Radiation Belt Electron Dynamics Research Articles

Dynamics of electron fluxes in the slot between radiation belts in November–December 2014 according to data of the Vernov satellite

The variations in the spatial structure and time in electron fluxes with E = 235–300 keV in the slot region (2 –60 nT) are analyzed based on the data of the RELEC complex on board the Vernov satellite (the height and inclination of the orbit are from 640 to 830 km and 98.4°, respectively). Irregular increases in the fluxes of such electrons and formation of a local maximum at L ~ 2.2–3.0 were observed. It has been shown that the intensity of this maximum is inversely proportional to the L value and grows with an increase in the geomagnetic activity level. New features discovered for the first time in the dynamics of radiation belt electrons manifest in the variations in the local structure and dynamics of fluxes of subrelativistic electrons in the slot region.

A statistical analysis of the global distribution of plasmaspheric hiss based on Van Allen Probes wave observations

Plasmaspheric hiss plays an important role in driving the precipitation loss of radiation belt electrons via pitch angle scattering, which is also known as the major cause of the formation of the slot region between the inner and outer radiation belt. Therefore, it is of scientific importance to acquire a complete picture of the global distribution of plasmaspheric hiss. Using the thirty-three month high-quality wave data of the Van Allen Probes from September 2012 to May 2015, which provide excellent coverage in the entire inner magnetosphere, we investigate in detail the characteristics of the global distribution of plasmaspheric hiss bin-averaged wave amplitude and occurrence rate with respect to the geomagnetic activity level, L-shell, geomagnetic latitude, and magnetic local time. It is demonstrated that the bin-averaged hiss amplitude strongly depends on the level of geomagnetic activity and exhibits a pronounced day-night asymmetry. Dayside hiss shows a tendency intensifying with the disturbed geomagnetic condition, which is primarily confined to L=2.5-4.0. In contrast, the average hiss amplitude on the nightside tends to decrease. It should also be noted that plasmaspheric hiss at different amplitude levels varies distinctly with geomagnetic condition. As the geomagnetic disturbance increases, the occurrence rate of hiss wave at a smaller amplitude level (i.e., 5-30 pT) increases on the nightside but decreases on the dayside, while the occurrence pattern of higher amplitude ( 30 pT) hiss wave is opposite. For high amplitude hiss wave, the occurrence rate increases on the dayside during intense geomagnetic activities while decreases on the nightside. This is probably because during active times, suprathermal electron fluxes are larger on the nightside, which causes stronger Landau damping of whistler mode waves and thus limits the ability of chorus waves to propagate into the plasmasphere and evolve into plasmaspheric hiss. In addition, plasmaspheric hiss waves with the amplitudes ranging from 5 to 30 pT have the highest occurrence probability both around the geomagnetic equator and at higher latitudes. Our statistical results can provide a reasonable and accurate cognition complementary to the current knowledge of the global features of plasmaspheric hiss, especially in the inner magnetosphere of L=2-6, thereby offering essential input parameters of hiss wave distribution for future simulations of the dynamic spatiotemporal variations of radiation belt electrons at different energies and pitch angles under the influence of diverse solar wind and magentospheric circumstances. Therefore, we suggest that these new properties of hiss wave should be incorporated into the future modeling of radiation belt electron dynamics.

Radiation belt electron acceleration during the 17 March 2015 geomagnetic storm: Observations and simulations

AbstractVarious physical processes are known to cause acceleration, loss, and transport of energetic electrons in the Earth's radiation belts, but their quantitative roles in different time and space need further investigation. During the largest storm over the past decade (17 March 2015), relativistic electrons experienced fairly rapid acceleration up to ~7 MeV within 2 days after an initial substantial dropout, as observed by Van Allen Probes. In the present paper, we evaluate the relative roles of various physical processes during the recovery phase of this large storm using a 3‐D diffusion simulation. By quantitatively comparing the observed and simulated electron evolution, we found that chorus plays a critical role in accelerating electrons up to several MeV near the developing peak location and produces characteristic flat‐top pitch angle distributions. By only including radial diffusion, the simulation underestimates the observed electron acceleration, while radial diffusion plays an important role in redistributing electrons and potentially accelerates them to even higher energies. Moreover, plasmaspheric hiss is found to provide efficient pitch angle scattering losses for hundreds of keV electrons, while its scattering effect on > 1 MeV electrons is relatively slow. Although an additional loss process is required to fully explain the overestimated electron fluxes at multi‐MeV, the combined physical processes of radial diffusion and pitch angle and energy diffusion by chorus and hiss reproduce the observed electron dynamics remarkably well, suggesting that quasi‐linear diffusion theory is reasonable to evaluate radiation belt electron dynamics during this big storm.

Intense low‐frequency chorus waves observed by Van Allen Probes: Fine structures and potential effect on radiation belt electrons

AbstractFrequency distribution is a vital factor in determining the contribution of whistler mode chorus to radiation belt electron dynamics. Chorus is usually considered to occur in the frequency range 0.1–0.8fce_eq (with the equatorial electron gyrofrequency fce_eq). We here report an event of intense low‐frequency chorus with nearly half of wave power distributed below 0.1fce_eq observed by Van Allen Probe A on 27 August 2014. This emission propagated quasi‐parallel to the magnetic field and exhibited hiss‐like signatures most of the time. The low‐frequency chorus can produce the rapid loss of low‐energy (∼0.1 MeV) electrons, different from the normal chorus. For high‐energy (≥0.5 MeV) electrons, the low‐frequency chorus can yield comparable momentum diffusion to that of the normal chorus but much stronger (up to 2 orders of magnitude) pitch angle diffusion near the loss cone.

Solar wind conditions leading to efficient radiation belt electron acceleration: A superposed epoch analysis

AbstractDetermining preferential solar wind conditions leading to efficient radiation belt electron acceleration is crucial for predicting radiation belt electron dynamics. Using Van Allen Probes electron observations (>1 MeV) from 2012 to 2015, we identify a number of efficient and inefficient acceleration events separately to perform a superposed epoch analysis of the corresponding solar wind parameters and geomagnetic indices. By directly comparing efficient and inefficient acceleration events, we clearly show that prolonged southward Bz, high solar wind speed, and low dynamic pressure are critical for electron acceleration to >1 MeV energies in the heart of the outer radiation belt. We also evaluate chorus wave evolution using the superposed epoch analysis for the identified efficient and inefficient acceleration events and find that chorus wave intensity is much stronger and lasts longer during efficient electron acceleration events, supporting the scenario that chorus waves play a key role in MeV electron acceleration.

Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their effects on radiation belt electron dynamics

AbstractPlasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550 Hz), which was in frequent use, and that the hiss wave power frequently extends below 100 Hz, particularly at larger L shells (> ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100 Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100 keV at L~5, although their effect is negligible at L ≤ 3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.

Radiation belt electron acceleration by chorus waves during the 17 March 2013 storm

AbstractLocal acceleration driven by whistler‐mode chorus waves is fundamentally important for accelerating seed electron populations to highly relativistic energies in the outer radiation belt. In this study, we quantitatively evaluate chorus‐driven electron acceleration during the 17 March 2013 storm, when the Van Allen Probes observed very rapid electron acceleration up to several MeV within ~12 hours. A clear radial peak in electron phase space density (PSD) observed near L* ~4 indicates that an internal local acceleration process was operating. We construct the global distribution of chorus wave intensity from the low‐altitude electron measurements made by multiple Polar Orbiting Environmental Satellites (POES) satellites over a broad region, which is ultimately used to simulate the radiation belt electron dynamics driven by chorus waves. Our simulation results show remarkable agreement in magnitude, timing, energy dependence, and pitch angle distribution with the observed electron PSD near its peak location. However, radial diffusion and other loss processes may be required to explain the differences between the observation and simulation at other locations away from the PSD peak. Our simulation results, together with previous studies, suggest that local acceleration by chorus waves is a robust and ubiquitous process and plays a critical role in accelerating injected seed electrons with convective energies (~100 keV) to highly relativistic energies (several MeV).

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 (~100–2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ~20–200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ~50–200 keV electrons and produces more pronounced pancake distributions of ~50–100 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 ~50–400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections.

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

AbstractWe 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 100–300 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.

Modeling radiation belt electron dynamics during GEM challenge intervals with the DREAM3D diffusion model

AbstractAs a response to the Geospace Environment Modeling (GEM) “Global Radiation Belt Modeling Challenge,” a 3D diffusion model is used to simulate the radiation belt electron dynamics during two intervals of the Combined Release and Radiation Effects Satellite (CRRES) mission, 15 August to 15 October 1990 and 1 February to 31 July 1991. The 3D diffusion model, developed as part of the Dynamic Radiation Environment Assimilation Model (DREAM) project, includes radial, pitch angle, and momentum diffusion and mixed pitch angle‐momentum diffusion, which are driven by dynamic wave databases from the statistical CRRES wave data, including plasmaspheric hiss, lower‐band, and upper‐band chorus. By comparing the DREAM3D model outputs to the CRRES electron phase space density (PSD) data, we find that, with a data‐driven boundary condition at Lmax = 5.5, the electron enhancements can generally be explained by radial diffusion, though additional local heating from chorus waves is required. Because the PSD reductions are included in the boundary condition at Lmax = 5.5, our model captures the fast electron dropouts over a large L range, producing better model performance compared to previous published results. Plasmaspheric hiss produces electron losses inside the plasmasphere, but the model still sometimes overestimates the PSD there. Test simulations using reduced radial diffusion coefficients or increased pitch angle diffusion coefficients inside the plasmasphere suggest that better wave models and more realistic radial diffusion coefficients, both inside and outside the plasmasphere, are needed to improve the model performance. Statistically, the results show that, with the data‐driven outer boundary condition, including radial diffusion and plasmaspheric hiss is sufficient to model the electrons during geomagnetically quiet times, but to best capture the radiation belt variations during active times, pitch angle and momentum diffusion from chorus waves are required.

Radiation belt electron dynamics driven by large‐amplitude whistlers

Acceleration of radiation belt electrons driven by oblique large‐amplitude whistler waves is studied. We show analytically and numerically that this is a stochastic process; the intensity of which depends on the wave power modified by Bessel functions. The type of this dependence is determined by the character of the nonlinear interaction due to coupling between action and phase. The results show that physically significant quantities have a relatively weak dependence on the wave power.

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

AbstractWe adopt a physics‐based technique to infer chorus wave amplitudes from the low‐altitude electron population (30–100 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 30–100 keV electron population at the conjugate low altitudes. We apply this technique to construct the chorus wave distributions during the 8–9 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.

Characteristics of the Poynting flux and wave normal vectors of whistler‐mode waves observed on THEMIS

The characteristics of the Poynting flux and wave normal vectors of whistler‐mode waves outside the plasmapause are investigated for the lower (0.1–0.5 fce) and upper bands (0.5–0.8 fce), where fce is the equatorial electron cyclotron frequency. To analyze the wave properties, we utilized high‐resolution waveform data from multiple THEMIS spacecraft in the near‐equatorial magnetosphere from June 2008 to November 2012. Full measurements of the wave electric and magnetic fields are used to calculate the Poynting fluxes and construct the wave normal vectors, which are then used to calculate the polar and azimuthal angles with respect to the background magnetic field. Statistical results show that the majority of whistler‐mode waves propagate away from the magnetic equator, suggesting that the major source region is very close to the equator. The lower band wave normal angle distribution shows a major peak close to the field line direction and a secondary peak near the resonance cone. In contrast, the wave normal distribution of upper band waves exhibits a broad distribution between 0° and 60° with the largest probability at ~0°. The azimuthal component of the wave normal vector predominantly points radially outward for both lower and upper band waves, but a tendency for azimuthal propagation is observed for lower band waves in the day and dusk sectors probably due to pronounced azimuthal density gradients in the afternoon sector. Our statistical results provide crucial information on the Poynting fluxes and wave normal vectors of whistler‐mode waves, which play a significant role in radiation belt electron dynamics.

Normal-Angle Dependence of the Interaction between Radiation Belt Electrons and Fast Magnetosonic Waves

Fast magnetosonic (MS) waves have been suggested to contribute significantly to radiation belt electron dynamics via Landau resonance and transit time scattering. The corresponding quasi-linear diffusion coefficients in pitch angle, energy and particularly cross-pitch-angle-energy are calculated from the gyro-averaged test-particle simulations. It is found that the cross diffusion coefficient is an effective indicator to differentiate between the contributions of resonant and non-resonant mechanisms. Furthermore, the dependence of diffusion coefficients on the normal angle of MS waves are parametrically investigated. Numerical results show that the increasing normal angle can lead to the shrinking of the Landau resonance region and the expansion of the transit-time scattering region. With the increasing normal angle, the pitch angle diffusion coefficients decrease significantly (by about two orders of magnitude), while the other two diffusion coefficients have a relatively limited decrease (within one order of magnitude). For arbitrary normal angles, the magnitude of cross diffusion coefficients is comparable to the other diffusion coefficients, suggesting that the cross diffusion is indispensable in the kinetic simulation works.

Atmospheric scattering and decay of inner radiation belt electrons

The dynamics of inner radiation belt electrons are governed by competing source, loss, and transport processes. However, during the recent extended solar minimum period the source was inactive and electron intensity was characterized by steady decay. This provided an opportunity to determine contributions to the decay rate of losses by precipitation into the atmosphere and of diffusive radial transport. To this end, a stochastic simulation of inner radiation belt electron transport is compared to data taken by the IDP instrument on the DEMETER satellite during 2009. For quasi‐trapped, 200 keV electrons atL= 1.3, observed in the drift loss cone (DLC), results are consistent with electron precipitation losses by atmospheric scattering alone, provided account is taken of non‐diffusive wide‐angle scattering. Such scattering is included in the stochastic simulation using a Markov jump process. Diffusive small‐angle atmospheric scattering, while causing most of the precipitation losses, is too slow relative to azimuthal drift to contribute significantly to DLC intensity. Similarly there is no contribution from scattering by VLF plasma waves. Energy loss, energy diffusion, and azimuthal drift are also included in the model. Even so, observed decay rates of stably‐trapped electrons withL < 1.5 are slower than predicted by scattering losses alone, requiring radial diffusion with coefficient DLL ∼ 3 × 10−10 s−1 to replenish electrons lost to the atmosphere at low L values.

Resonance zones and quasi-linear diffusion coefficients for radiation belt energetic electron interaction with oblique chorus waves in the Dungey magnetosphere

The resonance regions for resonant interactions of radiation belt electrons with obliquely propagating whistler-mode chorus waves are investigated in detail in the Dungey magnetic fields that are parameterized by the intensity of uniform southward interplanetary magnetic field (IMF) Bz or, equivalently, by the values of D=(M/Bz,0)1/3 (where M is the magnetic moment of the dipole and Bz,0 is the uniform southward IMF normal to the dipole’s equatorial plane). Adoption of background magnetic field model can considerably modify the determination of resonance regions. Compared to the results for the case of D = 50 (very close to the dipole field), the latitudinal coverage of resonance regions for 200 keV electrons interacting with chorus waves tends to become narrower for smaller D-values, regardless of equatorial pitch angle, resonance harmonics, and wave normal angle. In contrast, resonance regions for 1 MeV electrons tend to have very similar spatial lengths along the field line for various Dungey magnetic field models but cover different magnetic field intervals, indicative of a strong dependence on electron energy. For any given magnetic field line, the resonance regions where chorus-electron resonant interactions can take place rely closely on equatorial pitch angle, resonance harmonics, and kinetic energy. The resonance regions tend to cover broader latitudinal ranges for smaller equatorial pitch angles, higher resonance harmonics, and lower electron energies, consistent with the results in Ni and Summers [Phys. Plasmas 17, 042902, 042903 (2010)]. Calculations of quasi-linear bounce-averaged diffusion coefficients for radiation belt electrons due to nightside chorus waves indicate that the resultant scattering rates differ from using different Dungey magnetic field models, demonstrating a strong dependence of wave-induced electron scattering effect on the adoption of magnetic field model. Our results suggest that resonant wave-particle interaction processes should be implemented into a sophisticated, accurate global magnetic field model to pursue comprehensive and complete models of radiation belt electron dynamics.

Rapid acceleration of radiation belt energetic electrons by Z‐mode waves

We present the first simulation of the effect of Z‐mode waves on the outer radiation belt electron dynamics. We calculate bounce‐averaged diffusion rates in pitch angle and momentum and then use them as inputs to solve a 2‐D momentum‐pitch angle diffusion equation. Numerical results show that the phase space density (PSD) of 1 MeV electrons can enhance substantially and very rapidly (e.g., 30 minutes). In particular, the momentum diffusion rate exceeds the pitch angle and cross diffusion rates at 0.5 MeV and above, a behavior completely different from that for EMIC and chorus waves. Consequently, momentum (instead of pitch angle or cross) diffusion plays a dominant role in the dynamic evolution of energetic electrons. Moreover, the PSD evolution is found to be very dependent upon the assumed initial particle distributions. These results provide further insights on the interplay between acceleration mechanisms of outer radiation belt electrons.

Long-term radiation belt simulation with the VERB 3-D code: Comparison with CRRES observations

[1] Highly energetic electrons in the Earth’s radiation belts are hazardous for satellite equipment. Fluxes of relativistic electrons can vary by orders of magnitude during geomagnetic storms. The evolution of relativistic electron fluxes in the radiation belts is described by the 3-D Fokker-Planck equation in terms of the radial distance, energy, and equatorial pitch angle. To better understand the mechanisms that control radiation belt acceleration and loss and particle flux dynamics, we present a long-term radiation belt simulation for 100 days from 29 July to 6 November 1990 with the 3-D Versatile Electron Radiation Belt (VERB) code and compare the results with the electron fluxes observed by the Combined Release and Radiation Effects Satellite (CRRES). We also perform a comparison of Phase Space Density with a multisatellite reanalysis obtained by using Kalman filtering of observations from CRRES, Geosynchronous (GEO), GPS, and Akebono satellites. VERB 3-D simulations include radial, energy, and pitch angle diffusion and mixed energy and pitch angle diffusion driven by electromagnetic waves inside the magnetosphere with losses to the atmosphere. Boundary conditions account for the convective source of electrons and loss to the magnetopause. The results of the simulation that include all of the above processes show a good agreement with the data. The agreement implies that these processes are important for the radiation belt electron dynamics and therefore should be accounted for in outer radiation belt simulations. We also show that the results are very sensitive to the assumed wave model. Our simulations are driven only by the variation of the Kp index and variations of the seed electron population around geosynchronous orbit, which allows the model to be used for forecasting and nowcasting.

Typical properties of rising and falling tone chorus waves

[1] Chorus waves, which have received intense attention recently due to their significant role in radiation belt electron dynamics, frequently exhibit rising and falling tones. Lower-band chorus waves, observed using THEMIS wave burst data, are analyzed to obtain the typical properties of either class of chorus emissions. Our results show that rising tones are more likely to be quasi field-aligned, whereas falling tones are typically very oblique, close to the resonance cone. Furthermore, rising tones occur significantly more often than falling tones, and magnetic amplitudes of rising tones are generally much larger than those of falling tones. We also show the preferential regions of rising and falling tones dependent on MLT and magnetic latitude. Our new findings suggest that two separate mechanisms may be responsible for the generation and nonlinear evolution of rising and falling tone chorus.

Radiation belt electron dynamics driven by adiabatic transport, radial diffusion, and wave-particle interactions

The adiabatic transport process is introduced into our recently developed three-dimensional physics-based electron radiation belt model (STEERB, Storm-Time Evolution of Electron Radiation Belt) via adopting a time-varying Hilmer-Voigt geomagnetic field. The current STEERB model contains more complete physical processes: adiabatic transport, radial diffusion, and various in situ wave-particle interactions. In particular, the influence of adiabatic transport on storm time radiation belt electron dynamics is investigated by some idealized simulations. It is found that the adiabatic transport alone (without plume hiss and electromagnetic ion cyclotron (EMIC) waves) is unable to reproduce the observed main phase loss of energetic outer radiation belt electron fluxes in the presence of a strong chorus-driven acceleration process. However, these adiabatic and nonadiabatic processes for radiation belt electron dynamics are coupled to each other. The adiabatic transport, together with radial diffusion and cyclotron resonant interactions with chorus, plume hiss, and EMIC waves, contributes significantly to the main phase loss and the recovery phase enhancement of energetic electron fluxes. In the absence of adiabatic transport, the energetic outer radiation belt electron fluxes are found to be overestimated by a factor of 5-30 over all the pitch angles during the main phase and to be underestimated by a factor of 2-5 at larger pitch angles (alpha(e) > 50 degrees) during the recovery phase. These numerical results suggest that the adiabatic transport in a time-varying geomagnetic field model should be incorporated into the future radiation belt models for space weather application.