NASA's Van Allen Probes observed significant, long‐lived fluxes of inner belt electrons up to ∼1 MeV after geomagnetic storms in March and June 2015. Reanalysis of Magnetic Electron Ion Spectrometer ...(MagEIS) data with improved background correction showed a clearer picture of the relativistic electron population that persisted through 2016 and into 2017 above the Fennell et al. (2015, https://doi.org/10.1002/2014gl062874) limit. The intensity and duration of these enhancements allow estimation of decay timescales for comparison with simulated decay rates and theoretical lifetimes. We compare decay timescales from these data and DREAM3D simulations based on them using geomagnetic activity‐dependent pitch angle diffusion coefficients derived from plasmapause‐indexed wave data (Malaspina et al., 2016, https://doi.org/10.1002/2016gl069982, 2018, https://doi.org/10.1029/2018gl078564) and phase space densities derived from MagEIS observations. Simulated decay rates match observed decay rates more closely than the theoretical lifetime due to significantly nonequilibrium pitch angle distributions in simulation and data. We conclude that nonequilibrium effects, rather than a missing diffusion or loss process, account for observed short decay rates.
Plain Language Summary
Earth's radiation belts are influenced by a wide variety of source and loss processes, so it is difficult to model and forecast its evolution or predict its effects on spaceborne assets. Decay timescales for loss processes are essential to understanding this balance, but the theoretical predictions for these timescales in the inner radiation belt can exceed the observed decay times by an order of magnitude or more. We have observed and simulated an exceptional period of radiation belt injection and decay to understand this discrepancy. We have found that changes in the wave properties due to geomagnetic activity can account for the difference: changes in the equilibrium distribution associated with the wave environment result in consistent refilling of non‐equilibrium modes that decay much faster than the equilibrium mode.
Key Points
DREAM3D simulations of Earth's inner electron belt, based on Van Allen Probes observations, are carried out to evaluate model decay rates
Pitch angle diffusion using coefficients reflecting geomagnetic activity demonstrates realistic decay rates
Decay rates extracted with a Random Sample Consensus‐based algorithm from modeled and observed fluxes agree, while theoretical lifetimes are too long
Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10-30 kilohertz for submarine communications. While it has been of intense scientific interest and practical ...importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8-1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.
Long‐lasting second‐harmonic poloidal standing Alfvén waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the ...plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (∼10 mHz) were also observed in the energy (W) range 50–300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of ∼−200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180° (southward moving protons) at W ∼ 120 keV. This feature is explained by drift‐bounce resonance (mωd ∼ ωb) of ∼120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density (
FH+) exhibits a bump‐on‐tail structure with
∂FH+/∂W>0 occurring in the 1–10 keV energy range. This
FH+ is unstable and can excite P2 waves through bounce resonance (ω ∼ ωb), where ω is the wave frequency.
Key Points
Second‐harmonic poloidal standing Alfvén waves were observed by Van Allen Probes
The waves were propagating westward with an azimuthal wave number of ‐200
Proton data exhibit signatures of bounce and drift‐bounce resonances
During geomagnetic storms the intensities of the outer radiation belt electron population can exhibit dramatic variability. Deep depletions in intensity during the main phase are followed by ...increases during the recovery phase, often to levels that significantly exceed their prestorm values. To study these processes, we simulate the evolution of the outer radiation belt during the 17 March 2013 geomagnetic storm using our newly developed radiation belt model (Conservative Hamiltonian Integrator for Magnetospheric Particles) based on test particle and coupled 3‐D ring current and global magnetohydrodynamic (MHD) simulations, and driven solely with solar wind and F10.7 flux data. Our approach differs from previous work in that we use MHD information to identify regions of strong, bursty, and azimuthally localized earthward convection in the magnetotail where test particles are then seeded. We validate our model using in situ Van Allen Probe electron intensities over a multiday period and show that our model is able to reproduce meaningful qualitative and quantitative agreement. Analysis of our model enables us to study the processes that govern the transition from the prestorm to poststorm outer belt. Our analysis demonstrates that during the early main phase of the storm the preexisting outer belt is largely wiped out via magnetopause losses, and subsequently, a new outer belt is created during a handful of discrete, mesoscale injections. Finally, we demonstrate the potential importance of magnetic gradient trapping in the transport and energization of outer belt electrons using a controlled numerical experiment.
Plain Language Summary
During geomagnetic storms the intensity of the outer radiation belt, a population of high‐energy electrons trapped within the near‐Earth space environment, can vary by a factor of 1,000 over a day. Often observed is the near disappearance of the belt at the onset of the storm and its intensification in the aftermath. Not yet understood is the relationship between the prestorm and poststorm belt and the mechanisms that create the more energetic poststorm electrons or equivalently: are the electrons that disappeared the ones that came back? and where did this new energy come from? We introduce our new radiation belt model, Conservative Hamiltonian Integrator for Magnetospheric Particles, and study the 2013 St. Patrick's Day Storm to address these questions. We find that at the onset of the storm the existing belt is annihilated and swept into interplanetary space and that during the course of the storm a new, stronger belt is created. Traditional thinking on belt energization has focused on processes acting on either individual electrons or the global magnetosphere. We show that there is room in the middle by demonstrating, using our high‐resolution simulations, that electrons can become trapped and energized within bursty, confined flow channels that erupt near Earth during the course of the storm.
Key Points
Our radiation belt model broadly reproduces observed electron intensity throughout the 17 March 2013 storm
During the storm the existing belt is lost and rebuilt via a handful of intense injections
Mesoscale structure in the inner magnetosphere can play a role in electron transport and energization
We present observations of the radiation belts from the Helium Oxygen Proton Electron and Magnetic Electron Ion Spectrometer particle detectors on the Van Allen Probes satellites that illustrate the ...energy dependence and L shell dependence of radiation belt enhancements and decays. We survey events in 2013 and analyze an event on 1 March in more detail. The observations show the following: (a) at all L shells, lower energy electrons are enhanced more often than higher energies; (b) events that fill the slot region are more common at lower energies; (c) enhancements of electrons in the inner zone are more common at lower energies; and (d) even when events do not fully fill the slot region, enhancements at lower energies tend to extend to lower L shells than higher energies. During enhancement events the outer zone extends to lower L shells at lower energies while being confined to higher L shells at higher energies. The inner zone shows the opposite with an outer boundary at higher L shells for lower energies. Both boundaries are nearly straight in log(energy) versus L shell space. At energies below a few 100 keV, radiation belt electron penetration through the slot region into the inner zone is commonplace, but the number and frequency of “slot filling” events decreases with increasing energy. The inner zone is enhanced only at energies that penetrate through the slot. Energy‐ and L shell‐dependent losses (that are consistent with whistler hiss interactions) return the belts to more quiescent conditions.
Key Points
Radiation belt dynamics are a strong function of energy and L shell
Events that fill the slot region are common at lower energies and rare at higher energies
During enhancement events different energies are enhanced in different spatial regions
In this study, we analyze an electromagnetic ion cyclotron (EMIC) wave event of rising tone elements recorded by the Van Allen Probes. The pitch angle distributions of relativistic electrons exhibit ...a direct response to the two elements of EMIC waves: at the intermediate pitch angle, the fluxes are lower; and at the low pitch angle, the fluxes are higher than those when no EMIC was observed. In particular, the observed changes in the pitch angle distributions are most likely to be caused by nonlinear wave‐particle interaction. The calculation of the minimum resonant energy and a test‐particle simulation based on the observed EMIC waves support the role of the nonlinear wave‐particle interaction in the pitch angle scattering. This study provides direct evidence for the nonlinear pitch angle scattering of electrons by EMIC waves.
Key Points
Direct evidence of EMIC‐induced pitch angle scattering of relativistic electrons is observed
The nondiffusive pitch angle distributions of electrons indicate the importance of nonlinear wave‐particle interaction
The calculation of minimum resonance energy and a test‐particle simulation support the observations
Whistler mode wave properties inside the plasmasphere and plumes are systematically investigated using 5‐year data from Van Allen Probes. The occurrence and intensity of whistler mode waves in the ...plasmasphere and plumes exhibit dependences on magnetic local time, L, and AE. Based on the dependence of the wave normal angle and Poynting flux direction on L shell and normalized wave frequency to electron cyclotron frequency (fce), whistler mode waves are categorized into four types. Type I: ~0.5 fce with oblique wave normal angles mostly in plumes; Type II: 0.01–0.5 fce with small wave normal angles in the outer plasmasphere or inside plumes; Type III: <0.01 fce with oblique wave normal angles mostly within the plasmasphere or plumes; Type IV: 0.05–0.5 fce with oblique wave normal angles deep inside the plasmasphere. The Poynting fluxes of Type I and II waves are mostly directed away from the equator, suggesting local amplification, whereas the Poynting fluxes of Type III and IV are directed either away from or toward the equator, and may originate from other source regions. Whistler mode waves in plumes have relatively small wave normal angles with Poynting flux mostly directed away from the equator and are associated with high electron fluxes from ~30 keV to hundreds of keV, all of which support local amplification. Whistler mode wave amplitudes in plumes can be stronger than typical plasmaspheric hiss, particularly during active times. Our results provide critical insights into understanding whistler mode wave generation inside the plasmasphere and plumes.
Key Points
Whistler mode waves are statistically analyzed both inside the plasmasphere and in the plumes based on Van Allen Probes observations
The occurrence rate and amplitudes of whistler mode waves inside the plasmasphere and plumes show dependence on L, MLT, and geomagnetic activity
The majority of whistler mode waves in plumes are suggested to be locally amplified due to energetic electron injection
Many Machine Learning (ML) systems, especially deep neural networks, are fundamentally regarded as black boxes since it is difficult to fully grasp how they function once they have been trained. ...Here, we tackle the issue of the interpretability of a high‐accuracy ML model created to model the flux of Earth's radiation belt electrons. The Outer RadIation belt Electron Neural net (ORIENT) model uses only solar wind conditions and geomagnetic indices as input features. Using the Deep SHAPley additive explanations (DeepSHAP) method, for the first time, we show that the “black box” ORIENT model can be successfully explained. Two significant electron flux enhancement events observed by Van Allen Probes during the storm interval of 17–18 March 2013 and non‐storm interval of 19–20 September 2013 are investigated using the DeepSHAP method. The results show that the feature importance calculated from the purely data‐driven ORIENT model identifies physically meaningful behavior consistent with current physical understanding. This work not only demonstrates that the physics of the radiation belt was captured in the training of our previous model, but that this method can also be applied generally to other similar models to better explain the results and to potentially discover new physical mechanisms.
Plain Language Summary
A neural network is regarded as a black box model since it can approximate any function but its structure won't give any insights on the nature of the function being approximated. A set of neural network models named Outer RadIation belt Electron Neural net have been developed previously to model the electron flux of the outer radiation belt. In this work, we demonstrate the general flow of explaining the machine learning (ML) model of radiation belts and investigate two typical events during the storm and non‐storm times. The results identify physically meaningful behavior and are consistent with current physical understanding, additionally providing new insight into radiation belt dynamics. Furthermore, the proposed framework can be generalized for a variety of other ML models, including various plasma parameters in the Earth's magnetosphere.
Key Points
We demonstrate the feature attribution method for a machine learning model of electron flux
We quantify the effects of geomagnetic indices and solar wind parameters on electron flux during a storm time event and a non‐storm event
Our feature importance results identify physical effects that are consistent with our current understanding
We present the first ray tracing results of chorus waves generated in minimum‐B pockets. A new ray tracer is developed to accommodate an arbitrary magnetic field model, which enables us to construct ...minimum‐B pocket geometries using the Tsyganenko (1989) model. Rays with different frequencies and initial wave normal angles are launched from minimum‐B pockets and traced under different geomagnetic activity and hot electron density conditions. Our results indicate that chorus waves generated from minimum‐B pockets are usually highly localized and thus are unlikely to be detected unless the spacecraft is very close to the source region. These waves are also very unlikely to propagate into the plasmasphere. The propagation distances generally decrease with increasing geomagnetic activity.
Plain Language Summary
Chorus waves are an important kind of electromagnetic waves in the near‐Earth environment. We trace the ray paths of chorus waves generated in regions of low magnetic field intensity in the high‐latitude dayside magnetosphere (referred to as minimum‐B pockets) using a newly developed ray tracer which can deal with more complex geomagnetic field than the simplest model of magnetic field produced by a dipole magnet. Rays are launched with a range of frequencies, initial wave normal angles and space weather conditions. Our results indicate that chorus waves generated from minimum‐B pockets are usually highly localized and thus are very unlikely to be detected unless the spacecraft is very close to the source region. They are also very unlikely to propagate into the plasmasphere. The propagation distances in general decrease with enhanced geomagnetic activity.
Key Points
Ray tracing under a minimum‐B pocket geometry for chorus waves is conducted
Under average hot electron condition, MBPG chorus waves get damped within 1 or 2 Re of distance and thus are highly localized
Even when the hot electron proportion is only several ten thousands, MBPG chorus waves are very unlikely to propagate into the plasmapause
Abstract
Excitation of toroidal mode standing Alfvén waves in the midnight sector of the inner magnetosphere in association with substorms is well documented, but studies are sparse on dayside ...sources for the waves. This paper reports observation of midnight toroidal waves by the Van Allen Probe B spacecraft during a geomagnetically quiet period on 12–13 May 2013. The spacecraft detected toroidal waves excited at odd harmonics below 30 mHz as it moved within the plasmasphere from
2100 magnetic local time to
0030 magnetic local time through midnight in the dipole
range 4.2–6.1. The frequencies and the relationship between the electric and magnetic field components of the waves are consistent with theoretical toroidal waves for a reflecting ionosphere. At the time of the nightside toroidal waves, compressional waves were observed by geostationary satellites located on the dayside, and the amplitudes of both types of waves varied with the cone angle of the interplanetary magnetic field. The nightside toroidal waves were likely driven by fast mode waves that resulted from transmission of upstream ultralow frequency waves into the magnetosphere. Ground magnetometers located near the footprint of the spacecraft did not detect toroidal waves.
Key Points
Multiharmonic toroidal standing Alfvén waves were detected in the midnight sector of the plasmasphere
Interplanetary magnetic field cone angle was small, which suggests that foreshock ULF waves were the energy source
The toroidal waves were not detected on the ground at stations located in the midnight sector
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