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
The Van Allen Probe observations during the recovery phase of a large storm that occurred on 17 March 2015 showed that the ultrarelativistic electrons at the inner boundary of the outer radiation ...belt (L* = 2.6–3.7) exhibited butterfly pitch angle distributions, while the inner belt and the slot region also showed evidence of sub‐MeV electron butterfly distributions. Strong magnetosonic waves were observed in the same regions and at the same time periods as these butterfly distributions. Moreover, when these magnetosonic waves extended to higher altitudes (L* = 4.1), the butterfly distributions also extended to the same region. Combining test particle calculations and Fokker‐Planck diffusion simulations, we successfully reproduced the formation of the ultrarelativistic electron butterfly distributions, which primarily result from parallel acceleration caused by Landau resonance with magnetosonic waves. The coexistence of ultrarelativistic electron butterfly distributions with magnetosonic waves was also observed in the 24 June 2015 storm, providing further support that the magnetosonic waves play a key role in forming butterfly distributions.
Key Points
Ultrarelativistic electron butterfly distributions observed in association with magnetosonic wave
Simulations produce butterfly distributions consistent with observations
Butterfly distributions are caused by parallel acceleration via Landau resonance with MS waves
Radiation belt electrons can exhibit different types of pitch angle distributions in response to various magnetospheric processes. Butterfly distributions, characterized by flux minima at pitch ...angles around 90°, are broadly observed in both the outer and inner belts and the slot region. Butterfly distributions close to the outer magnetospheric boundary have been attributed to drift shell splitting and losses to the magnetopause. However, their occurrence in the inner belt and the slot region has hitherto not been resolved. By analyzing the particle and wave data collected by the Van Allen Probes during a geomagnetic storm, we combine test particle calculations and Fokker‐Planck simulations to reveal that scattering by equatorial magnetosonic waves is a significant cause for the formation of energetic electron butterfly distributions in the inner magnetosphere. Another event shows that a large‐amplitude magnetosonic wave in the outer belt can create electron butterfly distributions in just a few minutes.
Key Points
Electron butterfly distributions of ~100 keV are quickly formed in the radiation belts
Electron butterfly distributions are observed in direct association with magnetosonic waves
Numerical simulations produce butterfly distributions similar to the observations
A statistical survey of electron pitch angle distributions (PADs) is performed based on the pitch angle‐resolved flux observations from the Magnetic Electron Ion Spectrometer (MagEIS) instrument on ...board the Van Allen Probes during the period from 1 October 2012 to 1 May 2015. By fitting the measured PADs to a sinnα form, where α is the local pitch angle and n is the power law index, we investigate the dependence of PADs on electron kinetic energy, magnetic local time (MLT), the geomagnetic Kp index, and L shell. The difference in electron PADs between the inner and outer belt is distinct. In the outer belt, the common averaged n values are less than 1.5, except for large values of the Kp index and high electron energies. The averaged n values vary considerably with MLT, with a peak in the afternoon sector and an increase with increasing L shell. In the inner belt, the averaged n values are much larger, with a common value greater than 2. The PADs show a slight dependence on MLT, with a weak maximum at noon. A distinct region with steep PADs lies in the outer edge of the inner belt where the electron flux is relatively low. The distance between the inner and outer belt and the intensity of the geomagnetic activity together determine the variation of PADs in the inner belt. Besides being dependent on electron energy, magnetic activity, and L shell, the results show a clear dependence on MLT, with higher n values on the dayside.
Key Points
At the outer edge of the inner belt low‐energy electrons primarily possess a steep distribution
The steep distributions are dependent on electron energies, geomagnetic activity, and MLT
In the inner belt, the PADs are weakly dependent on MLT
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.
Key Points
Unusual hiss scatters ~50‐200 keV electrons more rapidly than normal hiss
Resultant electron lifetimes can be of ≤ 1 hour for energetic electrons
Unusual hiss scattering should be incorporated into radiation belt modeling
Electromagnetic ion cyclotron (EMIC) waves were observed at multiple observatory locations for several hours on 17 January 2013. During the wave activity period, a duskside relativistic electron ...precipitation (REP) event was observed by one of the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) balloons and was magnetically mapped close to Geostationary Operational Environmental Satellite (GOES) 13. We simulate the relativistic electron pitch angle diffusion caused by gyroresonant interactions with EMIC waves using wave and particle data measured by multiple instruments on board GOES 13 and the Van Allen Probes. We show that the count rate, the energy distribution, and the time variation of the simulated precipitation all agree very well with the balloon observations, suggesting that EMIC wave scattering was likely the cause for the precipitation event. The event reported here is the first balloon REP event with closely conjugate EMIC wave observations, and our study employs the most detailed quantitative analysis on the link of EMIC waves with observed REP to date.
Key PointsQuantitative analysis of the first balloon REP with closely conjugate EMIC wavesOur simulation suggests EMIC waves to be a viable cause for the REP eventThe adopted model is proved to be applicable to simulating the REP event
Energetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric ...Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of “flux‐reduced injections” resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection.
Plain Language Summary
Energetic particles are suddenly and abruptly pushed earthward through near‐Earth space via processes called “injections.” These are commonly seen by spacecraft as sudden time‐ and energy‐dependent increases in the number of observed particles and are known to play an important role in sourcing the planet's radiation belts. Investigating the processes governing energetic particle transport is critical to better understand fundamental plasma processes, the dynamics of Earth's radiation belts, and the effects of space weather. This paper investigates a set of similar, but unique, signatures where the number of particles suddenly decreases instead of increasing. The analysis presented here surprisingly reveals that the particles observed in these signatures come from the magnetotail, which raises new questions about what generates these signatures. We investigate two hypotheses to explain the observed decrease in particles: (1) that a process removes the particles from the system and (2) that a process similar to “normal” injections moves a population of decreased particles into an area where there are more particles (hence generating a local decrease). The analysis finds that the secondary hypothesis is far more likely to be the primary driver while the first cannot be ruled out entirely as a secondary contributor.
Key Points
MMS/FEEPS has repeatedly observed energy/time‐dispersed transient reductions in energetic electron flux in the dawnside magnetosphere
Drift tracing consistently indicates that these signatures are from a postmidnight magnetotail source
Results suggest that these features result from earthward injection in the presence of a negative radial gradient in phase space density
We present a set of neural network models that reproduce the dynamics of electron fluxes in the range of 50 keV ∼1 MeV in the outer radiation belt. The Outer Radiation belt Electron Neural net model ...for Medium energy electrons uses only solar wind conditions and geomagnetic indices as input. The models are trained on electron flux data from the Magnetic Electron Ion Spectrometer instrument onboard Van Allen Probes, and they can reproduce the dynamic variations of electron fluxes in different energy channels. The model results show high coefficient of determination (R2 ∼ 0.78–0.92) on the test data set, an out‐of‐sample 30‐day period from 25 February to 25 March in 2017, when a geomagnetic storm took place, as well as an out‐of‐sample one year period after March 2018. In addition, the models are able to capture electron dynamics such as intensifications, decays, dropouts, and the Magnetic Local Time dependence of the lower energy (∼<100 keV) electron fluxes during storms. The models have reliable prediction capability and can be used for a wide range of space weather applications. The general framework of building our model is not limited to radiation belt fluxes and could be used to build machine learning models for a variety of other plasma parameters in the Earth's magnetosphere.
Plain Language Summary
The Earth's radiation belts consist of energetic particles trapped by the geomagnetic field. This radiation environment is known to be particularly hazardous to spacecrafts and difficult to predict given the complex dynamics of electrons at different energy states. This paper presents a set of neural‐network‐based models that use measurements of geomagnetic and solar activities as drivers to reconstruct radiation belt electron fluxes ranging from 50 keV to 1 MeV. The models can determine the flux with high accuracy and capture the electron dynamics with long‐ and short‐term time scales. The models provide reliable prediction capability and can be used for a wide range of space weather applications. The approach through which our models are built is not limited to radiation belt fluxes and can be generalized for a variety of other plasma parameters in the Earth's magnetosphere.
Key Points
A set of neural network models was developed to reconstruct the 50 keV‐1 MeV electron fluxes in the outer radiation belt
The models reproduce fluxes with a high overall accuracy (R2 ∼ 0.78–0.92) during out‐of‐sample periods, including long‐ and short‐term dynamics
The models reproduce the time‐varying Magnetic Local Time dependence exhibited during storms by the lower energy electrons (∼<100 keV)
We present detailed observations and reconstructed magnetic field topologies from a magnetic reconnection event observed directly by NASA's Magnetospheric Multiscale (MMS) mission. Comparing and ...contrasting energy and angular distributions of >50 keV electrons observed by three MMS spacecraft separated by only ∼18 km revealed a combination of chaotic motion and surprisingly coherent gyrophase bunching within a few thousand kilometers of the reconnection site. Such gyrophase‐bunched electrons have not been observed previously at these energies in relation to magnetic reconnection. Results indicate that the magnetic topology around the electron diffusion region was relatively stable over several seconds (i.e., more than a thousand electron gyro‐periods), and the observed gyrophase bunching may be the highest‐energy extent of coherent gyrophase structures observed at lower‐energies closer to the reconnection site. We hypothesize that the coherent gyrophase bunching was evidence of these energetic electrons being accelerated at the reconnection site and possibly also in the outflowing exhaust jets of the active reconnection. This case study exemplifies how energetic electrons can carry coherent signals of remote magnetic topologies and processes relevant to the physics of magnetic reconnection.
Plain Language Summary
We examined data from NASA's Magnetospheric Multiscale mission to study how very energetic electrons behave near special sites where magnetic field lines break and merge back together, altering the topology of the magnetic field. We found unexpected features in the electron distributions that indicate how the changing magnetic topology might result in coherent scattering and acceleration of those electrons
Key Points
Combo of chaotic and coherent >50 keV electron motion observed near active reconnection site in Earth's magnetotail
Energetic electrons can carry coherent signals of remote magnetic topologies and processes relevant to the physics of magnetic reconnection
Coherent, gyro‐phase bunching in outflow exhaust very near the electron diffusion region may indicate relativistic electron acceleration
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