditions, when the background field asymmetry is large, the diffusion rates due to the radial field component are small. At the same time, the resonant scattering of energetic electrons by the azimuthal electric field of the waves provides an efficient form of radial diffusion and therefore can play an important role in the dynamics of the outer radiation belt. Ukhorskiy, A; Takahashi, K; Anderson, B.; Korth, H.; Published by: Journal of Geophysical Research Published on: 10/2005 YEAR: 2005 DOI: 10.1029/2005JA011017 |
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Timescale for MeV electron microburst loss during geomagnetic storms Energetic electrons in the outer radiation belt can resonate with intense bursts of whistler-mode chorus emission leading to microburst precipitation into the atmosphere. The timescale for removal of outer zone MeV electrons during the main phase of the October 1998 magnetic storm has been computed by comparing the rate of microburst loss observed on SAMPEX with trapped flux levels observed on Polar. Effective lifetimes are comparable to a day and are relatively independent of L shell. The lifetimes have also been evaluated by theoretical calculations based on quasi-linear scattering by field-aligned waves. Agreement with the observations requires average wide-band wave amplitudes comparable to 100 pT, which is consistent with the intensity of chorus emissions observed under active conditions. MeV electron scattering is most efficient during first-order cyclotron resonance with chorus emissions at geomagnetic latitudes above 30 degrees. Consequently, the zone of MeV microbursts tends to maximize in the prenoon (0400\textendash1200 MLT) sector, since nightside chorus is more strongly confined to the equator. Thorne, R.; O\textquoterightBrien, T.; Shprits, Y; Summers, D.; Horne, R.; Published by: Journal of Geophysical Research Published on: 09/2005 YEAR: 2005 DOI: 10.1029/2004JA010882 |
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Wave acceleration of electrons in the Van Allen radiation belts The Van Allen radiation belts1 are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth\textquoterights magnetic field. Their properties vary according to solar activity2, 3 and they represent a hazard to satellites and humans in space4, 5. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth6, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields. Horne, Richard; Thorne, Richard; Shprits, Yuri; Meredith, Nigel; Glauert, Sarah; Smith, Andy; Kanekal, Shrikanth; Baker, Daniel; Engebretson, Mark; Posch, Jennifer; Spasojevic, Maria; Inan, Umran; Pickett, Jolene; Decreau, Pierrette; Published by: Nature Published on: 09/2005 YEAR: 2005 DOI: 10.1038/nature03939 |
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One of the main questions concerning radiation belt research is the origin of very high energy (>1 MeV) electrons following many space storms. Under the hypothesis that the plasma sheet electron population is the source of these electrons, which are convected to the outer radiation belt region during substorms, we estimate the flux of particles generated at geosynchronous orbit. We use the test particle method of following guiding center electrons as they drift in the electromagnetic fields during substorm dipolarization. The dipolarization pulse model electromagnetic fields are taken from the Li et al. (1998) substorm particle injection model. We find that a substorm dipolarization can produce enough electrons within geosynchronous orbit to account for the electrons seen following storms. To do this, we compute transport ratios of plasma sheet electrons, that is, the relative ratio of plasma sheet electrons that are transported and trapped in the inner magnetosphere during substorms, as well as the change in energy of the electrons. Since high fluxes of MeV electrons are only seen following storms and not isolated substorms, it is likely that these electrons may serve as a source population for other energization mechanisms which accelerate the electrons to MeV energies. Furthermore, we do parametric studies of the dipolarization model to understand physically what conditions enable the generation of this source population. Published by: Journal of Geophysical Research Published on: 07/2005 YEAR: 2005 DOI: 10.1029/2004JA010511 |
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Techniques are presented for efficiently evaluating quasi-linear diffusion coefficients for whistler mode waves propagating according to the full cold plasma index of refraction. In particular, the density ratio ωpe/Ωe can be small, which favors energy diffusion. This generalizes an approach, previously used for high-density hiss and electromagnetic ion cyclotron waves, of identifying (and omitting) ranges of wavenormal angle θ that are incompatible with cyclotron resonant frequencies ω occurring between sharp cutoffs of the modeled wave frequency spectrum. This requires a detailed analysis of the maximum and minimum values of the refractive index as a function of ω and θ, as has previously been performed in the high-density approximation. Sample calculations show the effect of low-density ratio on the pitch angle and energy diffusion coefficients modeling the effect of chorus waves on radiation belt electrons. The high-density approximation turns out to be quite robust, especially when the upper frequency cutoff is small compared with Ωe. The techniques greatly reduce the amount of computation needed for a sample calculation, while taking into account all resonant harmonic numbers n up to \textpm$\infty$. Published by: Journal of Geophysical Research Published on: 03/2005 YEAR: 2005 DOI: 10.1029/2004JA010844 |
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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. Perry, K.; Hudson, M.; Elkington, S.; Published by: Journal of Geophysical Research Published on: 03/2005 YEAR: 2005 DOI: 10.1029/2004JA010760 |
| 2004 |
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The Earth\textquoterights radiation belts\textemdashalso known as the Van Allen belts1\textemdashcontain high-energy electrons trapped on magnetic field lines2, 3. The centre of the outer belt is usually 20,000\textendash25,000 km from Earth. The region between the belts is normally devoid of particles2, 3, 4, and is accordingly favoured as a location for spacecraft operation because of the benign environment5. Here we report that the outer Van Allen belt was compressed dramatically by a solar storm known as the \textquoteleftHallowe\textquoterighten storm\textquoteright of 2003. From 1 to 10 November, the outer belt had its centre only ~10,000 km from Earth\textquoterights equatorial surface, and the plasmasphere was similarly displaced inwards. The region between the belts became the location of high particle radiation intensity. This remarkable deformation of the entire magnetosphere implies surprisingly powerful acceleration and loss processes deep within the magnetosphere. Baker, D.; Kanekal, S.; Li, X.; Monk, S.; Goldstein, J.; Burch, J.; Published by: Nature Published on: 12/2004 YEAR: 2004 DOI: 10.1038/nature03116 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
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Time dependent radial diffusion modeling of relativistic electrons with realistic loss rates Model simulations are compared to the typically observed evolution of MeV electron fluxes during geomagnetic storms to investigate whether radial diffusion alone can account for the observed variability and to estimate the effect of electron lifetimes. We demonstrate that knowledge of lifetimes is crucial for understanding the radial structure of the storm-time radiation belts and their temporal evolution. Our model results suggest that outer zone lifetimes at 1 MeV are on the order of few days during quite-times and less than a day during storm-time conditions. Losses outside plasmasphere should be included in the modeling of electron fluxes since effective lifetimes are much shorter than that of plasmaspheric losses. Simulations with variable outer boundary conditions show that the depletion of the main phase relativistic electron fluxes at L <= 4 can not be explained only by variations in fluxes near geosynchronous orbit and require local lifetimes as short as 0.5 day. Radial diffusion alone is unable to account for either the gradual build up of relativistic electron fluxes or the maxima in phase space density near L = 4 - 5 observed during the recovery phase of many storms, which suggests that an additional local acceleration source is also required. Published by: Geophysical Research Letters Published on: 04/2004 YEAR: 2004 DOI: 10.1029/2004GL019591 |
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Relativistic electrons in the outer radiation belt: Differentiating between acceleration mechanisms Many theoretical models have been developed to explain the rapid acceleration to relativistic energies of electrons that form the Earth\textquoterights radiation belts. However, after decades of research, none of these models has been unambiguously confirmed by comparison to observations. Proposed models can be separated into two types: internal and external source acceleration mechanisms. Internal source acceleration mechanisms accelerate electrons already present in the inner magnetosphere (L < 6.6), while external source acceleration mechanisms transport and accelerate a source population of electrons from the outer to the inner magnetosphere. In principle, the two types of acceleration mechanisms can be differentiated because they imply that different radial gradients of electron phase space density expressed as a function of the three adiabatic invariants will develop. Model predictions can be tested by transforming measured electron flux (given as a function of pitch angle, energy, and position) to phase space density as a function of the three invariants, μ, K, and Φ. The transformation requires adoption of a magnetic field model. Phase space density estimates have, in the past, produced contradictory results because of limited measurements and field model errors. In this study we greatly reduce the uncertainties of previous work and account for the contradictions. We use data principally from the Polar High Sensitivity Telescope energetic detector on the Polar spacecraft and the Tsyganenko and Stern [1996] field model to obtain phase space density. We show how imperfect magnetic field models produce phase space density errors and explore how those errors modify interpretations. On the basis of the analysis we conclude that the data are best explained by models that require acceleration of an internal source of electrons near L* \~ 5. We also suggest that outward radial diffusion from a phase space density peak near L* \~ 5 can explain the observed correspondence between flux enhancements at geostationary orbit and increases in ULF wave power. Published by: Journal of Geophysical Research Published on: 03/2004 YEAR: 2004 DOI: 10.1029/2003JA010153 |
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Quantification of relativistic electron microburst losses during the GEM storms Bursty precipitation of relativistic electrons has been implicated as a major loss process during magnetic storms. One type of precipitation, microbursts, appears to contain enough electrons to empty the prestorm outer radiation belt in approximately a day. During storms that result in high fluxes of trapped relativistic electrons, microbursts continue for several days into the recovery phase, when trapped fluxes are dramatically increasing. The present study shows that this apparent inconsistency is resolved by observations that the number of electrons lost through microbursts is 10\textendash100 times larger during the main phase than during the recovery phase of several magnetic storms chosen by the Geospace Environment Modeling (GEM) program. O\textquoterightBrien, T.; Looper, M.; Blake, J.; Published by: Geophysical Research Letters Published on: 02/2004 YEAR: 2004 DOI: 10.1029/2003GL018621 |
| 2003 |
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We examine signatures of two types of waves that may be involved in the acceleration of energetic electrons in Earth\textquoterights outer radiation belts. We have compiled a database of ULF wave power from SAMNET and IMAGE ground magnetometer stations for 1987\textendash2001. Long-duration, comprehensive, in situ VLF/ELF chorus wave observations are not available, so we infer chorus wave activity from low-altitude SAMPEX observations of MeV electron microbursts for 1996\textendash2001 since microbursts are thought to be caused by interactions between chorus and trapped electrons. We compare the ULF and microburst observations to in situ trapped electrons observed by high-altitude satellites from 1989\textendash2001. We find that electron acceleration at low L shells is closely associated with both ULF activity and MeV microbursts and thereby probably also with chorus activity. Electron flux enhancements across the outer radiation belt are, in general, related to both ULF and VLF/ELF activity. However, we suggest that electron flux peaks observed at L \~ 4.5 are likely caused by VLF/ELF wave acceleration, while ULF activity probably produces the dominant electron acceleration at geosynchronous orbit and beyond. O\textquoterightBrien, T.; Lorentzen, K.; Mann, I.; Meredith, N.; Blake, J.; Fennell, J.; Looper, M.; Milling, D.; Anderson, R.; Published by: Journal of Geophysical Research Published on: 08/2003 YEAR: 2003 DOI: 10.1029/2002JA009784 |
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Evaluation of quasi-linear diffusion coefficients for EMIC waves in a multispecies plasma Quasi-linear velocity-space diffusion coefficients due to L-mode electromagnetic ion cyclotron (EMIC) waves are considered in a multispecies plasma. It is shown, with slight approximations to exact cold plasma theory, that within EMIC pass bands the index of refraction is a monotonically increasing function of frequency. Analytical criteria are then derived which identify ranges of latitude, wavenormal angle, and resonance number consistent with resonance in a prescribed wave population. This leads to computational techniques which allow very efficient calculation of the diffusion coefficients, along the lines previously developed for whistler and ion cyclotron waves in an electron-proton plasma. The techniques are applied to radiation belt electrons at L = 4, for EMIC waves in the hydrogen, helium, and oxygen bands representative of different phases of a magnetic storm. Finally, diffusion coefficients for recovery-phase helium-band EMIC waves are combined with those for typical whistler hiss, resulting in electron precipitation lifetimes substantially less than those due to hiss alone. Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009792 |
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We perform a survey of the plasma wave and particle data from the CRRES satellite during 26 geomagnetically disturbed periods to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by Doppler-shifted cyclotron resonant interactions with whistler mode chorus. Relativistic electron flux enhancements associated with moderate or strong storms may be seen over the whole outer zone (3 < L < 7), typically peaking in the range 4 < L < 5, whereas those associated with weak storms and intervals of prolonged substorm activity lacking a magnetic storm signature (PSALMSS) are typically observed further out in the regions 4 < L < 7 and 4.5 < L < 7, respectively. The most significant relativistic electron flux enhancements are seen outside of the plasmapause and are associated with periods of prolonged substorm activity with AE greater than 100 nT for a total integrated time greater than 2 days or greater than 300 nT for a total integrated time greater than 0.7 days. These events are also associated with enhanced fluxes of seed electrons and enhanced lower-band chorus wave power with integrated lower-band chorus wave intensities of greater than 500 pT2 day. No significant flux enhancements are seen unless the level of substorm activity is sufficiently high. These results are consistent with a local, stochastic, chorus-driven electron acceleration mechanism involving the energization of a seed population of electrons with energies of a few hundred keV to relativistic energies operating on a timescale of the order of days. Meredith, Nigel; Cain, Michelle; Horne, Richard; Thorne, Richard; Summers, D.; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009764 |
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Electromagnetic ion cyclotron (EMIC) waves which propagate at frequencies below the proton gyrofrequency can undergo cyclotron resonant interactions with relativistic electrons in the outer radiation belt and cause pitch-angle scattering and electron loss to the atmosphere. Typical storm-time wave amplitudes of 1\textendash10 nT cause strong diffusion scattering which may lead to significant relativistic electron loss at energies above the minimum energy for resonance, Emin. A statistical analysis of over 800 EMIC wave events observed on the CRRES spacecraft is performed to establish whether scattering can occur at geophysically interesting energies (<=2 MeV). While Emin is well above 2 MeV for the majority of these events, it can fall below 2 MeV in localized regions of high plasma density and/or low magnetic field (fpe/fce,eq > 10) for wave frequencies just below the hydrogen or helium ion gyrofrequencies. These lower energy scattering events, which are mainly associated with resonant L-mode waves, are found within the magnetic local time range 1300 < MLT < 1800 for L > 4.5. The average wave spectral intensity of these events (4\textendash5 nT2/Hz) is sufficient to cause strong diffusion scattering. The spatial confinement of these events, together with the limited set of these waves that resonate with <=2 MeV electrons, suggest that these electrons are only subject to strong scattering over a small fraction of their drift orbit. Consequently, drift-averaged scattering lifetimes are expected to lie in the range of several hours to a day. EMIC wave scattering should therefore significantly affect relativistic electron dynamics during a storm. The waves that resonate with the \~MeV electrons are produced by low-energy (\~keV) ring current protons, which are expected to be injected into the inner magnetosphere during enhanced convection events. Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009700 |
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1] Resonant interactions with whistler-mode chorus waves provide an important process for electron loss and acceleration during storm times. We demonstrate that wave propagation significantly affects the electron scattering rates. We show that stormtime chorus waves outside the plasmapause can scatter equatorial electrons <=60 keV into the loss cone and accelerate trapped electrons up to \~ MeV energies at large pitch-angles. Using ray tracing to map the waves to higher latitudes, we show that the decrease in the ratio between the electron plasma and gyro frequencies, along with the normalized chorus frequency bandwidth, enable much higher energy electrons \~1 MeV to be scattered into the loss cone. We suggest that off equatorial pitch-angle scattering by chorus waves is responsible for relativistic micro-burst precipitation seen on SAMPEX. Off-equatorial scattering at pitch-angles well away from the loss cone also contributes to the acceleration of higher energy >=3 MeV electrons. Published by: Geophysical Research Letters Published on: 05/2003 YEAR: 2003 DOI: 10.1029/2003GL016973 |
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[1] During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch-angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch-angle diffusion coefficients Dαα for EMIC waves in a multi-ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi-linear theory that is valid for parallel-propagating, small-amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (1\textendash10nT), the rates of resonant pitch-angle diffusion are close to the limit of \textquotedblleftstrong\textquotedblright diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/electron gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) >=10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy >=1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time-scale of several hours to a day. Published by: Journal of Geophysical Research Published on: 04/2003 YEAR: 2003 DOI: 10.1029/2002JA009489 |
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Using the data from the NOAA and Exos-D satellites during the 3 November 1993 magnetic storm, the dynamic behavior of electrons with energies from a few tens of kiloelectronvolts to a few and its relation to plasma waves were examined. After the late main phase, relativistic electron flux started to recover from the heart of the outer radiation belt, where the cold plasma density was extremely low, and intense whistler mode chorus emissions were detected. The phase space density showed a peak in the outer belt, and the peak increased gradually. The simulation of the inward radial diffusion process could not reproduce the observed energy spectrum and phase space density variation. On the other hand, the simulated energy diffusion due to the gyroresonant electron-whistler mode wave interactions, under the assumption of the Kolmogorov turbulence spectrum, could generate the relativistic electrons without the flux transport from the outer region. The present study suggested that the seed population of relativistic electrons, which appeared in the heart of the outer radiation belt during the late main phase, was the ring current electrons injected from the plasma sheet, and that the acceleration by whistler mode chorus via gyroresonant wave\textendashparticle interactions outside the plasmapause could play an important role to generate the relativistic electrons. Published by: Journal of Geophysical Research Published on: 03/2003 YEAR: 2003 DOI: 10.1029/2001JA007542 |
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Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field [1] 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 |
| 2002 |
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X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer The high-resolution germanium detector aboard the MAXIS (MeV Auroral X-ray Imaging and Spectroscopy) balloon payload detected nine X-ray bursts with significant flux extending above 0.5 MeV during an 18 day flight over Antarctica. These minutes-to-hours-long events are characterized by an extremely flat spectrum (\~E-2) similar to the first MeV event discovered in 1996, indicating that the bulk of parent precipitating electrons is at relativistic energies. The MeV bursts were detected between magnetic latitudes 58\textdegree\textendash68\textdegree (L-values of 3.8\textendash6.7) but only in the late afternoon/dusk sectors (14:30\textendash00:00 MLT), suggesting scattering by EMIC (electromagnetic ion cyclotron) waves as a precipitation mechanism. We estimate the average flux of precipitating E >= 0.5 MeV electrons to be \~360 cm-2s-1, corresponding to about 5 \texttimes 1025 such electrons precipitated during the eight days at L = 3.8\textendash6.7, compared to \~2 \texttimes 1025 trapped 0.5\textendash3.6 MeV electrons estimated from dosimeter measurements on a GPS spacecraft. These observations show that MeV electron precipitation events are a primary loss mechanism for outer zone relativistic electrons. Published by: Geophysical Research Letters Published on: 12/2002 YEAR: 2002 DOI: 10.1029/2002GL015922 |
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MHD/particle simulations of radiation belt dynamics 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. ELKINGTON, S; Hudson, M.; Wiltberger, M.J; Lyon, J.; Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 04/2002 YEAR: 2002 DOI: 10.1016/S1364-6826(02)00018-4 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
| 2001 |
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The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50\textendash300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron ε \texttimes B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm. Ingraham, J.; Cayton, T.; Belian, R.; Christensen, R.; Friedel, R.; Meier, M.; Reeves, G.; Takahashi, K; Published by: Journal of Geophysical Research Published on: 11/2001 YEAR: 2001 DOI: 10.1029/2000JA000458 |
| 2000 |
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In this paper we study the variation of the relativistic electron fluxes in the Earth\textquoterights outer radiation belt during the March 26, 1995, magnetic storm. Using observations by the radiation environment monitor (REM) on board the space technology research vehicle (STRV-Ib), we discuss the flux decrease and possible loss of relativistic electrons during the storm main phase. In order to explain the observations we have performed fully adiabatic and guiding center simulations for relativistic equatorial electrons in the nonstationary Tsygarienko96 magnetospheric magnetic field model. In our simulations the drift of electrons through the magnetopause was considered as a loss process. We present our model results and discuss their dependence on the magnetospheric magnetic and electric field model, as well as on the prestorm fluxes used in the simulations. Desorgher, L.; ühler, P.; Zehnder, A.; ückiger, E.; Published by: Journal of Geophysical Research Published on: 09/2000 YEAR: 2000 DOI: 10.1029/2000JA900060 |
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The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral-electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 <= L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3\textpm1.2 and 4.6\textpm0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave-particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 <= L < 6.0 the timescale for wave decay at the ECH equator is 2.3 \textpm 0.6 and 1.1 \textpm 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time. Meredith, Nigel; Horne, Richard; Johnstone, Alan; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2000 YEAR: 2000 DOI: 10.1029/2000JA900010 |
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The response of outer radiation belt relativistic electrons to the October 9, 1990, magnetic storm is analyzed in detail using a radial diffusion model and data from the Combined Release and Radiation Effects Satellite (CRRES) and the Los Alamos National Laboratory (LANL) geosynchronous satellite 1989-046. Electron measurements are expressed in terms of phase space density as a function of the three adiabatic invariants determined from CRRES magnetic field data and the Tsyganenko 1989 Kp-dependent magnetic field model. The radial diffusion model is implemented with a time-dependent radial diffusion coefficient parameterized by Kp, and a time-dependent outer boundary condition scaled by geosynchronous electron data. The results show that radial diffusion propagates outer boundary variations into the heart of the outer radiation belt, accounting for both significant decreases and increases in the <1 MeV electron flux throughout that region. It is further shown that the gradual increase throughout the recovery phase of the >1 MeV electrons is inconsistent with the radial diffusion model given the parameter regime chosen for this study. Greatly enhanced whistler chorus waves observed by CRRES throughout the recovery phase suggest that a possible explanation for the inconsistency may be electron acceleration via wave-particle interaction. Published by: Journal of Geophysical Research Published on: 01/2000 YEAR: 2000 DOI: 10.1029/1999JA900344 |
| 1999 |
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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. Elkington, Scot; Hudson, M.; Chan, Anthony; Published by: Geophysical Research Letters Published on: 11/1999 YEAR: 1999 DOI: 10.1029/1999GL003659 |
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Simulation of Radiation Belt Dynamics Driven by Solar Wind Variations 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 Hudson, M.; Elkington, S.; Lyon, J.; Goodrich, C.; Rosenberg, T.; Published by: Published on: YEAR: 1999 DOI: 10.1029/GM10910.1029/GM109p0171 |
| 1998 |
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Resonant diffusion curves for electron cyclotron resonance with field-aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single-ion plasma, and a numerical scheme is developed for the more realistic case of a multi-ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth\textquoterights magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler (R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere. Summers, D.; Thorne, Richard; Xiao, Fuliang; Published by: Journal of Geophysical Research Published on: 09/1998 YEAR: 1998 DOI: 10.1029/98JA01740 |
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Substorm electron injections: Geosynchronous observations and test particle simulations We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (\~ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two-species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn-dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn-dusk direction of the tail collapse and the associated cross-tail electric field, in combination with a difference in the relative importance of E \texttimes B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b]. Birn, J.; Thomsen, M.; Borovsky, J.; Reeves, G.; McComas, D.; Belian, R.; Hesse, M.; Published by: Journal of Geophysical Research Published on: 05/1998 YEAR: 1998 DOI: 10.1029/97JA02635 |
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Electron scattering loss in Earth\textquoterights inner magnetosphere 1. Dominant physical processes Pitch angle diffusion rates due to Coulomb collisions and resonant interactions with plasmaspheric hiss, lightning-induced whistlers and anthropogenic VLF transmissions are computed for inner magnetospheric electrons. The bounce-averaged, quasi-linear pitch angle diffusion coefficients are input into a pure pitch angle diffusion equation to obtain L and energy dependent equilibrium distribution functions and precipitation lifetimes. The relative effects of each scattering mechanism are considered as a function of electron energy and L shell. Model calculations accurately describe the enhanced loss rates in the slot region, as well as reduced scattering in the heavily populated inner radiation belt. Predicted electron distribution function calculations in the slot region display a characteristic \textquotedbllefttop hat\textquotedblright distribution which is supported by observations. Inner zone electron lifetimes based on observed decay rates of the Starfish electron population are in approximate agreement with model predictions. Published by: Journal of Geophysical Research Published on: 02/1998 YEAR: 1998 DOI: 10.1029/97JA02919 |
| 1997 |
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The disappearance and reappearance of outer zone energetic electrons during the November 3\textendash4, 1993, magnetic storm is examined utilizing data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), the Global Positioning System (GPS) series, and the Los Alamos National Laboratory (LANL) sensors onboard geosynchronous satellites. The relativistic electron flux drops during the main phase of the magnetic storm in association with the large negative interplanetary Bz and rapid solar wind pressure increase late on November 3. Outer zone electrons with E > 3 MeV measured by SAMPEX disappear for over 12 hours at the beginning of November 4. This represents a 3 orders of magnitude decrease down to the cosmic ray background of the detector. GPS and LANL sensors show similar effects, confirming that the flux drop of the energetic electrons occurs near the magnetic equator and at all pitch angles. Enhanced electron precipitation was measured by SAMPEX at L >= 3.5. The outer zone electron fluxes then recover and exceed prestorm levels within one day of the storm onset and the inner boundary of the outer zone moves inward to smaller L (<3). These multiple-satellite measurements provide a data set which is examined in detail and used to determine the mechanisms contributing to the loss and recovery of the outer zone electron flux. The loss of the inner part of the outer zone electrons is partly due to the adiabatic effects associated with the decrease of Dst, while the loss of most of the outer part (those electrons initially at L >= 4.0) are due to either precipitation into the atmosphere or drift to the magnetopause because of the strong compression of the magnetosphere by the solar wind. The recovery of the energetic electron flux is due to the adiabatic effects associated with the increase in Dst, and at lower energies (<0.5 MeV) due to rapid radial diffusion driven by the strong magnetic activity during the recovery phase of the storm. Heating of the electrons by waves may contribute to the energization of the more energetic part (>1.0 MeV) of the outer zone electrons. Li, Xinlin; Baker, D.; Temerin, M.; Cayton, T.; Reeves, E.; Christensen, R.; Blake, J.; Looper, M.; Nakamura, R.; Kanekal, S.; Published by: Journal of Geophysical Research Published on: 01/1997 YEAR: 1997 DOI: 10.1029/97JA01101 |
| 1994 |
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Electric and magnetic fields were measured by the CRRES spacecraft at an L-value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross-L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field. Wygant, J.; Mozer, F.; Temerin, M.; Blake, J.; Maynard, N.; Singer, H.; Smiddy, M.; Published by: Geophysical Research Letters Published on: 08/1994 YEAR: 1994 DOI: 10.1029/94GL00375 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
| 1993 |
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We model the rapid (\~ 1 min) formation of a new electron radiation belt at L ≃ 2.5 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the CRRES satellite. Guided by the observed electric and magnetic fields, we represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earth\textquoterights magnetic field. We follow the electrons using a relativistic guiding center code. The test-particle simulations show that electrons with energies of a few MeV at L > 6 were energized up to 40 MeV and transported to L ≃ 2.5 during a fraction of their drift period. The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field. Our simulation results, with an initial W-8 energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT. Li, Xinlin; Roth, I.; Temerin, M.; Wygant, J.; Hudson, M.; Blake, J.; Published by: Geophysical Research Letters Published on: 11/1993 YEAR: 1993 DOI: 10.1029/93GL02701 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
| 1988 |
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Simultaneous Radial and Pitch Angle Diffusion in the Outer Electron Radiation Belt A solution of the bimodal (radial and pitch angle) diffusion equation for the radiation belts is developed with special regard for the requirements of satellite radiation belt data analysis. In this paper, we use this solution to test the bimodal theory of outer electron belt diffusion by confronting it with satellite data. Satellite observations, usually over finite volumes of (L, t) space, are seldom sufficient in space-time duration to cover the relaxation to equilibrium of the entire radiation belt. Since time scales of continuous data coverage are often comparable to that of radiation belt disturbances, it is therefore inappropriate to apply impulsive semi-infinite time response solutions of diffusion theory to interpret data from a finite window of (L, t) space. Observational limitations indicate that appropriate solutions for the interpretation of satellite data are general solutions for a finite-volume boundary value problem in bimodal diffusion. Here we test such a solution as the prime candidate for comprehensive radiation belt dynamic modeling by applying the solution and developing a method of analysis to radiation belt electron data obtained by the SCATHA satellite at moderate geomagnetic activity. The results and the generality of our solution indicate its promise as a new approach to dynamic modeling of the radiation belts. Chiu, Y.; Nightingale, R.; Rinaldi, M.; Published by: Journal of Geophysical Research Published on: 04/1988 YEAR: 1988 DOI: 10.1029/JA093iA04p02619 |
| 1981 |
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An account is given of measurements of electrons made by the LLNL magnetic electron spectrometer (60\textendash3000 keV in seven differential energy channels) on the Ogo 5 satellite in the earth\textquoterights outer-belt regions during 1968 and early 1969. The data were analyzed to identify those features dominated by pitch angle and radial diffusion; in doing so all aspects of phase space covered by the data were studied, including pitch angle distributions and spectral features, as well as decay rates. The pitch angle distributions are reported elsewhere. The spectra observed in the weeks after a storm at L \~3\textendash4.5 show the evolution of a peak at \~1.5 MeV and pronounced minima at \~0.5 MeV. The observed pitch angle diffusion lifetimes are identified as being the shortest decays observed and are found to be highly energy and L dependent with minimum lifetimes of \~1\textendash2 days occurring at L \~3\textendash4.5. Two contiguous periods of decay, following the intense storm injection on October 31 and November 1, were analyzed in terms of radial diffusion. Significant differences were found between the derived values of DLL for the two periods; also significant energy dependence shows in the results. Although the values of DLL vary by about a factor of 10, representative values are 0.3 day-1 at L=6, 0.06 at L=4, 0.015 at L=3, and 0.001 at L=2.5. Despite the wide variation of many prior results in the literature, there is a family of results in approximate agreement with the present results. By noting the variations in DLL, as a function of the invariant quantities, we are able to order a fair body of previous results with our new results. West, H.; Buck, R.; Davidson, G.; Published by: Journal of Geophysical Research Published on: 04/1981 YEAR: 1981 DOI: 10.1029/JA086iA04p02111 |
| 1979 |
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Direct Evaluation of the Radial Diffusion Coefficient near L = 6 Due to Electric Field Fluctuations The radial diffusion coefficient for radiation belt particles near L=6 has been calculated from the measured electric field fluctuations. Simultaneous balloon flights in August 1974 from six auroral zone sites ranging 180\textdegree in magnetic longitude produced the electric field data. The large scale slowly varying ionospheric electric fields from these flights have been mapped to the equator during the quiet magnetic conditions of this campaign. These mapped equatorial electric fields were then Fourier transformed in space and time to produce power spectra of the first two terms of the global azimuthal electric field. From these power spectra the radial diffusion coefficient has been calculated. Published by: Journal of Geophysical Research Published on: 06/1979 YEAR: 1979 DOI: 10.1029/JA084iA06p02559 |
| 1973 |
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Measurements at conjugate points on the ground near L = 4 of the power spectra of magnetic-field fluctuations in the frequency range 0.5 to 20 mHz are used as a means of estimating daily values for the relativistic-electron radial-diffusion coefficient DLL for two periods in December 1971 and January 1972. The values deduced for L-10 DLL show a strong variation with magnetic activity, as measured by the Fredricksburg magnetic index KFR. The radial-diffusion coefficient typically increases by a factor of \~10 for a unit increase in KFR. When KFR ≲ 2, it is generally found that DLL ≲ 2 \texttimes 10-9 L10 day-1 for equatorially mirroring electrons having a first invariant M = 750 Mev/gauss; a value of DLL \~4 \texttimes 10-7 L10 day-1 is deduced for one day on which the mean KFR was 4.5. The quantity L-10 DLL theoretically depends on energy and L as (L/M)(s-2)/2 for relativistic particles, where s is the logarithmic slope of the power-law spectrum of magnetic fluctuations observed on the ground. For the time period analyzed, s typically had values between 1 and 3. Lanzerotti, L.; Morgan, Caroline; Published by: Journal of Geophysical Research Published on: 08/1973 YEAR: 1973 DOI: 10.1029/JA078i022p04600 |
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Equilibrium Structure of Radiation Belt Electrons The detailed quiet time structure of energetic electrons in the earth\textquoterights radiation belts is explained on the basis of a balance between pitch angle scattering loss and inward radial diffusion from an average outer zone source. Losses are attributed to a combination of classical Coulomb scattering at low L and whistler mode turbulent pitch angle diffusion throughout the outer plasmasphere. Radial diffusion is driven by substorm associated fluctuations of the magnetospheric convection electric field. Lyons, Lawrence; Thorne, Richard; Published by: Journal of Geophysical Research Published on: 05/1973 YEAR: 1973 DOI: 10.1029/JA078i013p02142 |
| 1972 |
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Parasitic Pitch Angle Diffusion of Radiation Belt Particles by Ion Cyclotron Waves The resonant pitch angle scattering of protons and electrons by ion cyclotron turbulence is investigated. The analysis is analogous to that recently performed for electron interactions with whistler mode waves. The role played by the intense band of ion cyclotron waves, predicted to be generated just within the plasmapause during the decay of the magnetospheric ring current, is evaluated in detail. Loss rates resulting from parasitic interactions with this turbulence are determined for energetic protons and relativistic electrons. Lyons, Lawrence; Thorne, Richard; Published by: Journal of Geophysical Research Published on: 10/1972 YEAR: 1972 DOI: 10.1029/JA077i028p05608 |
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Inner-Zone Energetic-Electron Repopulation by Radial Diffusion A quantitative study of the intrusion of natural electrons into the inner radiation zone during and after the geomagnetic storm of September 2, 1966, shows that the transport is consistent with a radial-diffusion mechanism in which the first two invariants are conserved. Except for the 3-day period of the storm main phase when data were missing, the radial-diffusion coefficient is D = 2.7 \texttimes 10-5 L7.9 μ-0.5 day-1 in the range 1.7 <= L <= 2.6 and 13.3 <= μ <= 27.4 Mev gauss-1. This value could be produced by variation of a large-scale electric field across the magnetosphere having an amplitude of 0.28 mv / m and a period of 1600 sec. Electric fields having approximately these characteristics have been inferred from previous observations of the motion of whistler ducts within the plasmapause. If fields of this amplitude and period exist throughout the magnetosphere, the radial diffusion of all geomagnetically trapped particles except the high-energy inner-zone protons is strongly influenced by electric-field variations. A comprehensive review of previously reported radial-diffusion coefficients shows reasonable agreement for L less than about 3.0, but serious discrepancies among reported values exist for determinations made in the outer zone. These discrepancies cannot be explained by the simple theory of radial diffusion due to variation of large-scale electric or magnetic fields. Tomassian, Albert; Farley, Thomas; Vampola, Alfred; Published by: Journal of Geophysical Research Published on: 07/1972 YEAR: 1972 DOI: 10.1029/JA077i019p03441 |
| 1970 |
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Radial Diffusion of Outer-Zone Electrons: An Empirical Approach to Third-Invariant Violation The near-equatorial fluxes of outer-zone electrons (E>0.5 Mev and E>1.9 Mev) measured by an instrument on the satellite Explorer 15 following the geomagnetic storm of December 17\textendash18, 1962, are used to determine the electron radial diffusion coefficients and electron lifetimes as functions of L for selected values of the conserved first invariant \textmu. For each value of \textmu, the diffusion coefficient is assumed to be time-independent and representable in the form D = DnLn. The diffusion coefficients and lifetimes are then simultaneously obtained by requiring that the L-dependent reciprocal electron lifetime, as determined from the Fokker-Planck equation, deviate minimally from a constant in time. Applied to the data, these few assumptions yield a value of D that is smaller by approximately a factor of 10 than the value recently found by Newkirk and Walt in a separate analysis of 1.6-Mev electron data obtained during the same time period on another satellite. The electron lifetimes are found to be strong functions of L, with 4- to 6-day lifetimes observed at the higher L values (4.6\textendash4.8). Lanzerotti, L.; Maclennan, C.; Schulz, Michael; Published by: Journal of Geophysical Research Published on: 10/1970 YEAR: 1970 DOI: 10.1029/JA075i028p05351 |