Bibliography

Can Earth’s magnetotail plasma sheet produce a source of relativistic electrons for the radiation belts?

Abstract Simultaneous observations from Van Allen Probes (RBSP) in Earth’s outer radiation belt (∼4-6 RE) and Magnetospheric Multiscale (MMS) in the magnetotail plasma sheet at >20 RE geocentric distance are used to compare relative levels of relativistic electron phase space density (PSD) for constant values of the first adiabatic invariant, M. We present new evidence from two events showing: i) at times, there is sufficient PSD in the central plasma sheet to provide a source of >1 MeV electrons into the outer belt; ii) the most intense levels of relativistic electrons are not accelerated in the solar wind or transported from the inner magnetosphere and thus must be accelerated rapidly (within ∼minutes or less) and efficiently across a broad region of the magnetotail itself; and iii) the highest intensity relativistic electrons observed by MMS were confined within only the central plasma sheet. The answer to the title question here is: yes, it can, however whether Earth’s plasma sheet actually does provide a source of several 100s keV to >1 MeV electrons to the outer belt and how often it does so remain important outstanding questions.

Turner, Drew; Cohen, Ian; Michael, Adam; Sorathia, Kareem; Merkin, Slava; Mauk, Barry; Ukhorskiy, Sasha; Murphy, Kyle; Gabrielse, Christine; Boyd, Alexander; Fennell, Joseph; Blake, Bernard; Claudepierre, Seth; Drozdov, Alexander; Jaynes, Allison; Ripoll, Jean-Francois; Reeves, Geoffrey;

Published by: Geophysical Research Letters      Published on: 09/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2021GL095495

Radiation belts; plasma sheet; Particle acceleration; relativistic electrons; inner magnetosphere; magnetotail; Van Allen Probes

Electromagnetic power of lightning superbolts from Earth to space

Lightning superbolts are the most powerful and rare lightning events with intense optical emission, first identified from space. Superbolt events occurred in 2010-2018 could be localized by extracting the high energy tail of the lightning stroke signals measured by the very low frequency ground stations of the World-Wide Lightning Location Network. Here, we report electromagnetic observations of superbolts from space using Van Allen Probes satellite measurements, and ground measurements, and with two events measured both from ground and space. From burst-triggered measurements, we compute electric and magnetic power spectral density for very low frequency waves driven by superbolts, both on Earth and transmitted into space, demonstrating that superbolts transmit 10-1000 times more powerful very low frequency waves into space than typical strokes and revealing that their extreme nature is observed in space. We find several properties of superbolts that notably differ from most lightning flashes; a more symmetric first ground-wave peak due to a longer rise time, larger peak current, weaker decay of electromagnetic power density in space with distance, and a power mostly confined in the very low frequency range. Their signal is absent in space during day times and is received with a long-time delay on the Van Allen Probes. These results have implications for our understanding of lightning and superbolts, for ionosphere-magnetosphere wave transmission, wave propagation in space, and remote sensing of extreme events.

Ripoll, J.-F.; Farges, T.; Malaspina, D.; Cunningham, G.; Lay, E.; Hospodarsky, G.; Kletzing, C.; Wygant, J.;

Published by: Nature Communications      Published on: 06/2021

YEAR: 2021     DOI: https://doi.org/10.1038/s41467-021-23740-6

Van Allen Probes

Observations and simulations of dropout events and flux decays in October 2013: Comparing MEO equatorial with LEO polar orbit

Abstract We compare ESA PROBA-V observations of electron flux at LEO with those from the NASA Van Allen Probes mostly at MEO for October 2013. Dropouts are visible at all energy during 4 storms from both satellites. Equatorial trapped electron fluxes are higher than at LEO by 102 (<1 MeV) to 105 (>2.5 MeV). We observe a quite isotropic structure of the outer belt during quiet times, contrary to the inner belt, and pitch angle dependence of high energy injection. We find very good overlap of the outer belt at MEO and LEO at ∼0.5 MeV. We use test-particle simulations of the energetic electrons trapped in the terrestrial magnetic field to study the outer radiation belt electron flux changes during geomagnetic storms. We show that the Dst (Disturbance storm time) effect during the main phase of a geomagnetic storm results in a betatron mechanism causing outward radial drift and a deceleration of the electrons. This outward drift motion is energy independent, pitch angle dependent, and represent a significant distance (∼1 L-shell at L=5 for moderate storms). At fixed L-shell, this causes a decay of the LEO precipitating flux (adiabatic outward motion), followed by a return to the normal state (adiabatic inward motion) during main and recovery phases. Dst effect, associated with magnetopause shadowing and radial diffusion can explain the main characteristics of outer radiation belt electron dropouts in October 2013. We also use Fokker-Planck simulations with event-driven diffusion coefficients at high temporal resolution, in order to distinguish instantaneous loss from the gradual scattering that depopulates the slot region and the outer belt after storms. Simulations reproduce the slot formation and the gradual loss in the outer belt. The typical energy-dependence of these losses leads to the absence of scattering for relativistic and ultra-relativistic electrons in the outer belt, oppositely to dropouts.

Pierrard, V.; Ripoll, J.-F.; Cunningham, G.; Botek, E.; Santolik, O.; Thaller, S.; Kurth, W.; Cosmides, M.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA028850

Radiation belts; relativistic electrons; Geomagnetic storms; energetic particles; Van Allen Probes

First Direct Observations of Propagation of Discrete Chorus Elements From the Equatorial Source to Higher Latitudes, Using the Van Allen Probes and Arase Satellites

Whistler mode chorus waves have recently been established as the most likely candidate for scattering relativistic electrons to produce the electron microbursts observed by low altitude satellites and balloons. These waves would have to propagate from the equatorial source region to significantly higher magnetic latitude in order to scatter electrons of these relativistic energies. This theoretically proposed propagation has never been directly observed. We present the first direct observations of the same discrete rising tone chorus elements propagating from a near equatorial (Van Allen Probes) to an off-equatorial (Arase) satellite. The chorus is observed first on the more equatorial satellite and is found to be more oblique and significantly attenuated at the off-equatorial satellite. This is consistent with the prevailing theory of chorus propagation and with the idea that chorus must propagate from the equatorial source region to higher latitudes. Ray tracing of chorus at the observed frequencies confirms that these elements could be generated parallel to the field at the equator, and propagate through the medium unducted to Van Allen Probes A and then to Arase with the observed time delay, and have the observed obliquity and intensity at each satellite.

Colpitts, Chris; Miyoshi, Yoshizumi; Kasahara, Yoshiya; Delzanno, Gian; Wygant, John; Cattell, Cynthia; Breneman, Aaron; Kletzing, Craig; Cunningham, Greg; Hikishima, Mitsuru; Matsuda, Shoya; Katoh, Yuto; Ripoll, Jean-Francois; Shinohara, Iku; Matsuoka, Ayako;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2020

YEAR: 2020     DOI: https://doi.org/10.1029/2020JA028315

Chorus; wave; propagation; Simultaneous observations; Radiation belt; Van Allen Probes

Analysis of Electric and Magnetic Lightning-Generated Wave Amplitudes Measured by the Van Allen Probes

Abstract We provide a statistical analysis of both electric and magnetic field wave amplitudes of very low frequency lightning-generated waves (LGWs) based on the equivalent of 11.5 years of observations made by the Van Allen Probes encompassing ~24.6 × 106 survey mode measurements. We complement this analysis with data from the ground-based World Wide Lightning Location Network to explore differences between satellite and ground-based measurements. LGW mean amplitudes are found to be low compared with other whistler mode waves (1 ± 1.6 pT and 19 ± 59 μV/m). Extreme events (1/5,000) can reach 100 pT and contributes strongly to the mean power below L = 2. We find excellent correlations between World Wide Lightning Location Network-based power and wave amplitudes in space at various longitudes. We reveal strong dayside ionospheric damping of the LGW electric field. LGW amplitudes drop for L < 2, contrary to the Earth s intense equatorial lightning activity. We conclude that it is difficult for equatorial LGW to propagate and remain at L < 2.

Ripoll, J.-F.; Farges, T.; Malaspina, D.; Lay, E.; Cunningham, G.; Hospodarsky, G.; Kletzing, C.; Wygant, J.;

Published by: Geophysical Research Letters      Published on: 03/2020

YEAR: 2020     DOI: 10.1029/2020GL087503

lightning-generated waves; electric wave power; magnetic wave power; WWLLN database; Radiation belts; Van Allen Probes

Particle Dynamics in the Earth\textquoterights Radiation Belts: Review of Current Research and Open Questions

The past decade transformed our observational understanding of energetic particle processes in near-Earth space. An unprecedented suite of observational systems were in operation including the Van Allen Probes, Arase, MMS, THEMIS, Cluster, GPS, GOES, and LANL-GEO magnetospheric missions. They were supported by conjugate low-altitude measurements on spacecraft, balloons, and ground-based arrays. Together these significantly improved our ability to determine and quantify the mechanisms that control the build-up and subsequent variability of energetic particle intensities in the inner magnetosphere. The high-quality data from NASA\textquoterights Van Allen Probes are the most comprehensive in-situ measurements ever taken in the near-Earth space radiation environment. These observations, coupled with recent advances in radiation belt theory and modeling, including dramatic increases in computational power, has ushered in a new era, perhaps a \textquotedblleftgolden era,\textquotedblright in radiation belt research. We have edited a Journal of Geophysical Research: Space Science Special Collection dedicated to Particle Dynamics in the Earth\textquoterights Radiation Belts in which we gather the most recent scientific findings and understanding of this important region of geospace. This collection includes the results presented at the American Geophysical Union Chapman International Conference in Cascais, Portugal (03/2018) and many other recent and relevant contributions. The present article introduces and review the context, current research, and main questions that motivate modern radiation belt research divided into the following topics: (1) particle acceleration and transport, (2) particle loss, (3) the role of nonlinear processes, (4) new radiation belt modeling capabilities and the quantification of model uncertainties, and (5) laboratory plasma experiments.

Ripoll, Jean-Francois; Claudepierre, Seth; Ukhorskiy, Sasha; Colpitts, Chris; Li, Xinlin; Fennell, Joe; Crabtree, Chris;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2019

YEAR: 2019     DOI: 10.1029/2019JA026735

inner magnetosphere; laboratory plasma experiments; Particle acceleration; particle loss; Radiation belts; Van Allen Probes

Local and Statistical Maps of Lightning-Generated Wave Power Density Estimated at the Van Allen Probes Footprints From the World-Wide Lightning Location Network Database

We propose a new method that uses the World-Wide Lightning Location Network (WWLLN) to estimate both the local and the drift lightning power density at the Van Allen Probes footprints during 4.3 years (~2 \texttimes 108 strokes.). The ratio of the drift power density to the local power density defines a time-resolved WWLLN-based model of lightning-generated wave (LGW) power density ratio, RWWLLN. RWWLLNis computed every ~34 s. This ratio multiplied by the time-resolved LGW intensity measured by the Probes allows direct computation of pitch angle diffusion coefficients used in radiation belt codes. Statistical analysis shows the median power density ratio is urn:x-wiley:00948276:media:grl58808:grl58808-math-0001 over the Americas. Elsewhere, urn:x-wiley:00948276:media:grl58808:grl58808-math-0002 in general. Over oceans, urn:x-wiley:00948276:media:grl58808:grl58808-math-0003 is larger than ~10. urn:x-wiley:00948276:media:grl58808:grl58808-math-1003 varies with season, urn:x-wiley:00948276:media:grl58808:grl58808-math-0083 ~ 2.5 from winter to summer. The yearly-median urn:x-wiley:00948276:media:grl58808:grl58808-math-0004 decays as urn:x-wiley:00948276:media:grl58808:grl58808-math-0005. The strong geographical and temporal variation should be kept in assessing effects in space. RWWLLN > 1 suggests significant LGW effects in the inner belt.

Ripoll, J.-F.; Farges, T.; Lay, E.; Cunningham, G.;

Published by: Geophysical Research Letters      Published on: 03/2019

YEAR: 2019     DOI: 10.1029/2018GL081146

drift wave power density; lightning power density; lightning-generated waves; occurrence rate; Radiation belts; Van Allen Probes; WWLLN database

The Response of Earth\textquoterights Electron Radiation Belts to Geomagnetic Storms: Statistics From the Van Allen Probes Era Including Effects From Different Storm Drivers

A statistical study was conducted of Earth\textquoterights radiation belt electron response to geomagnetic storms using NASA\textquoterights Van Allen Probes mission. Data for electrons with energies ranging from 30 keV to 6.3 MeV were included and examined as a function of L-shell, energy, and epoch time during 110 storms with SYM-H <=-50 nT during September 2012 to September 2017 (inclusive). The radiation belt response revealed clear energy and L-shell dependencies, with tens of keV electrons enhanced at all L-shells (2.5 <= L <= 6) in all storms during the storm commencement and main phase and then quickly decaying away during the early recovery phase, low hundreds of keV electrons enhanced at lower L-shells (~3 <= L <= ~4) in upward of 90\% of all storms and then decaying gradually during the recovery phase, and relativistic electrons throughout the outer belt showing main phase dropouts with subsequent and generally unpredictable levels of replenishment during the recovery phase. Compared to prestorm levels, electrons with energies >1 MeV also revealed a marked increase in likelihood of a depletion at all L-shells through the outer belt (3.5 <= L <= 6). Additional statistics were compiled revealing the storm time morphology of the radiation belts, confirming the aforementioned qualitative behavior. Considering storm drivers in the solar wind: storms driven by coronal mass ejection (CME) shocks/sheaths and CME ejecta only are most likely to result in a depletion of >1-MeV electrons throughout the outer belt, while storms driven by full CMEs and stream interaction regions are most likely to produce an enhancement of MeV electrons at lower (L < ~5) and higher (L > ~4.5) L-shells, respectively. CME sheaths intriguingly result in a distinct enhancement of ~1-MeV electrons around L~5.5, and on average, CME sheaths and stream interaction regions result in double outer belt structures.

Turner, D.; Kilpua, E.; Hietala, H.; Claudepierre, S.; O\textquoterightBrien, T.; Fennell, J.; Blake, J.; Jaynes, A.; Kanekal, S.; Baker, D.; Spence, H.; Ripoll, J.-F.; Reeves, G.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2019

YEAR: 2019     DOI: 10.1029/2018JA026066

energetic particles; Geomagnetic storms; inner magnetosphere; Radiation belts; relativistic electrons; Van Allen Probes; wave-particle interactions

Observations and Fokker-Planck simulations of the L-shell, energy, and pitch-angle structure of Earth\textquoterights electron radiation belts during quiet times

The evolution of the radiation belts in L-shell (L), energy (E), and equatorial pitch-angle (α0) is analyzed during the calm 11-day interval (March 4 \textendashMarch 15) following the March 1 storm 2013. Magnetic Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker-Planck simulations combined with consistent event-driven scattering modeling from whistler mode hiss waves. Three (L, E, α0)-regions persist through 11 days of hiss wave scattering; the pitch-angle dependent inner belt core (L~<2.2 and E<700 keV), pitch-angle homogeneous outer belt low-energy core (L>~5 and E~<100 keV), and a distinct pocket of electrons (L~[4.5, 5.5] and E~[0.7, 2] MeV). The pitch-angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α0~<60\textdegree, E>100 keV, 3.5

Ripoll, -F.; Loridan, V.; Denton, M.; Cunningham, G.; Reeves, G.; ik, O.; Fennell, J.; Turner, D.; Drozdov, A; Villa, J.; Shprits, Y; Thaller, S.; Kurth, W.; Kletzing, C.; Henderson, M.; Ukhorskiy, A;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2018

YEAR: 2018     DOI: 10.1029/2018JA026111

electron lifetime; hiss waves; pitch-angle diffusion coefficient; Radiation belts; Van Allen Probes; wave particle interactions

Variation in Plasmaspheric Hiss Wave Power With Plasma Density

Plasmaspheric hiss waves are commonly observed in the inner magnetosphere. These waves efficiently scatter electrons, facilitating their precipitation into the atmosphere. Predictive inner magnetosphere simulations often model hiss waves using parameterized empirical maps of observed hiss power. These maps nearly always include parameterization by magnetic L value. In this work, data from the Van Allen Probes are used to compare variation in hiss wave power with variation in both L value and cold plasma density. It is found that for L> 2.5, plasmaspheric hiss wave power increases with plasma density. For L> 3, this increase is stronger and occurs regardless of L value and for all local times. This result suggests that the current paradigm for parameterizing hiss wave power in many magnetospheric simulations may need to be revisited and that a new parameterization in terms of plasma density rather than L value should be explored.

Malaspina, David; Ripoll, Jean-Francois; Chu, Xiangning; Hospodarsky, George; Wygant, John;

Published by: Geophysical Research Letters      Published on: 09/2018

YEAR: 2018     DOI: 10.1029/2018GL078564

inner magnetosphere; Plasmaspheric Hiss; Radiation belts; Van Allen Probes; Wave models

Effects of whistler mode hiss waves in March 2013

We present simulations of the loss of radiation belt electrons by resonant pitch angle diffusion caused by whistler mode hiss waves for March 2013. Pitch angle diffusion coefficients are computed from the wave properties and the ambient plasma data obtained by the Van Allen Probes with a resolution of 8 hours and 0.1 L-shell. Loss rates follow a complex dynamic structure, imposed by the wave and plasma properties. Hiss effects can be strong, with minimum lifetimes (of ~1 day) moving from energies of ~100 keV at L~5 up to ~2 MeV at L~2, and stop abruptly, similarly to the observed energy-dependent inner belt edge. Periods when the plasmasphere extends beyond L~5 favor long-lasting hiss losses from the outer belt. Such loss rates are embedded in a reduced Fokker-Planck code and validated against MagEIS observations of the belts at all energy. Results are complemented with a sensitivity study involving different radial diffusion and lifetime models. Validation is carried out globally at all L-shells and energies. The good agreement between simulations and observations demonstrates that hiss waves drive the slot formation during quiet times. Combined with transport, they sculpt the energy-structure of the outer belt into an "S-shape". Low energy electrons (<0.3 MeV) are less subject to hiss scattering below L=4. In contrast, 0.3-1.5 MeV electrons evolve in a environment that depopulates them as they migrate from L~5 to L~2.5. Ultra-relativistic electrons are not affected by hiss losses until L~2-3.

Ripoll, J.-F.; Santol?k, O.; Reeves, G.; Kurth, W.; Denton, M.; Loridan, V.; Thaller, S.; Kletzing, C.; Turner, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2017

YEAR: 2017     DOI: 10.1002/2017JA024139

diffusion coefficients; electron lifetimes; energy-structure; Radiation belts; Van Allen Probes; Whistler-mode hiss

On the Time Needed to Reach an Equilibrium Structure of the Radiation Belts

In this study, we complement the notion of equilibrium states of the radiation belts with a discussion on the dynamics and time needed to reach equilibrium. We solve for the equilibrium states obtained using 1D radial diffusion with recently developed hiss and chorus lifetimes at constant values of Kp = 1, 3 and 6. We find that the equilibrium states at moderately low Kp, when plotted vs L-shell (L) and energy (E), display the same interesting S-shape for the inner edge of the outer belt as recently observed by the Van Allen Probes. The S-shape is also produced as the radiation belts dynamically evolve toward the equilibrium state when initialized to simulate the buildup after a massive dropout or to simulate loss due to outward diffusion from a saturated state. Physically, this shape, intimately linked with the slot structure, is due to the dependence of electron loss rate (originating from wave-particle interactions) on both energy and L-shell. Equilibrium electron flux profiles are governed by the Biot number (τDiffusion/τloss), with large Biot number corresponding to low fluxes and low Biot number to large fluxes. The time it takes for the flux at a specific (L, E) to reach the value associated with the equilibrium state, starting from these different initial states, is governed by the initial state of the belts, the property of the dynamics (diffusion coefficients), and the size of the domain of computation. Its structure shows a rather complex scissor form in the (L, E) plane. The equilibrium value (phase space density or flux) is practically reachable only for selected regions in (L, E) and geomagnetic activity. Convergence to equilibrium requires hundreds of days in the inner belt for E > 300 keV and moderate Kp (<=3). It takes less time to reach equilibrium during disturbed geomagnetic conditions (Kp >= 3), when the system evolves faster. Restricting our interest to the slot region, below L = 4, we find that only small regions in (L, E) space can reach the equilibrium value: E ~ [200, 300] keV for L = [3.7, 4] at Kp = 1, E ~ [0.6, 1] MeV for L = [3, 4] at Kp = 3, and E ~ 300 keV for L = [3.5, 4] at Kp = 6 assuming no new incoming electrons.

Ripoll, J.; Loridan, V.; Cunningham, G.; Reeves, G.; Shprits, Y;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2016

YEAR: 2016     DOI: 10.1002/2015JA022207

Radiation belts; Van Allen Probes

Reproducing the observed energy-dependent structure of Earth s electron radiation belts during storm recovery with an event-specific diffusion model

We present dynamic simulations of energy-dependent losses in the radiation belt " slot region" and the formation of the two-belt structure for the quiet days after the March 1st storm. The simulations combine radial diffusion with a realistic scattering model, based data-driven spatially and temporally-resolved whistler mode hiss wave observations from the Van Allen Probes satellites. The simulations reproduce Van Allen Probes observations for all energies and L-shells (2 to 6) including (a) the strong energy-dependence to the radiation belt dynamics (b) an energy-dependent outer boundary to the inner zone that extends to higher L-shells at lower energies and (c) an " S-shaped" energy-dependent inner boundary to the outer zone that results from the competition between diffusive radial transport and losses. We find that the characteristic energy-dependent structure of the radiation belts and slot region is dynamic and can be formed gradually in ~15 days, although the " S-shape" can also be reproduced by assuming equilibrium conditions. The highest energy electrons (E > 300 keV) of the inner region of the outer belt (L ~ 4-5) also constantly decay, demonstrating that hiss wave scattering affects the outer belt during times of extended plasmasphere. Through these simulations, we explain the full structure in energy and L-shell of the belts and the slot formation by hiss scattering during storm recovery. We show the power and complexity of looking dynamically at the effects over all energies and L-shells and the need for using data-driven and event-specific conditions.

Ripoll, J.; Reeves, G.; Cunningham, G.; Loridan, V.; Denton, M.; ik, O.; Kurth, W.; Kletzing, C.; Turner, D.; Henderson, M.; Ukhorskiy, A;

Published by: Geophysical Research Letters      Published on: 05/2016

YEAR: 2016     DOI: 10.1002/2016GL068869

electron lifetimes; electron losses; hiss waves; Radiation belts; Slot region; Van Allen Probes; wave particle interactions

Electron lifetimes from narrowband wave-particle interactions within the plasmasphere

This paper is devoted to the systematic study of electron lifetimes from narrowband wave-particle interactions within the plasmasphere. It relies on a new formulation of the bounce-averaged quasi-linear pitch angle diffusion coefficients parameterized by a single frequency, ω, and wave normal angle, θ. We first show that the diffusion coefficients scale with ω/Ωce, where Ωce is the equatorial electron gyrofrequency, and that maximal pitch angle diffusion occurs along the line α0 = π/2\textendashθ, where α0 is the equatorial pitch angle. Lifetimes are computed for L shell values in the range [1.5, 3.5] and energies, E, in the range [0.1, 6] MeV as a function of frequency and wave normal angle. The maximal pitch angle associated with a given lifetime is also given, revealing the frequencies that are able to scatter nearly equatorial pitch angle particles. The lifetimes are relatively independent of frequency and wave normal angle after taking into consideration the scaling law, with a weak dependence on wave normal angle up to 60\textendash70\textdegree, increasing to infinity as the wave normal angle approaches the resonance cone. We identify regions in the (L, E) plane in which a single wave type (hiss, VLF transmitters, or lightning-generated waves) is dominant relative to the others. We find that VLF waves dominate the lifetime for 0.2\textendash0.4 MeV at L ~ 2 and for 0.5\textendash0.8 MeV at L ~ 1.5, while hiss dominates the lifetime for 2\textendash3 MeV at L = 3\textendash3.5. The influence of lightning-generated waves is always mixed with the other two and cannot be easily differentiated. Limitations of the method for addressing effects due to restricted latitude or pitch angle domains are also discussed. Finally, for each (L, E) we search for the minimum lifetime and find that the optimal frequency that produces this lifetime increases as L diminishes. Restricting the search to very oblique waves, which could be emitted during the Demonstration and Science Experiments satellite mission, we find that the optimal frequency is always close to 0.16Ωce.

Ripoll, J.-F.; Albert, J.; Cunningham, G.;

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

YEAR: 2014     DOI: 10.1002/2014JA020217

DSX; electron; narrowband; plasmasphere; wave-particle interactions

On long decays of electrons in the vicinity of the slot region observed by HEO3

Long decay periods of electron counts, which follow abrupt rises and last from weeks to months, have been observed by the HEO3 spacecraft in the vicinity of the slot region between the years 1998 and 2007. During the most stable decay periods as selected, e-folding timescales are extracted and statistically analyzed from observations as a function of L-shell and electron energy. A challenge is to reproduce the observed timescales from simulations of pitch angle diffusion by three acting waves\textendashthe plasmaspheric hiss, lightning-generated whistlers, and VLF transmitter waves. We perform full numerical simulations to accurately compute electron lifetimes. We choose to use the method and wave parameters proposed by Abel \& Thorne [1998] with the goal to assess whether they can reproduce lifetimes extracted from HEO observations. We show how hiss dominantly affect high energy electrons (E > 2 MeV) for L in [2, 3.5] and VLF transmitter waves control residency times of low energy electrons (<0.4 MeV) around L = 2. These interactions induce characteristic shapes of the lifetime profiles that will be discussed. We show how the wave amplitudes can be adjusted for the particular energy particles that are dominantly affected by one wave type only. Using these amplitudes, mean HEO lifetimes are reproduced within a factor 2 to 5. VLF occurrence rates and hiss amplitude turn out significantly higher than those proposed by Abel \& Thorne [1998]. The wide energy response of the sensors complicates the analysis because it blurs the electron lifetime dependence on energy, increases the overall lifetimes and reduces the differences between the different channel lifetimes. In particular, our simulations suggest the flux measured by an integrated energy sensor aboard HEO has a variable slope, i.e. a variable lifetime, during 10-20 days in our data, due to the faster decay of the low residency time particles while slower decaying particles control the steady decay. It can explain some of the multi-slopes decays observed by HEO. HEO electron long decay timescales are also compared to the timescales previously observed from SAMPEX and CRRES with differences attributed to factors such as instrument characteristic and different satellite orbits.

Ripoll, J.-F.; Chen, Y.; Fennell, J.; Friedel, R.;

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

YEAR: 2014     DOI: 10.1002/2014JA020449

electron; HEO; Slot region