Radiation Belt Electron Dynamics Research Articles

Effects of Superthermal Plasmas on Hiss Wave‐Driven Scattering Loss of Radiation Belt Electrons

AbstractPlasmaspheric hiss plays an important role in the loss of radiation belt electrons via cyclotron resonant interactions. The cold plasma approximation is widely used in the evaluation of hiss‐driven electron losses, which however can break down during disturbed periods of geomagnetic storms and substorms. The kappa particle velocity distribution, characterized by a pronounced high‐energy tail, is well‐established to model the profile of superthermal plasma under disturbed geomagnetic conditions. In the present study, by calculating the electron bounce‐averaged pitch angle diffusion coefficients with kappa plasma dispersion relations, we investigate the sensitivity of hiss‐induced cyclotron‐resonant electron scattering loss to the spectral index κ under a variety of superthermal plasma conditions. Our results demonstrate that, with increasing κ, the diffusion coefficients of ∼20–100 keV radiation belt electrons significantly decrease at lower pitch angles and increase at higher pitch angles. In contrast, for electrons at higher energies, the diffusion coefficients tend to increase at lower pitch angles and decrease at relatively higher pitch angles. We also find that decrease of L‐shell and increase of α* and temperature anisotropy tend to weaken the hiss‐driven pitch angle scattering efficiency of electrons at energies from tens to hundreds of keV with a dip at ∼30–50 keV, while the scattering of higher energy electrons can be enhanced. This study confirms the important role of superthermal plasmas in the hiss‐driven electron loss processes and should be carefully incorporated in future modeling of radiation belt electron dynamics.

ULF Wave Transport of Relativistic Electrons in the Van Allen Belts: Criteria for Transition to Radial Diffusion

AbstractRelativistic electrons in the radiation belts can be transported as a result of wave‐particle interactions (WPI) with ultra‐low frequency (ULF) waves. Such WPI are often assumed to be diffusive, parametric models for the radial diffusion coefficient often being used to assess the rates of radial transport. However, these WPI transition from initially coherent interactions to the diffusive regime over a finite time, this time depending on the ULF wave power spectral density, and local resonance conditions. Further, in the real system on the timescales of a single storm, interactions with finite discrete modes may be more realistic. Here, we use a particle‐tracing model to simulate the dynamics of outer radiation belt electrons in the presence of a finite number of discrete frequency modes. We characterize the point of the onset of diffusion as a transition from separate discrete interactions in terms of wave parameters by using the “two‐thirds” overlap criterion (Lichtenberg & Lieberman, 1992, https://doi.org/10.1007/978‐1‐4757‐2184‐3), a comparison between the distance between, and the widths of, the electron's primary resonant islands in phase space. Further, we find the particle decorrelation time in our model system with typical parameters to be on the timescale of hours, which only afterward can the system be modeled by one‐dimensional radial diffusion. Direct comparison of particle transport rates in our model with previous analytic diffusion coefficient formulations show good agreement at times beyond the decorrelation time. These results are critical for determining the time periods and conditions under which ULF wave radial diffusion theory can be applied.

Rapid Acceleration Bursts in the Van Allen Radiation Belt

AbstractThe fast Van Allen radiation belt electron dynamics during geomagnetic storms have not yet been fully explained, in part due to limitations of standard satellite missions in both orbit and the number of spacecraft. Here we overcome these limitations using measurements from the Global Positioning System (GPS) constellation during an acceleration event on 26 August 2018. We show that the acceleration of relativistic electrons occurs in two distinct bursts, each dominated by a different acceleration mechanism. The first burst enhances the radiation belt electrons by four orders of magnitude in 2 hr and is consistent with ULF‐wave radial diffusion. The second burst is likely caused by the local acceleration and delivers an order‐of‐magnitude increase in 20 min. This work demonstrates how distributed, operational measurements can be used to resolve phenomena not observable with previous capabilities, and that rapid energization of the radiation belt can occur much faster than previously reported.

Statistical Properties of Pc4‐5 ULF Waves in Plasmaspheric Plumes

AbstractUltra‐low‐frequency (ULF) waves emerge as pivotal factors in elucidating the mechanisms that drive the intricate dynamics of radiation belt electrons within the plasmasphere and plasmaspheric plumes. Utilizing THEMIS data from September 2012 to September 2017, we conducted a comprehensive statistical analysis of Pc4‐5 ULF waves within and outside the plasmaspheric plume. Our findings reveal a distinctive dawn‐dusk asymmetry in occurrence rate and wave power of poloidal mode waves in the absence of the plume, resembling the toroidal mode asymmetry observed. Poloidal mode waves exhibit a higher likelihood of formation within the plume, while the toroidal mode waves show the opposite trend, contributing to the elevated dusk‐side occurrence rate of poloidal mode waves. Moreover, both wave modes within the plume demonstrate lower peak frequencies compared to waves outside the plume. The global distribution of wave power within the plume suggests higher power at noon than on the dusk side.

Properties of Magnetohydrodynamic Normal Modes in the Earth's Magnetosphere

AbstractThe Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies <80 mHz. We identify persistent normal mode structure in observed dawn sector power spectra with frequency‐dependent wave power peaks like those obtained from simulation ensemble averages, where the simulations assume different radial Alfvén speed profiles and magnetopause locations. We further show with both observations and simulations how frequency‐dependent wave power peaks at L > 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves.

Design and Instrument Characterization of the High‐Energy Resolution Relativistic Electron Telescope (HERT) Using Geant4 Simulation

AbstractEarth's outer radiation belt is filled with relativistic (∼100s of keV–3 MeV) and ultrarelativistic (∼>3 MeV) electrons. The acceleration of electrons to these high energies are believed to be caused by inward radial transport and local acceleration, but the relative importance of the two mechanisms to electrons with different energies are still under considerate debate. In the post Van Allen Probes era, measurements of radiation belt populations must rely on small space missions such as CubeSats and SmallSats. The Miniaturized High‐Energy‐Resolution relativistic electron Telescope (HERT) is a compact (≤3U) telescope designed for a CubeSat mission in geosynchronous transfer orbit (GTO). HERT's main objective is to provide high‐energy‐resolution measurements of outer belt electrons in an energy range of ∼1–7 MeV to help differentiate the contribution of inward radial transport and local acceleration. Geant4 simulations were conducted to characterize the instrument responses. A novel method of using a spherical cap particle source in Geant4 simulation was developed for more efficient instrument characterization. Combined with Bow tie analysis, it is demonstrated that HERT will have an energy resolution of 5% for ∼1.5–3 MeV electrons and 12% for ∼3–7 MeV with current electronics design. In addition, using the AE9 model, we showed the instrument would have statistically sufficient count rates in the outer belt while not saturating the electronics. With a compact configuration and higher energy resolution in comparison to previous instruments, HERT will significantly contribute to the quantitative understanding of the radiation belt electron dynamics.

Deep learning model of hiss waves in the plasmasphere and plumes and their effects on radiation belt electrons

Hiss waves play an important role in removing energetic electrons from Earth’s radiation belts by precipitating them into the upper atmosphere. Compared to plasmaspheric hiss that has been studied extensively, the evolution and effects of plume hiss are less understood due to the challenge of obtaining their global observations at high cadence. In this study, we use a neural network approach to model the global evolution of both the total electron density and the hiss wave amplitudes in the plasmasphere and plume. After describing the model development, we apply the model to a storm event that occurred on 14 May 2019 and find that the hiss wave amplitude first increased at dawn and then shifted towards dusk, where it was further excited within a narrow region of high density, namely, a plasmaspheric plume. During the recovery phase of the storm, the plume rotated and wrapped around Earth, while the hiss wave amplitude decayed quickly over the nightside. Moreover, we simulated the overall energetic electron evolution during this storm event, and the simulated flux decay rate agrees well with the observations. By separating the modeled plasmaspheric and plume hiss waves, we quantified the effect of plume hiss on energetic electron dynamics. Our simulation demonstrates that, under relatively quiet geomagnetic conditions, the region with plume hiss can vary from L = 4 to 6 and can account for up to an 80% decrease in electron fluxes at hundreds of keV at L > 4 over 3 days. This study highlights the importance of including the dynamic hiss distribution in future simulations of radiation belt electron dynamics.

Resonant Scattering of Radiation Belt Electrons at Saturn by Ion Cyclotron Waves

AbstractBy constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves on the basis of Cassini datasets, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. Calculations based on quasi‐linear bounce‐averaged diffusion coefficients show that at Saturn ion cyclotron waves can efficiently pitch angle scatter >∼1 MeV to tens of MeV electrons into the loss cone thereby inducing precipitation loss, while the mixed and momentum scattering effects are typically negligible. The resultant electron loss timescales range from a few to tens of minutes, which in fact decrease significantly with increasing L‐shell at L = 4–6. We also find that the kinetic effects introduced by pick‐up ring particles cause distinct changes in pitch angle scattering efficiency for lower energy electrons (<3 MeV at L = 5). Our results demonstrate that ion cyclotron waves play a significant role in the dynamics of Saturn's radiation belt electrons.

Magnetosonic Waves Above the Lower Hybrid Frequency in Cyclotron Resonance With the Van Allen Radiation Belt Electrons

AbstractCyclotron resonance with magnetosonic waves was usually considered unimportant for the Van Allen radiation belt electron dynamics because it occurs at energies that are too high. Here, we show that for magnetosonic waves with frequencies above the lower hybrid frequency and normal angles below 87° in the high‐density plasmasphere, the electron cyclotron resonances of ±1, ±2, and ±3 orders can extend down to energies of 10–100s keV. These high‐frequency, non‐quasi‐perpendicular magnetosonic waves are found to occur predominantly in the duskside outer plasmasphere, with an amplitude on the order of 10 pT following strong substorms. They probably originate from midlatitudes of the outer plasmasphere, different from the commonly known magnetosonic waves generated near the equator.

Inter‐Calibration Between the Electron Flux Measurements of FengYun‐3B and Van Allen Probe‐A Based on Electron Phase Space Density Conjunctions

AbstractCross‐satellite calibration of energetic particle fluxes is essential to understanding Earth's radiation belt dynamics and modeling the space radiation environment. Using the method of comparing phase space density (PSD) conjunctions in the same set of phase space coordinates, we perform a cross‐satellite calibration of energetic electron fluxes measured by the Low Earth Orbit Chinese FengYun‐3B satellite (FY‐3B) and the near‐equatorial Highly Eccentric Orbit Van Allen Probe‐A (VAP‐A), respectively. VAP‐A provides pitch angle resolved electron (directional) differential fluxes while FY‐3B provides omnidirectional electron differential fluxes. Before calculating the PSDs from FY‐3B high energy electron detector (HEED), a method is introduced to estimate the local pitch angle of FY‐3B based on the installation direction information of HEED and the T89c magnetic field model. We calculate the calibration factors by using the PSD conjunctions inferred from the two satellites for three fixed sets of (μ, K, L*) during the geomagnetic storm times. The calibration factors range from 0.76 to 0.86 varying with fixed sets of (μ, K, L*), which indicates that the electron differential fluxes from FY‐3B are higher than that from VAP‐A. During geomagnetic quiet times, we perform calibration by using independent Gaussian fitting analysis of the number of points with the unaveraged PSDs of FY‐3B and VAP‐A for three fixed sets of (μ, K, L*), with the corresponding calibration factors of 0.78, 0.72, and 0.46. The calibrated electron differential fluxes of FY‐3B measured at the high magnetic latitudes, can be used in the future modeling of the radiation belt electron dynamics.

A machine learning-based electron density (MLED) model in the inner magnetosphere

The plasma density is an important factor in determining wave-particle interaction in the magnetosphere. We develop a machine learning-based electron density (MLED) model in the inner magnetosphere using a data set of electron density from Van Allen Probes between September 25, 2012 and August 30, 2019. This MLED model is a physics-based nonlinear network that employs fundamental physical principles to represent the variation of electron density. It predicts the plasmapause location under different geomagnetic activities and models the electron density of the plasmasphere and trough separately. We train the model using gradient descent and backpropagation algorithms, which can be widely used to deal effectively with nonlinear relationships among physical quantities in space plasma environments. The model gives explicit expressions with few parameters and describes the association of electron density with geomagnetic activity, solar cycle, and seasonal effects. Under various geomagnetic conditions, the electron densities calculated by this model agree well with the observation and provide a good description of the plasmapause movement. This MLED model, which can be easily incorporated into the previously developed radiation belt model, would be very helpful in modeling and forecasting the radiation belt electron dynamics.

Energy-dependent Boundaries of Earth's Radiation Belt Electron Slot Region

Abstract The variations in radiation belt boundaries reflect competition between acceleration and loss physical processes of energetic electrons, which is an important issue for radiation belts of planets with an internal magnetic field (e.g., Earth, Jupiter, and Saturn). Based on high-quality measurements from Van Allen Probes spanning the years 2014–2018, we develop an empirical model of the energy-dependent boundaries of Earth's electron radiation belt slot region, showing that the lower boundary follows a logarithmic function of the electron energy while the upper boundary is controlled by two competing energy-dependent processes, namely compression and recovery. The compression process relates linearly to a 15 hr averaged Kp index, while the recovery process is found to be approximately proportional to time. Detailed data-model comparisons demonstrate that our model, using only the Kp index and time epoch as inputs, reconstructs the slot region boundaries in real time for 200 keV to 2 MeV electrons under varying geomagnetic conditions. Such a data-driven empirical model is prerequisite to understanding the dynamic changes of the slot region in response to both solar and geomagnetic activities. The model can be readily incorporated into future global simulations of radiation belt electron dynamics in Earth's inner magnetosphere and provide new insights into the study of Saturn's and Jupiter's radiation belt variability.

A Strong Correlation Between Relativistic Electron Microbursts and Patchy Aurora

AbstractIn this letter, we present the results of a conjunction between the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite and a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager in Gillam, Canada, showing a high correlation between relativistic, MeV, electron microbursts and a type of pulsating aurora called patchy aurora. The correlation was 0.8, and is not serendipitous. While the relationship between pulsating aurora and 10–100s keV microbursts has been previously predicted, here we show a strong association between keV and MeV electron dynamics, possibly spanning two orders of magnitude. Importantly, this result shows that the dynamics of relativistic radiation belt electrons are at times intimately tied to keV electron precipitation, and cannot be studied in isolation.

Modeling the Dynamics of Radiation Belt Electrons With Source and Loss Driven by the Solar Wind

AbstractA radial diffusion model directly driven by the solar wind is developed to reproduce MeV electron variations between L = 2–12 (L is L* in this study) from October 2012 to April 2015. The radial diffusion coefficient, internal source rate, quick loss due to EMIC waves, and slow loss due to hiss waves are all expressed in terms of the solar wind speed, dynamic pressure, and interplanetary magnetic field (IMF). The model achieves a prediction efficiency (PE) of 0.45 at L = 5 and 0.51 at L = 4 after converting the electron phase space densities to differential fluxes and comparing with Van Allen Probes measurements of 2 and 3 MeV electrons at L = 5 and L = 4, respectively. Machine learning techniques are used to tune parameters to get higher PE. By tuning parameters for every 60‐day period, the model obtains PE values of 0.58 and 0.82 at L = 5 and L = 4, respectively. Inspired by these results, we divide the solar wind activity into three categories based on the condition of solar wind speed, IMF Bz, and dynamic pressure, and then tune these three sets of parameters to obtain the highest PE. This experiment confirms that the solar wind speed has the greatest influence on the electron flux variations, particularly at higher L, while the dynamic pressure has more influence at lower L. Also, the PE at L = 4 is mostly higher than those at L = 5, suggesting that the electron loss due to the magnetopause shadowing combined with the outward radial diffusion is not well captured in the model.

Off‐Equatorial Source of Magnetosonic Waves Extending Above the Lower Hybrid Resonance Frequency in the Inner Magnetosphere

AbstractInner magnetospheric magnetosonic waves are usually considered to grow from the near‐equatorial ion Bernstein instability below the lower hybrid resonance frequency and contribute significantly to the radiation belt electron dynamics. We here present unusual observations of magnetosonic waves extending above the local lower hybrid resonance frequency in the midlatitude ∼15° plasmasphere after the substorm proton injection. Linear instability analyses and ray‐tracing simulations show that, because of the latitudinal variation of normal angles, these waves have little growth near the equator but are amplified accumulatively to the observable level over broad latitudes |λ| < 30° during the bounce‐drift propagation. Our data and modeling illustrate a previously unexplored scenario that off‐equatorial proton ring distributions could be a significant source of magnetosonic waves in the inner magnetosphere. Such off‐equatorially generated magnetosonic waves might differ from the near‐equatorially generated ones in latitudinal coverage, wavevector distribution, and then effect on the radiation belt electrons.

Impact of EMIC‐Wave Driven Electron Precipitation on the Radiation Belts and the Atmosphere

AbstractIn recent years, there has been a growing body of direct experimental evidence demonstrating electromagnetic ion cyclotron (EMIC) waves driving energetic electron precipitation (EEP) at unexpectedly low, sub‐MeV energies—as low as only a few hundred keV. EMIC‐wave driven scattering at these energies has important ramifications for our understanding of not only radiation belt electron dynamics, but also the importance of EMIC‐driven EEP to the chemical balance of the Earth's atmosphere. In this study, we use three experimentally derived EMIC‐driven EEP flux spectra to investigate the impact of this precipitation on trapped radiation belt fluxes. In doing so, we resolve an apparent contradiction with earlier results derived from trapped electron flux populations that suggested EMIC waves only caused significant scattering at ultrarelativistic energies. We show that strong sub‐MeV EEP measurements are not necessarily mutually exclusive with a strongly relativistic‐only trapped flux response, as the sub‐MEV peak precipitation is comparatively much smaller than the trapped population at those energies. Using a further six EEP spectra, we also demonstrate that EMIC‐driven EEP can generate significant ionization of the Earth's atmosphere above 40 km, leading to the loss of mesospheric ozone. We find poor correlation between EMIC‐driven EEP fluxes and geomagnetic activity proxies, such that EMIC‐driven EEP is likely to be poorly specified in the forcing factors of modern coupled‐climate models.

Scattering by whistler-mode waves during a quiet period perturbed by substorm activity

We study the dynamics of radiation belt electrons during a 10-day quiet period perturbed by substorm activity and preceding a high-speed stream (HSS), aiming at a global description of the radiation belts in L-shell, L in [2, 6], and energy [0.1, 10] MeV. We combine Van Allen Probes observations and Fokker-Planck numerical simulations of pitch-angle diffusion. The Fokker-Planck model uses event-driven pitch angle diffusion coefficients from whistler-mode waves, built from the wave properties and the ambient plasma density measurements from the Van Allen Probes. We first find this event has some similar characteristics to regular quiet times previously studied; a widely extended plasmasphere within which we observe strong and varying whistler-mode waves. These ambient conditions lead to strong pitch-angle scattering, which contributes to the creation of a wide slot region as well as a significant decay of the outer radiation belts, which are observed and qualitatively well simulated. In addition, we find the substorm activity causes short duration (within ± 4h) decay of the plasma density and a lowering amplitude of the whistler-mode waves within the plasmasphere, both causing opposite effects in terms of pitch angle diffusion. This leads to a diminution of pitch-angle diffusion at the time of the main substorm activity. Conversely, whistler-mode waves become enhanced in the time periods between the substorm injections. All effects cumulated, we find an enhancement of pitch angle diffusion by whistler-mode waves above L~4.7 during the 10-day period. This directly relates to the combination of quietness and substorm activity which allows pitch angle diffusing of up to 1 MeV electrons in the outer belt. Relativistic electrons of 1–2 MeV remain trapped in the outer belt, from L~4.7 to L~5.2, forming, in both the observations and the simulations, a distinct pocket of remnant electrons.

Dominant Roles of High Harmonics on Interactions Between AKR and Radiation Belt Relativistic Electrons

AbstractAuroral kilometric radiations (AKR) are widely existing superluminous waves in the magnetosphere. Due to rarely available measurements of AKR magnetic field components, we derive the explicit expression of diffusion coefficients in terms of electric field components, which can be applied to various space plasma waves. Using Van Allen Probes data, we calculate diffusion coefficients with different harmonic orders for two AKR events. It is found that the first (higher) harmonic order contributes to diffusion coefficients at small (larger) pitch angles. The effect of higher harmonic orders becomes more significant as the electron energy increases. These results are contrary to the case of chorus wave, in which the first order usually contributes much to wave‐particle interactions. Estimations show that the phase space density of 0.5 MeV electrons can increase ∼10 times within 6 hr by AKR. This study is important for better modeling of AKR‐induced radiation belt electron dynamics.

Two-step Dropouts of Radiation Belt Electron Phase Space Density Induced by a Magnetic Cloud Event

Abstract We report a two-step dropout event of radiation belt electron phase space density (PSD) induced by a typical magnetic cloud (MC) that drove an intense geomagnetic storm. The first and second steps of PSD dropout occurred, respectively, in the initial and main phases of the storm with a short-time partial recovery between the two dropouts. In this event, the initial phase after the sudden commencement lasted for near 21 hr, which gives an ideal opportunity to investigate the nature of the radiation belt electron dropout by isolating the main phase from any losses occurring during the initial phase. Detailed analysis shows that the first step of the dropout in the initial phase is likely associated with the magnetopause shadowing effect in combination with ultra-low frequency wave-induced outward transport caused by sustaining enhanced dynamic pressure activity before the MC. Comparably, the prolonged strong southward interplanetary magnetic field inside the MC that resulted in the storm main phase is supposed to play an important role in the second step of significant electron losses to the interplanetary space. Additionally, the partial recovery of electron PSD between the two steps of the dropout is possibly due to the acceleration processes via wave-particle interactions with whistler-mode chorus waves. Our study demonstrated that persistently enhanced solar wind dynamic pressure, which is frequently observed inside interplanetary coronal mass ejections and corotating interaction regions, can play an important role in modulating the radiation belt electron dynamics before the storm main phase driven by these solar wind disturbances.

Hemispheric Distribution of Lower-band Chorus Waves Observed by Van Allen Probes

Whistler mode chorus waves are important electromagnetic emissions due to their dual roles in acceleration and loss processes of Earth’s radiation belt electrons. A detailed global survey of lower-band chorus is performed using EMFISIS data from Van Allen Probes in near-equatorial orbits. In addition to the confirmation of the positive correlation of chorus wave intensities to geomagnetic activity and dayside-nightside distribution asymmetry of wave amplitude and occurrence probability, the analysis results find that in statistics lower-band chorus emissions exhibit higher wave occurrence rates and larger normalized peak wave frequencies in the magnetically northern hemisphere but somehow stronger peak wave intensities in the magnetically southern hemisphere. While overall the differences between the two magnetically hemispheric distributions tend to be not significant, it is important to establish the magnetically hemispheric distribution profiles of lowerband chorus with respect to <i>L</i>-shell, magnetic local time, and geomagnetic latitude for improved understanding of chorus-induced dynamics of radiation belt electrons.