Bibliography

Competing source and loss mechanisms due to wave-particle interactions in Earth\textquoterights outer radiation belt during the 30 September to 3 October 2012 geomagnetic storm

Drastic variations of Earth\textquoterights outer radiation belt electrons ultimately result from various competing source, loss, and transport processes, to which wave-particle interactions are critically important. Using 15 spacecraft including NASA\textquoterights Van Allen Probes, THEMIS, and SAMPEX missions and NOAA\textquoterights GOES and POES constellations, we investigated the evolution of the outer belt during the strong geomagnetic storm of 30 September to 3 October 2012. This storm\textquoterights main phase dropout exhibited enhanced losses to the atmosphere at L* < 4, where the phase space density (PSD) of multi-MeV electrons dropped by over an order of magnitude in <4 h. Based on POES observations of precipitating >1 MeV electrons and energetic protons, SAMPEX >1 MeV electrons, and ground observations of band-limited Pc1-2 wave activity, we show that this sudden loss was consistent with pitch angle scattering by electromagnetic ion cyclotron waves in the dusk magnetic local time sector at 3 < L* < 4. At 4 < L* < 5, local acceleration was also active during the main and early recovery phases, when growing peaks in electron PSD were observed by both Van Allen Probes and THEMIS. This acceleration corresponded to the period when IMF Bz was southward, the AE index was >300 nT, and energetic electron injections and whistler-mode chorus waves were observed throughout the inner magnetosphere for >12 h. After this period, Bz turned northward, and injections, chorus activity, and enhancements in PSD ceased. Overall, the outer belt was depleted by this storm. From the unprecedented level of observations available, we show direct evidence of the competitive nature of different wave-particle interactions controlling relativistic electron fluxes in the outer radiation belt.

Turner, D.; Angelopoulos, V.; Li, W.; Bortnik, J.; Ni, B.; Ma, Q.; Thorne, R.; Morley, S.; Henderson, M.; Reeves, G.; Usanova, M.; Mann, I.; Claudepierre, S.; Blake, J.; Baker, D.; Huang, C.-L.; Spence, H.; Kurth, W.; Kletzing, C.; Rodriguez, J.;

Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014

YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2014JA019770

Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations

The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth\textquoterights radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 \textpm 0.5). This reveals graphically that both \textquotedblleftcompeting\textquotedblright mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.

Baker, D.; Jaynes, A.; Li, X.; Henderson, M.; Kanekal, S.; Reeves, G.; Spence, H.; Claudepierre, S.; Fennell, J.; Hudson, M.; Thorne, R.; Foster, J.; Erickson, P.; Malaspina, D.; Wygant, J.; Boyd, A.; Kletzing, C.; Drozdov, A.; Shprits, Y;

Published by: Geophysical Research Letters Published on: 03/2014

YEAR: 2014 DOI: 10.1002/2013GL058942

Resonant scattering of energetic electrons by unusual low-frequency hiss

We quantify the resonant scattering effects of the unusual low-frequency dawnside plasmaspheric hiss observed on 30 September 2012 by the Van Allen Probes. In contrast to normal (~100\textendash2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ~20\textendash200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ~50\textendash200 keV electrons and produces more pronounced pancake distributions of ~50\textendash100 keV electrons. It is demonstrated that such unusual low-frequency hiss, even with a duration of a couple of hours, plays a particularly important role in the decay and loss process of energetic electrons, resulting in shorter electron lifetimes for ~50\textendash400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections.

Ni, Binbin; Li, Wen; Thorne, Richard; Bortnik, Jacob; Ma, Qianli; Chen, Lunjin; Kletzing, Craig; Kurth, William; Hospodarsky, George; Reeves, Geoffrey; Spence, Harlan; Blake, Bernard; Fennell, Joseph; Claudepierre, Seth;

Published by: Geophysical Research Letters Published on: 03/2014

YEAR: 2014 DOI: 10.1002/2014GL059389

Magnetosonic wave excitation by ion ring distributions in the Earth\textquoterights inner magnetosphere

Combining Time History of Events and Macroscale Interaction during Substorms (THEMIS) wave and particle observations and a quantitative calculation of linear wave growth rate, we demonstrate that magnetosonic (MS) waves can be locally excited by ion ring distributions in the Earth\textquoterights magnetosphere when the ion ring energy is comparable to the local Alfven energy. MS waves in association with ion ring distributions were observed by THEMIS A on 24 November 2010 in the afternoon sector, both outside the plasmapause where the wave spectrum varied with fLHR and inside the plasmapause where the wave frequency band remained nearly constant. Our plasma instability analysis in three different regions shows that higher and narrow frequency band MS waves are excited locally outside the plasmapause, and lower and broad frequency band MS waves are excited in the region where the density slightly increases. However, there is no evidence for wave excitation inside the plasmapause, and wave propagation from a distant source is needed to explain their existence. The simulation of the MS wave growth rate spectra during this event agrees reasonably well with the observed wave magnetic field power spectra. We also simulated a MS wave event on 19 October 2011 in the dusk sector and found that the ion ring distribution with an ion ring energy slightly higher than the local Alfven energy can excite the typical broad band MS waves outside the plasmapause.

Ma, Qianli; Li, Wen; Chen, Lunjin; Thorne, Richard; Angelopoulos, Vassilis;

Published by: Journal of Geophysical Research: Space Physics Published on: 02/2014

YEAR: 2014 DOI: 10.1002/2013JA019591

magnetosonic waves; ring current; THEMIS observation; wave excitation

Quantifying hiss-driven energetic electron precipitation: A detailed conjunction event analysis

We analyze a conjunction event between the Van Allen Probes and the low-altitude Polar Orbiting Environmental Satellite (POES) to quantify hiss-driven energetic electron precipitation. A physics-based technique based on quasi-linear diffusion theory is used to estimate the ratio of precipitated and trapped electron fluxes (R), which could be measured by the two-directional POES particle detectors, using wave and plasma parameters observed by the Van Allen Probes. The remarkable agreement between modeling and observations suggests that this technique is applicable for quantifying hiss-driven electron scattering near the bounce loss cone. More importantly, R in the 100\textendash300 keV energy channel measured by multiple POES satellites over a broad L magnetic local time region can potentially provide the spatiotemporal evolution of global hiss wave intensity, which is essential in evaluating radiation belt electron dynamics, but cannot be obtained by in situ equatorial satellites alone.

Li, W.; Ni, B.; Thorne, R.; Bortnik, J.; Nishimura, Y.; Green, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Blake, J.; Fennell, J.; Claudepierre, S.; Gu, X.;

Published by: Geophysical Research Letters Published on: 02/2014

YEAR: 2014 DOI: 10.1002/2013GL059132

Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus

2013

Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density1, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt2, 3, but are inconsistent with acceleration by inward radial diffusive transport4, 5. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration6, 7, 8, 9, 10, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations11 obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model12, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth\textquoterights outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Thorne, R.; Li, W.; Ni, B.; Ma, Q.; Bortnik, J.; Chen, L.; Baker, D.; Spence, H.; Reeves, G.; Henderson, M.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Blake, J.; Fennell, J.; Claudepierre, S.; Kanekal, S.;

Published by: Nature Published on: 12/2013

YEAR: 2013 DOI: 10.1038/nature12889

Resonant scattering and resultant pitch angle evolution of relativistic electrons by plasmaspheric hiss

We perform a comprehensive analysis to evaluate hiss-induced scattering effect on the pitch angle evolution and associated decay processes of relativistic electrons. The results show that scattering by the equatorial, highly oblique hiss component is negligible. Quasi-parallel approximation is good for evaluation of hiss-driven electron scattering rates <= 2 MeV. However, realistic wave propagation angles as a function of latitude must be considered to accurately quantify hiss scattering rates above 2 MeV, and ambient plasma density is also a critical parameter. While the first-order cyclotron and the Landau resonances are dominant for hiss scattering < 2 MeV electrons, higher-order resonances become important and even dominant at intermediate pitch angles for ultrarelativistic (>= 3 MeV) electrons. Hiss-induced electron pitch angle evolution shows an initially rapid transport from high to lower pitch angles, with a gradual approach toward equilibrium, and a final exponential decay as a whole. Although hiss scattering rates near the loss cone control the pitch angle evolution and the ultimate loss of ultrarelativistic electrons, the scattering bottleneck significantly affects the loss rate and leads to characteristic top hat-shaped pitch angle distributions at energies < 1 MeV. Decay timescales are on the order of a few days, tens of days, and > 100 days for 500 keV, 2 MeV, and 5 MeV electrons, respectively, consistent with recent observations from the Van Allen Probes and indicating that scattering by hiss can realistically account for the long-term loss process and the pitch angle evolution of relativistic electrons in the plasmasphere following storm time injections.

Ni, Binbin; Bortnik, Jacob; Thorne, Richard; Ma, Qianli; Chen, Lunjin;

Published by: Journal of Geophysical Research: Space Physics Published on: 12/2013

YEAR: 2013 DOI: 10.1002/2013JA019260

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

The Electric and Magnetic Field Instrument and Integrated Science (EMFISIS) investigation on the NASA Radiation Belt Storm Probes (now named the Van Allen Probes) mission provides key wave and very low frequency magnetic field measurements to understand radiation belt acceleration, loss, and transport. The key science objectives and the contribution that EMFISIS makes to providing measurements as well as theory and modeling are described. The key components of the instruments suite, both electronics and sensors, including key functional parameters, calibration, and performance, demonstrate that EMFISIS provides the needed measurements for the science of the RBSP mission. The EMFISIS operational modes and data products, along with online availability and data tools provide the radiation belt science community with one the most complete sets of data ever collected.

Kletzing, C.; Kurth, W.; Acuna, M.; MacDowall, R.; Torbert, R.; Averkamp, T.; Bodet, D.; Bounds, S.; Chutter, M.; Connerney, J.; Crawford, D.; Dolan, J.; Dvorsky, R.; Hospodarsky, G.; Howard, J.; Jordanova, V.; Johnson, R.; Kirchner, D.; Mokrzycki, B.; Needell, G.; Odom, J.; Mark, D.; Pfaff, R.; Phillips, J.; Piker, C.; Remington, S.; Rowland, D.; Santolik, O.; Schnurr, R.; Sheppard, D.; Smith, C.; Thorne, R.; Tyler, J.;

Published by: Space Science Reviews Published on: 11/2013

YEAR: 2013 DOI: 10.1007/s11214-013-9993-6

Science Goals and Overview of the Energetic Particle, Composition, and Thermal Plasma (ECT) Suite on NASA\textquoterights Radiation Belt Storm Probes (RBSP) Mission

The Radiation Belt Storm Probes (RBSP)-Energetic Particle, Composition, and Thermal Plasma (ECT) suite contains an innovative complement of particle instruments to ensure the highest quality measurements ever made in the inner magnetosphere and radiation belts. The coordinated RBSP-ECT particle measurements, analyzed in combination with fields and waves observations and state-of-the-art theory and modeling, are necessary for understanding the acceleration, global distribution, and variability of radiation belt electrons and ions, key science objectives of NASA\textquoterights Living With a Star program and the Van Allen Probes mission. The RBSP-ECT suite consists of three highly-coordinated instruments: the Magnetic Electron Ion Spectrometer (MagEIS), the Helium Oxygen Proton Electron (HOPE) sensor, and the Relativistic Electron Proton Telescope (REPT). Collectively they cover, continuously, the full electron and ion spectra from one eV to 10\textquoterights of MeV with sufficient energy resolution, pitch angle coverage and resolution, and with composition measurements in the critical energy range up to 50 keV and also from a few to 50 MeV/nucleon. All three instruments are based on measurement techniques proven in the radiation belts. The instruments use those proven techniques along with innovative new designs, optimized for operation in the most extreme conditions in order to provide unambiguous separation of ions and electrons and clean energy responses even in the presence of extreme penetrating background environments. The design, fabrication and operation of ECT spaceflight instrumentation in the harsh radiation belt environment ensure that particle measurements have the fidelity needed for closure in answering key mission science questions. ECT instrument details are provided in companion papers in this same issue. In this paper, we describe the science objectives of the RBSP-ECT instrument suite on the Van Allen Probe spacecraft within the context of the overall mission objectives, indicate how the characteristics of the instruments satisfy the requirements to achieve these objectives, provide information about science data collection and dissemination, and conclude with a description of some early mission results.

Spence, H.; Reeves, G.; Baker, D.; Blake, J.; Bolton, M.; Bourdarie, S.; Chan, A.; Claudpierre, S.; Clemmons, J.; Cravens, J.; Elkington, S.; Fennell, J.; Friedel, R.; Funsten, H.; Goldstein, J.; Green, J.; Guthrie, A.; Henderson, M.; Horne, R.; Hudson, M.; Jahn, J.-M.; Jordanova, V.; Kanekal, S.; Klatt, B.; Larsen, B.; Li, X.; MacDonald, E.; Mann, I.R.; Niehof, J.; O\textquoterightBrien, T.; Onsager, T.; Salvaggio, D.; Skoug, R.; Smith, S.; Suther, L.; Thomsen, M.; Thorne, R.;

Published by: Space Science Reviews Published on: 11/2013

YEAR: 2013 DOI: DOI: 10.1007/s11214-013-0007-5

Constructing the global distribution of chorus wave intensity using measurements of electrons by the POES satellites and waves by the Van Allen Probes

We adopt a physics-based technique to infer chorus wave amplitudes from the low-altitude electron population (30\textendash100 keV) measured by multiple Polar Orbiting Environmental Satellites (POES), which provide extensive coverage over a broad region in L-shell and magnetic local time (MLT). This technique is validated by analyzing conjunction events between the Van Allen Probes measuring chorus wave amplitudes near the equator and POES satellites measuring the 30\textendash100 keV electron population at the conjugate low altitudes. We apply this technique to construct the chorus wave distributions during the 8\textendash9 October storm in 2012 and demonstrate that the inferred chorus wave amplitudes agree reasonably well with conjugate measurements of chorus wave amplitudes from the Van Allen Probes. The evolution of the chorus wave intensity inferred from low-altitude electron measurements can provide real-time global estimates of the chorus wave intensity, which cannot be obtained from in situ chorus wave measurements by equatorial satellites alone, but is crucial in quantifying radiation belt electron dynamics.

Li, W.; Ni, B.; Thorne, R.; Bortnik, J.; Green, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.;

Published by: Geophysical Research Letters Published on: 09/2013

YEAR: 2013 DOI: 10.1002/grl.v40.1710.1002/grl.50920

An unusual enhancement of low-frequency plasmaspheric hiss in the outer plasmasphere associated with substorm-injected electrons

Both plasmaspheric hiss and chorus waves were observed simultaneously by the two Van Allen Probes in association with substorm-injected energetic electrons. Probe A, located inside the plasmasphere in the postdawn sector, observed intense plasmaspheric hiss, whereas Probe B observed chorus waves outside the plasmasphere just before dawn. Dispersed injections of energetic electrons were observed in the dayside outer plasmasphere associated with significant intensification of plasmaspheric hiss at frequencies down to ~20 Hz, much lower than typical hiss wave frequencies of 100\textendash2000 Hz. In the outer plasmasphere, the upper energy of injected electrons agrees well with the minimum cyclotron resonant energy calculated for the lower cutoff frequency of the observed hiss, and computed convective linear growth rates indicate instability at the observed low frequencies. This suggests that the unusual low-frequency plasmaspheric hiss is likely to be amplified in the outer plasmasphere due to the injected energetic electrons.

Li, W.; Thorne, R.; Bortnik, J.; Reeves, G.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Blake, J.; Fennell, J.; Claudepierre, S.; Wygant, J.; Thaller, S.;

Published by: Geophysical Research Letters Published on: 08/2013

YEAR: 2013 DOI: 10.1002/grl.50787

Electron Acceleration in the Heart of the Van Allen Radiation Belts

The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth\textquoterights magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA\textquoterights Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.

Reeves, G.; Spence, H.; Henderson, M.; Morley, S.; Friedel, R.; Funsten, H.; Baker, D.; Kanekal, S.; Blake, J.; Fennell, J.; Claudepierre, S.; Thorne, R.; Turner, D.; Kletzing, C.; Kurth, W.; Larsen, B.; Niehof, J.;

Published by: Science Published on: 07/2013

YEAR: 2013 DOI: 10.1126/science.1237743

Evolution and slow decay of an unusual narrow ring of relativistic electrons near L ~ 3.2 following the September 2012 magnetic storm

A quantitative analysis is performed on the decay of an unusual ring of relativistic electrons between 3 and 3.5 RE, which was observed by the Relativistic Electron Proton Telescope instrument on the Van Allen probes. The ring formed on 3 September 2012 during the main phase of a magnetic storm due to the partial depletion of the outer radiation belt for L > 3.5, and this remnant belt of relativistic electrons persisted at energies above 2 MeV, exhibiting only slow decay, until it was finally destroyed during another magnetic storm on 1 October. This long-term stability of the relativistic electron ring was associated with the rapid outward migration and maintenance of the plasmapause to distances greater than L = 4. The remnant ring was thus immune from the dynamic process, which caused rapid rebuilding of the outer radiation belt at L > 4, and was only subject to slow decay due to pitch angle scattering by plasmaspheric hiss on timescales exceeding 10\textendash20 days for electron energies above 3 MeV. At lower energies, the decay is much more rapid, consistent with the absence of a long-duration electron ring at energies below 2 MeV.

Thorne, R.; Li, W.; Ni, B.; Ma, Q.; Bortnik, J.; Baker, D.; Spence, H.; Reeves, G.; Henderson, M.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Turner, D.; Angelopoulos, V.;

Published by: Geophysical Research Letters Published on: 06/2013

YEAR: 2013 DOI: 10.1002/grl.50627

A Long-Lived Relativistic Electron Storage Ring Embedded in Earth\textquoterights Outer Van Allen Belt

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

Baker, D.; Kanekal, S.; Hoxie, V.; Henderson, M.; Li, X.; Spence, H.; Elkington, S.; Friedel, R.; Goldstein, J.; Hudson, M.; Reeves, G.; Thorne, R.; Kletzing, C.; Claudepierre, S.;

Published by: Science Published on: 04/2013

YEAR: 2013 DOI: 10.1126/science.1233518

Modeling ring current ion and electron dynamics and plasma instabilities during a high-speed stream driven storm

2012

1] The temporal and spatial development of the ring current is evaluated during the 23\textendash26 October 2002 high-speed stream (HSS) storm, using a kinetic ring current-atmosphere interactions model with self-consistent magnetic field (RAM-SCB). The effects of nondipolar magnetic field configuration are investigated on both ring current ion and electron dynamics. As the self-consistent magnetic field is depressed at large (>4RE) radial distances on the nightside during the storm main phase, the particles\textquoteright drift velocities increase, the ion and electron fluxes are reduced and the ring current is confined closer to Earth. In contrast to ions, the electron fluxes increase closer to Earth and the fractional electron energy reaches \~20\% near storm peak due to better electron trapping in a nondipolar magnetic field. The ring current contribution to Dst calculated using Biot-Savart integration differs little from the DPS relation except during quiet time. RAM-SCB simulations underestimate |SYM-H| minimum by \~25\% but reproduce very well the storm recovery phase. Increased anisotropies develop in the ion and electron velocity distributions in a self-consistent magnetic field due to energy dependent drifts, losses, and dispersed injections. There is sufficient free energy to excite whistler mode chorus, electromagnetic ion cyclotron (EMIC), and magnetosonic waves in the equatorial magnetosphere. The linear growth rate of whistler mode chorus intensifies in the postmidnight to noon sector, EMIC waves are predominantly excited in the afternoon to midnight sector, and magnetosonic waves are excited over a broad MLT range both inside and outside the plasmasphere. The wave growth rates in a dipolar magnetic field have significantly smaller magnitude and spatial extent.

Jordanova, V.; Welling, D.; Zaharia, S.; Chen, L.; Thorne, R.;

Published by: Journal of Geophysical Research Published on: 09/2012

YEAR: 2012 DOI: 10.1029/2011JA017433

2007

Dynamic evolution of energetic outer zone electrons due to wave-particle interactions during storms

[1] Relativistic electrons in the outer radiation belt are subjected to pitch angle and energy diffusion by chorus, electromagnetic ion cyclotron (EMIC), and hiss waves. Using quasi-linear diffusion coefficients for cyclotron resonance with field-aligned waves, we examine whether the resonant interactions with chorus waves produce a net acceleration or loss of relativistic electrons. We also examine the effect of pitch angle scattering by EMIC and hiss waves during the main and recovery phases of a storm. The numerical simulations show that wave-particle interactions with whistler mode chorus waves with realistic wave spectral properties result in a net acceleration of relativistic electrons, while EMIC waves, which provide very fast scattering near the edge of the loss cone, may be a dominant loss mechanism during the main phase of a storm. In addition, hiss waves are effective in scattering equatorially mirroring electrons and may be an important mechanism of transporting high pitch angle electrons toward the loss cone.

Li, W.; Shprits, Y; Thorne, R.;

Published by: Journal of Geophysical Research Published on: 10/2007

YEAR: 2007 DOI: 10.1029/2007JA012368

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.

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.

MILLAN, R; THORNE, R;

Published by: Journal of Atmospheric and Solar-Terrestrial Physics Published on: 03/2007

YEAR: 2007 DOI: 10.1016/j.jastp.2006.06.019

Observation of two distinct, rapid loss mechanisms during the 20 November 2003 radiation belt dropout event

2006

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

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

Magnetopause Losses

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

Timescale for MeV electron microburst loss during geomagnetic storms

2005

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

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

2004

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.

Shprits, Y; Thorne, R.;

Published by: Geophysical Research Letters Published on: 04/2004

YEAR: 2004 DOI: 10.1029/2004GL019591

Radial Transport

2003

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

1998

Relativistic theory of wave-particle resonant diffusion with application to electron acceleration in the magnetosphere

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

Local Acceleration due to Wave-Particle Interaction

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.

Abel, Bob; Thorne, Richard;

Published by: Journal of Geophysical Research Published on: 02/1998

YEAR: 1998 DOI: 10.1029/97JA02919

Equilibrium Structure of Radiation Belt Electrons

1973

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

Parasitic Pitch Angle Diffusion of Radiation Belt Particles by Ion Cyclotron Waves

1972

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