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Found 21 entries in the Bibliography.

Showing entries from 1 through 21


Multi-Parameter Chorus and Plasmaspheric Hiss Wave Models

Abstract The resonant interaction of energetic particles with plasma waves, such as chorus and plasmaspheric hiss waves, plays a direct and crucial role in the acceleration and loss of radiation belt electrons that ultimately affect the dynamics of the radiation belts. In this study, we use the comprehensive wave data measurements made by the Electric and Magnetic Field Instrument Suite and Integrated Science instruments on board the two Van Allen probes, to develop multi-parameter statistical chorus and plasmaspheric hiss wave models. The models of chorus and plasmaspheric hiss waves are presented as a function of combined geomagnetic activity (AE), solar wind velocity (V), and southward interplanetary magnetic field (Bs). The relatively smooth wave models reveal new features. Despite, the coupling between geomagnetic and solar wind parameters, the results show that each parameter still carries a sufficient amount of unique information to more accurately constrain the chorus and plasmaspheric hiss wave intensities. The new wave models presented here highlight the importance of multi-parameter wave models, and can improve radiation belt modeling.

Aryan, Homayon; Bortnik, Jacob; Meredith, Nigel; Horne, Richard; Sibeck, David; Balikhin, Michael;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2020

YEAR: 2020     DOI:

chorus waves; inner magnetosphere; multi parameter wave distribution; plasmaspheric hiss waves; Van Allen Probes; wave-particle interactions

The Implications of Temporal Variability in Wave-Particle Interactions in Earth s Radiation Belts

Changes in electron flux in Earth s outer radiation belt can be modeled using a diffusion-based framework. Diffusion coefficients D for such models are often constructed from statistical averages of observed inputs. Here, we use stochastic parameterization to investigate the consequences of temporal variability in D. Variability time scales are constrained using Van Allen Probe observations. Results from stochastic parameterization experiments are compared with experiments using D constructed from averaged inputs and an average of observation-specific D. We find that the evolution and final state of the numerical experiment depends upon the variability time scale of D; experiments with longer variability time scales differ from those with shorter time scales, even when the time-integrated diffusion is the same. Short variability time scale experiments converge with solutions obtained using an averaged observation-specific D, and both exhibit greater diffusion than experiments using the averaged-input D. These experiments reveal the importance of temporal variability in radiation belt diffusion.

Watt, C.; Allison, H.; Thompson, R.; Bentley, S.; Meredith, N.; Glauert, S.; Horne, R.; Rae, I.;

Published by: Geophysical Research Letters      Published on: 12/2020

YEAR: 2020     DOI:

probabilistic methods; stochastic parameterization; Van Allen Probes

A New Approach to Constructing Models of Electron Diffusion by EMIC Waves in the Radiation Belts

Electromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron losses in the radiation belts through diffusion via resonant wave-particle interactions. We present a new approach for calculating bounce and drift-averaged EMIC electron diffusion coefficients. We calculate bounce-averaged diffusion coefficients, using quasi-linear theory, for each individual Combined Release and Radiation Effects Satellite (CRRES) EMIC wave observation using fitted wave properties, the plasma density and the background magnetic field. These calculations are then combined into bounce-averaged diffusion coefficients. The resulting coefficients therefore capture the combined effects of individual spectra and plasma properties as opposed to previous approaches that use average spectral and plasma properties, resulting in diffusion over a wider range of energies and pitch angles. These calculations, and their role in radiation belt simulations, are then compared against existing diffusion models. The new diffusion coefficients are found to significantly improve the agreement between the calculated decay of relativistic electrons and Van Allen Probes data.

Ross, J.; Glauert, S.; Horne, R.; Watt, C.; Meredith, N.; Woodfield, E.;

Published by: Geophysical Research Letters      Published on: 10/2020

YEAR: 2020     DOI:

Radiation belts; EMIC waves; electron diffusion; Van Allen Probes

Global Model of Whistler Mode Chorus in the Near-Equatorial Region (|λm|<  18°)

We extend our database of whistler mode chorus, based on data from seven satellites, by including ∼3 years of data from Radiation Belt Storm Probes (RBSP)-A and RBSP-B and an additional ∼6 years of data from Time History of Events and Macroscale Interactions during Substorms (THEMIS)-A, THEMIS-D, and THEMIS-E. The new database allows us to probe the near-equatorial region in detail, revealing new features. In the equatorial source region, |λm|<6°, strong wave power is most extensive in the 0.1–0.4fce bands in the region 21–11 magnetic local time (MLT) from the plasmapause out to L∗ = 8 and beyond, especially near dawn. At higher frequencies, in the 0.4–0.6fce frequency bands, strong wave power is more tightly confined, typically being restricted to the postmidnight sector in the region 4

Meredith, Nigel; Horne, Richard; Shen, Xiao-Chen; Li, Wen; Bortnik, Jacob;

Published by: Geophysical Research Letters      Published on: 05/2020

YEAR: 2020     DOI:

whistler mode chorus; wave-particle interactions; Radiation belts; Van Allen Probes


Variability of Quasilinear Diffusion Coefficients for Plasmaspheric Hiss

In the outer radiation belt, the acceleration and loss of high-energy electrons is largely controlled by wave-particle interactions. Quasilinear diffusion coefficients are an efficient way to capture the small-scale physics of wave-particle interactions due to magnetospheric wave modes such as plasmaspheric hiss. The strength of quasilinear diffusion coefficients as a function of energy and pitch angle depends on both wave parameters and plasma parameters such as ambient magnetic field strength, plasma number density, and composition. For plasmaspheric hiss in the magnetosphere, observations indicate large variations in the wave intensity and wave normal angle, but less is known about the simultaneous variability of the magnetic field and number density. We use in situ measurements from the Van Allen Probe mission to demonstrate the variability of selected factors that control the size and shape of pitch angle diffusion coefficients: wave intensity, magnetic field strength, and electron number density. We then compare with the variability of diffusion coefficients calculated individually from colocated and simultaneous groups of measurements. We show that the distribution of the plasmaspheric hiss diffusion coefficients is highly non-Gaussian with large variance and that the distributions themselves vary strongly across the three phase space bins studied. In most bins studied, the plasmaspheric hiss diffusion coefficients tend to increase with geomagnetic activity, but our results indicate that new approaches that include natural variability may yield improved parameterizations. We suggest methods like stochastic parameterization of wave-particle interactions could use variability information to improve modeling of the outer radiation belt.

Watt, C.; Allison, H.; Meredith, N.; Thompson, R.; Bentley, S.; Rae, I.; Glauert, S.; Horne, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2019

YEAR: 2019     DOI: 10.1029/2018JA026401

empirical; Magnetosphere; parameterization; stochastic; Van Allen Probes; wave-particle interactions


Global model of plasmaspheric hiss from multiple satellite observations

We present a global model of plasmaspheric hiss, using data from eight satellites, extending the coverage and improving the statistics of existing models. We use geomagnetic activity dependent templates to separate plasmaspheric hiss from chorus. In the region 22-14 MLT the boundary between plasmaspheric hiss and chorus moves to lower L* values with increasing geomagnetic activity. The average wave intensity of plasmaspheric hiss is largest on the dayside and increases with increasing geomagnetic activity from midnight through dawn to dusk. Plasmaspheric hiss is most intense and spatially extended in the 200-500 Hz frequency band during active conditions, 400

Meredith, Nigel; Horne, Richard; Kersten, Tobias; Li, Wen; Bortnik, Jacob; Sicard-Piet, elica; Yearby, Keith;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2018

YEAR: 2018     DOI: 10.1029/2018JA025226

plasmasphere; Plasmaspheric Hiss; Radiation belts; Van Allen Probes


Electron losses from the radiation belts caused by EMIC waves

Electromagnetic Ion Cyclotron (EMIC) waves cause electron loss in the radiation belts by resonating with high-energy electrons at energies greater than about 500 keV. However, their effectiveness has not been fully quantified. Here we determine the effectiveness of EMIC waves by using wave data from the fluxgate magnetometer on CRRES to calculate bounce-averaged pitch angle and energy diffusion rates for L*=3.5\textendash7 for five levels of Kp between 12 and 18 MLT. To determine the electron loss, EMIC diffusion rates were included in the British Antarctic Survey Radiation Belt Model together with whistler mode chorus, plasmaspheric hiss, and radial diffusion. By simulating a 100 day period in 1990, we show that EMIC waves caused a significant reduction in the electron flux for energies greater than 2 MeV but only for pitch angles lower than about 60\textdegree. The simulations show that the distribution of electrons left behind in space looks like a pancake distribution. Since EMIC waves cannot remove electrons at all pitch angles even at 30 MeV, our results suggest that EMIC waves are unlikely to set an upper limit on the energy of the flux of radiation belt electrons.

Kersten, Tobias; Horne, Richard; Glauert, Sarah; Meredith, Nigel; Fraser, Brian; Grew, Russell;

Published by: Journal of Geophysical Research: Space Physics      Published on: 11/2014

YEAR: 2014     DOI: 10.1002/2014JA020366

electron losses; EMIC waves

Simulating the Earth\textquoterights radiation belts: Internal acceleration and continuous losses to the magnetopause

In the Earth\textquoterights radiation belts the flux of relativistic electrons is highly variable, sometimes changing by orders of magnitude within a few hours. Since energetic electrons can damage satellites it is important to understand the processes driving these changes and, ultimately, to develop forecasts of the energetic electron population. One approach is to use three-dimensional diffusion models, based on a Fokker-Planck equation. Here we describe a model where the phase-space density is set to zero at the outer L* boundary, simulating losses to the magnetopause, using recently published chorus diffusion coefficients for 1.5<=L*<=10. The value of the phase-space density on the minimum-energy boundary is determined from a recently published, solar wind-dependent, statistical model. Our simulations show that an outer radiation belt can be created by local acceleration of electrons from a very soft energy spectrum without the need for a source of electrons from inward radial transport. The location in L* of the peaks in flux for these steady state simulations is energy dependent and moves earthward with increasing energy. Comparisons between the model and data from the CRRES spacecraft are shown; flux dropouts are reproduced in the model by the increased outward radial diffusion that occurs during storms. Including the inward movement of the magnetopause in the model has little additional effect on the results. Finally, the location of the low-energy boundary is shown to be important for accurate modeling of observations.

Glauert, Sarah; Horne, Richard; Meredith, Nigel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2014

YEAR: 2014     DOI: 10.1002/jgra.v119.910.1002/2014JA020092

magnetopause; Radiation belt; wave-particle interactions

Three-dimensional stochastic modeling of radiation belts in adiabatic invariant coordinates

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.

Zheng, Liheng; Chan, Anthony; Albert, Jay; Elkington, Scot; Koller, Josef; Horne, Richard; Glauert, Sarah; Meredith, Nigel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2014

YEAR: 2014     DOI: 10.1002/jgra.v119.910.1002/2014JA020127

adiabatic invariant coordinates; diffusion equation; fully 3-D model; Radiation belt; stochastic differential equation


Review of modeling of losses and sources of relativistic electrons in the outer radiation belt II: Local acceleration and loss

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

Local Acceleration due to Wave-Particle Interaction

Review of modeling of losses and sources of relativistic electrons in the outer radiation belt I: Radial transport

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

Radial Transport


Slot region electron loss timescales due to plasmaspheric hiss and lightning-generated whistlers

[1] Energetic electrons (E > 100 keV) in the Earth\textquoterights radiation belts undergo Doppler-shifted cyclotron resonant interactions with a variety of whistler mode waves leading to pitch angle scattering and subsequent loss to the atmosphere. In this study we assess the relative importance of plasmaspheric hiss and lightning-generated whistlers in the slot region and beyond. Electron loss timescales are determined using the Pitch Angle and energy Diffusion of Ions and Electrons (PADIE) code with global models of the spectral distributions of the wave power based on CRRES observations. Our results show that plasmaspheric hiss propagating at small and intermediate wave normal angles is a significant scattering agent in the slot region and beyond. In contrast, plasmaspheric hiss propagating at large wave normal angles and lightning-generated whistlers do not contribute significantly to radiation belt loss. The loss timescale of 2 MeV electrons due to plasmaspheric hiss propagating at small and intermediate wave normal angles in the center of the slot region (L = 2.5) lies in the range 1\textendash10 days, consistent with recent Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) observations. Wave turbulence in space, which is responsible for the generation plasmaspheric hiss, thus leads to the formation of the slot region. During active periods, losses due to plasmaspheric hiss may occur on a timescale of 1 day or less for a wide range of energies, 200 keV < E < 1 MeV, in the region 3.5 < L < 4.0. Plasmaspheric hiss may thus also be a significant loss process in the inner region of the outer radiation belt during magnetically disturbed periods.

Meredith, Nigel; Horne, Richard; Glauert, Sarah; Anderson, Roger;

Published by: Journal of Geophysical Research      Published on: 08/2007

YEAR: 2007     DOI: 10.1029/2007JA012413

Local Loss due to VLF/ELF/EMIC Waves

Refilling of the slot region between the inner and outer electron radiation belts during geomagnetic storms

[1] Energetic electrons (>=50 keV) are injected into the slot region (2 < L < 4) between the inner and outer radiation belts during the early recovery phase of geomagnetic storms. Enhanced convection from the plasma sheet can account for the storm-time injection at lower energies but does not explain the rapid appearance of higher-energy electrons (>=150 keV). The effectiveness of either radial diffusion (driven by enhanced ULF waves) or local acceleration (during interactions with enhanced whistler mode chorus emissions), as a potential source for refilling the slot at higher energies, is analyzed for observed conditions during the early recovery phase of the 10 October 1990 storm. We demonstrate that local acceleration, driven by observed chorus emissions, can account for the rapid enhancement in 200\textendash700 keV electrons in the outer slot region near L = 3.3. Radial diffusion is much less effective but may partially contribute to the flux enhancement at lower L. Subsequent outward expansion of the plasmapause during the storm recovery phase effectively terminates local wave acceleration in the slot and prevents acceleration to energies higher than \~700 keV. A statistical analysis of energetic electron flux enhancements and wave and plasma properties over the entire CRRES mission supports the concept of local wave acceleration as a dominant process for refilling the slot during the main and early recovery phase of storms. For moderate storms, the injection process naturally becomes less effective at energies >=1 MeV, due to the longer wave acceleration times and additional precipitation loss from scattering by electromagnetic ion cyclotron waves. However, during extreme events when the plasmapause remains compressed for several days, conditions may occur to allow wave acceleration to multi-MeV energies at locations normally associated with the slot.

Thorne, R.; Shprits, Y; Meredith, N.; Horne, R.; Li, W.; Lyons, L.;

Published by: Journal of Geophysical Research      Published on: 06/2007

YEAR: 2007     DOI: 10.1029/2006JA012176

Shock-Induced Transport. Slot Refilling and Formation of New Belts.

Timescales for radiation belt electron acceleration and loss due to resonant wave-particle interactions: 1. Theory

Radiation belt electrons can interact with various modes of plasma wave in their drift orbits about the Earth, including whistler-mode chorus outside the plasmasphere, and both whistler-mode hiss and electromagnetic ion cyclotron waves inside the plasmasphere. Electrons undergo gyroresonant diffusion in their interactions with these waves. To determine the timescales for electron momentum diffusion and pitch angle diffusion, we develop bounce-averaged quasi-linear resonant diffusion coefficients for field-aligned electromagnetic waves in a hydrogen or multi-ion (H+, He+, O+) plasma. We assume that the Earth\textquoterights magnetic field is dipolar and that the wave frequency spectrum is Gaussian. Evaluation of the diffusion coefficients requires the solution of a sixth-order polynomial equation for the resonant wave frequencies in the case of a multi-ion (H+, He+, O+) plasma, compared to the solution of a fourth-order polynomial equation for a hydrogen plasma. In some cases, diffusion coefficients for field-aligned waves can provide a valuable approximation for diffusion rates for oblique waves calculated using higher-order resonances. Bounce-averaged diffusion coefficients for field-aligned waves can be evaluated generally in minimal CPU time and can therefore be profitably incorporated into comprehensive kinetic radiation belt codes.

Summers, D.; Ni, Binbin; Meredith, Nigel;

Published by: Journal of Geophysical Research      Published on: 04/2007

YEAR: 2007     DOI: 10.1029/2006JA011801

Local Acceleration due to Wave-Particle Interaction


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

Local Loss due to VLF/ELF/EMIC Waves

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

Local Acceleration due to Wave-Particle Interaction


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

Local Acceleration due to Wave-Particle Interaction


Energization of relativistic electrons in the presence of ULF power and MeV microbursts: Evidence for dual ULF and VLF acceleration

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

Local Acceleration due to Wave-Particle Interaction

Evidence for chorus-driven electron acceleration to relativistic energies from a survey of geomagnetically disturbed periods

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

Local Acceleration due to Wave-Particle Interaction

Statistical analysis of relativistic electron energies for cyclotron resonance with EMIC waves observed on CRRES

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.

Meredith, Nigel;

Published by: Journal of Geophysical Research      Published on: 06/2003

YEAR: 2003     DOI: 10.1029/2002JA009700

Local Loss due to VLF/ELF/EMIC Waves


The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere

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

Substorm Injections