The past decade transformed our observational understanding of energetic particle processes in near‐Earth space. An unprecedented suite of observational systems was in operation including the Van ...Allen Probes, Arase, Magnetospheric Multiscale, Time History of Events and Macroscale Interactions during Substorms, Cluster, GPS, GOES, and Los Alamos National Laboratory‐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 buildup and subsequent variability of energetic particle intensities in the inner magnetosphere. The high‐quality data from National Aeronautics and Space Administration's 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, have ushered in a new era, perhaps a “golden era,” in radiation belt research. We have edited a Journal of Geophysical Research: Space Science Special Collection dedicated to Particle Dynamics in the Earth's 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 (March 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.
Key Points
We review and discuss current research and open questions relative to Earth's radiation belts
Aspects of modern radiation belt research concern particle acceleration and transport, particle loss, and the role of nonlinear processes
We also discuss new radiation belt modeling capabilities, the quantification of model uncertainties, and laboratory plasma experiments
A statistical study was conducted of Earth's radiation belt electron response to geomagnetic storms using NASA's 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.
Key Points
A statistical model of electron radiation belt response to storms as a function of energy and L‐shell is developed
Storm‐time morphology of the electron radiation belts is qualitatively predictable
Results are better organized when the solar wind drivers of storms are identified
The behavior of trapped electrons in the Earth's radiation belts can be described in terms of relativistic quasi‐linear diffusion by cyclotron‐resonant plasma waves. Multidimensional dynamical ...simulations of the full belts require rapid and accurate evaluations of the diffusion coefficients. Recently developed approximations already allow replacing the integration over the wave‐normal distribution by an evaluation at a carefully chosen point. An approximate, but useful, analytical formulation of diffusion coefficients is derived here in the particular limit of low frequency and moderately oblique waves. It accounts for both the integration over the wave‐normal distribution and the summation over all the relevant n‐harmonic resonances. Analytical estimates of the diffusion coefficients averaged over the bounce motion along a geomagnetic field line are also given. From them, a simplified analytical expression of the electron lifetimes is deduced. Detailed comparisons between the analytical formulas and the full numerical solutions are presented for different cases of resonant interactions of electrons with hiss waves inside the plasmasphere, demonstrating a fair agreement and providing a better understanding of the simulation results.
Key Points
Analytical equatorial quasi‐linear diffusion coefficients
Analytical bounce‐averaged diffusion coefficients
Analytical electron lifetimes
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.
We deduce the electron plasma density from the NASA Van Allen Probes Electric Field and Waves and Electric and Magnetic Field Instrument Suite and Integrated Science measurements and extract the ...plasmasphere boundaries throughout 2012–2019. We use the gradient method for locating the plasmapause at Lpp and the 100 cm−3 density threshold for the plasmasphere outer edge at L100. We show how, where, and when both Lpp and L100 coincide when the plasmapause gradient exists. L100 is demonstrated to bound the plasmasphere at large L‐shell in the dusk. The plasmasphere expands farther out than predicted from the Carpenter and Anderson (1992, https://doi.org/10.1029/91JA01548) model. We generate statistics of the plasmasphere boundaries binned by L‐shell, magnetic local time (MLT), and geomagnetic indices, leading to new models for radiation belt codes. The L100 boundary commonly varies by ∼±0.5 L, increasing with activity up to ∼±1 L, becomes MLT‐dependent for Kp > ∼2, and is preferentially steep on the night side for non‐quiet times and a wider region in the afternoon sector.
Plain Language Summary
The plasmasphere is a region of plasma extending out from the ionized upper part of the atmosphere to distances of 2–6 Earth Radii. The plasmasphere plasma is the coldest plasma (1/100–1/1,000,000 of the energy of other plasma) in the space around Earth where the particle motions are regulated by Earth's magnetic field (the magnetosphere). It is also high density, 100–10,000 times higher than elsewhere in the magnetosphere. The outer edge of the plasmasphere, called the plasmapause, typically drops from >100 to <10 cm−3 over a relatively short distance. Waves that energize radiation belt particles (chorus) are found outside the plasmasphere. Inside the plasmasphere are different waves (hiss) that cause radiation belt particles to precipitate into Earth's atmosphere. Therefore, models predicting the radiation belt's behavior need to know the plasmapause location. To predict the plasmapause position, we analyze 7 years of Van Allen Probes data to find the plasma density in two different ways, using both the 100 cm−3 density and the density gradient. We look at how their locations change with the level of geomagnetic storm activity and deduce new plasmasphere boundaries models for space weather codes.
Key Points
We deduce the electron plasma density from Electric Field and Waves and Electric and Magnetic Field Instrument Suite and Integrated Science measurements (2012–2019) and extract the plasmasphere boundaries
New plasmasphere boundary statistics and laws, binned by L, magnetic local time, and geomagnetic indices are generated to be used in space weather codes
A density‐based boundary is more frequently defined than is a gradient‐based boundary, and yields a more frequently applicable model
The loss of relativistic electrons from the Earth's radiation belts can be described in terms of the quasi‐linear pitch angle diffusion by cyclotron‐resonant waves, provided that their frequency ...spectrum is broad enough. Chorus waves at large wave‐normal angles with respect to the magnetic field are often present in CLUSTER and THEMIS measurements in the outer belt at moderate to high latitudes. An approximate analytical formulation of diffusion coefficients has been derived in the low‐frequency limit, leading to a simplified analytical expression of diffusion coefficients and lifetimes for energetic trapped electrons. Large values of the wave‐normal angles between the Gendrin and resonance angles are shown to induce important increases in diffusion, thereby strongly reducing the particle lifetimes (by almost two orders of magnitude). The analytical diffusion coefficients and lifetimes obtained here are found to be in a good agreement with full numerical calculations based on CLUSTER chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 2 MeV. Such very oblique chorus waves could contribute to a predominantly perpendicular anisotropy of the global equatorial electron population on the dayside and to a relative isotropization at low energy under disturbed conditions. It is also suggested that they might play a significant role in pulsating auroras.
Key Points
Reduced electron lifetimes due to oblique chorus waves
Analytical estimates of electron lifetimes
Explanation of decreased lifetimes
Effects of whistler mode hiss waves in March 2013 Ripoll, J.‐F.; Santolík, O.; Reeves, G. D. ...
Journal of geophysical research. Space physics,
July 2017, 2017-07-00, 20170701, Letnik:
122, Številka:
7
Journal Article
Recenzirano
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 h 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 Magnetic Electron and Ion Spectrometer 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 an environment that depopulates them as they migrate from L ~ 5 to L ~ 2.5. Ultrarelativistic electrons are not affected by hiss losses until L ~ 2–3.
Key Points
Computations of daily pitch angle diffusion coefficients and electron lifetimes from properties of hiss waves observed in March 2013
Good agreement found between MagEIS flux observations and 1‐D Fokker‐Planck simulations based on our hiss loss term for quiet times
Combined with transport, hiss waves loss drives the daily energy structure of the radiation belts, with a typical S‐shaped outer belt
Electron flux variations for E > 500 keV during geomagnetic storms are investigated using the Energetic Particle Telescope (EPT). This detector launched in May 2013 on board the satellite PROBA‐V at ...an altitude of 820 km was designed to provide uncontaminated spectra of electrons, protons, and alpha particles. Electron flux dropout events are observed during the main phase of each storm and even during substorms: a rapid reduction of the electron flux is noted throughout the outer electron radiation belt at all energies above about 0.5 MeV on timescales of a few hours. The electron spectrograms measured by the EPT between 2013 and 2019 show that after each geomagnetic storm, dropout events are followed by a flux enhancement starting first at low L values, and reaching the slot or even the inner belt for the strongest storms. We determine the link between Disturbed Storm Time (Dst) and the minimum value of the L‐shell where the dropouts deplete the outer belt, as well as the nonlinear relation between Dst and the minimum L‐shell where the flux penetrates in the slot region or even the inner belt during the storms. Dropouts appear at all energies measured by EPT and penetrate down to L∼3.5 for the strongest events. Dropouts are observed at Low Earth Orbit each time Dst has an inverted peak < −40 nT. Flux enhancements appear at lower L only for big storm events with Dst < −50 nT. They penetrate down to an impenetrable barrier with a minimum L‐shell related to Dst and to the energy. For E > 1 MeV, this limit is also linked to the plasmapause position.
Key Points
Electron >500 keV flux measured by Energetic Particle Telescope at Low Earth Orbit from 2013 to 2019 shows dropouts followed by enhancements during all geomagnetic storms
The study shows that dropouts deplete the outer radiation belt down to a minimum L‐shell related to disturbed storm time (Dst)
Electron fluxes are enhanced in the slot down to an impenetrable barrier with a L‐shell related to Dst and to the plasmapause position
A new empirical density model is developed for the inner zone between 1 < L < 3 using plasma densities inferred from the upper hybrid resonance on Arase, and hiss‐inferred density values from Van ...Allen Probes. The Van Allen Probes hiss‐inferred densities are first recalibrated and validated against Arase observations, using both a conjunction event and statistical analyses. The newly developed density model includes dependencies on L, magnetic latitude, and magnetic local time (MLT). Between 1.5 < L < 3.0, the equatorial density variation with L is shown to be equivalent to that of the Ozhogin et al. (2012, https://doi.org/10.1029/2011JA017330) model. However, for L < 1.5, this dependence changes as the plasma density increases at a faster rate with decreasing L. The latitudinal dependence of the plasma density is shown to present a flatter profile than previous models, meaning lower densities extend to higher latitudes. This dependence is well‐modeled by updated fitting coefficients. A clear MLT dependence of the plasma density is identified, which was not found or included in some previous models. This variation is consistent with the diurnal variation of the ionosphere, peaking near MLT = 14 and becoming larger in amplitude with decreasing L. A function describing this MLT dependence is presented. Overall, the new L, latitude, and MLT‐dependent empirical model can provide density values in areas outside the validity region of many previous models, making it a useful resource for accurately determining diffusion coefficients and predicting electron dynamics and their lifetimes in the inner radiation belt.
Key Points
Radiation Belt Storm Probes and Arase data are used to build a new empirical plasma density model for the inner zone, including L, latitude, and magnetic local time (MLT) dependencies
MLT dependence consistent with diurnal variation of ionosphere. Variation is largest in amplitude at low L, but persists out to L = 3
New model provides density in areas outside previous model bounds, making it a useful resource for modeling inner radiation belt dynamics
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 (4–15 March) following the 1 March 2013 storm. 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°, E > 100 keV, 3.5 < L < Lpp ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV.
Plain Language Summary
We study the evolution of the radiation belts during quiet geomagnetic times from satellite observations and numerical codes. We reach a global understanding of the trapped electrons variation with time, space, energy, and pitch angle (the angle of the velocity vector with the magnetic field). We exhibit three stable regions, which are less sensitive to scattering from hiss waves, while, on the other hand, hiss causes flux decay over 12 days that forms the slot region between the inner and outer belt. The existing theory explains why the outer belt electron decay is independent of pitch angle but dependent upon energy. This implies that satellite observations can reveal local pitch angle diffusion rates, themselves intimately connected with the wave properties. Thus, a connection is made between observed wave properties and observed/computed scattered electron flux, consistent with theory. Regions where the flux is pitch angle dependent are isolated in the low‐energy slot region where we show that the real shape is a smoothed version of the ideal top‐hat distribution computed from theory. The impact of this work is improved understanding of the belt evolution for space weather prediction, with a proposed event‐driven method that accurately (within ×2) predicts the electron flux decay after storms.
Key Points
Global computations of the (L, E, α0) structure of the evolving radiation belt during quiet times agree well with observations
The inner belt decay is pitch angle dependent, while the outer belt is much more homogeneous with two distinct (L, E) regions
The homogeneity of the pitch angle diffusion coefficient due to hiss waves explains the uniform outer belt decay and why 1D and 3D simulations agree
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