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
It has been suggested that whistler mode chorus is responsible for both acceleration of MeV electrons and relativistic electron microbursts through resonant wave‐particle interactions. Relativistic ...electron microbursts have been considered as an important loss mechanism of radiation belt electrons. Here we report on the observations of relativistic electron microbursts and flux variations of trapped MeV electrons during the 8–9 October 2012 storm, using the SAMPEX and Van Allen Probes satellites. Observations by the satellites show that relativistic electron microbursts correlate well with the rapid enhancement of trapped MeV electron fluxes by chorus wave‐particle interactions, indicating that acceleration by chorus is much more efficient than losses by microbursts during the storm. It is also revealed that the strong chorus wave activity without relativistic electron microbursts does not lead to significant flux variations of relativistic electrons. Thus, effective acceleration of relativistic electrons is caused by chorus that can cause relativistic electron microbursts.
Key Points
Balance between MeV electron acceleration and loss by chorus is investigated
Microburst precipitations well coincides with large increase in MeV electron fluxes
MeV electron acceleration by chorus is much efficient than microburst losses
Using Van Allen Probes' observations and established plasmapause location (Lpp) models, we investigate the relationship between the location of the initial enhancement (IE) of energetic electrons and ...the innermost (among all magnetic local time sectors) Lpp over five intense storm periods. Our study reveals that the IE events for ~30‐keV to ~2‐MeV electrons always occurred outside of the innermost Lpp. On average, the inner extent of the IE events (LIE) for <800‐keV electrons was closer to the innermost Lpp when compared to the LIE for >800‐keV electrons that was found consistently at ~1.5 RE outside of the innermost Lpp. The IE of tens of kiloelectron volts electrons was observed before the IE of hundreds of kiloelectron volt electrons, and the IE of >800‐keV electrons was observed on average 12.6 ± 2.3 hr after the occurrence of the earliest IE event. In addition, we report an overall electron (~30 keV to ~2 MeV) flux increase outside the plasmasphere during the selected storm periods, in contrast to the little change of energy spectrum evolution inside the plasmasphere; this demonstrates the important role of the plasmasphere in shaping energetic electron dynamics. Our investigation of the LIE‐Lpp relationship also provides insights into the underlying physical processes responsible for the dynamics of ~30‐keV to ~2‐MeV electrons.
Key Points
The initial enhancements of ~30‐keV to 2‐MeV electrons are always located outside of the innermost plasmapause locations
The initial enhancement of >800‐keV electrons always occurs at ~1.5 RE away from the 12‐hr innermost plasmapause locations
A drastic difference of the energy spectrum evolution between inside and outside of the plasmasphere is observed during storm periods
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 Van Allen Probe observation of drift‐resonance interaction between energetic electrons and ultralow frequency (ULF) waves on 29 October 2013. Oscillations in electron flux were observed at ...the period of ∼450 s, which is also the dominant period of the observed ULF magnetic pulsations. The phase shift of the electron fluxes (∼50 to 150 keV) across the estimated resonant energy (∼104 keV) is ∼360°. This phase relationship is different from the characteristic 180° phase shift as expected from the drift‐resonance theory. We speculate that the additional 180° phase difference arises from the inversion of electron phase space density (PSD) gradient, which in turn is caused by the drift motion of the substorm injected electrons. This PSD gradient adjusts the characteristic particle signatures in the drift‐resonance theory, which indicates a coupling effect between the magnetotail and the radiation belt and helps to better understand the wave‐particle interaction in the magnetosphere.
Key Points
Both electron flux oscillations in response to ULF waves and drift echoes of injected electrons are observed
A 360° phase shift in the electron fluxes modulated by ULF waves via drift‐resonance interaction is observed for the first time
The unusual phase relationship is associated with the inversion of PSD gradient caused by the drift motion of the injected electrons
The present study addresses two basic questions related to banded chorus waves in the Earth’s magnetosphere: 1) are chorus spectral gaps formed near the equatorial source region or during propagation ...away from the equator? and 2) why are chorus spectral gaps usually located below 0.5 fce (fce: electron gyro‐frequency)? By analyzing Van Allen Probes data, we demonstrate that chorus spectral gaps are observed in the source region where chorus waves propagate both in the parallel and anti‐parallel directions to the magnetic field. Chorus spectral gaps below 0.5 fce are associated with electron parallel acceleration at energies above the equatorial Landau resonant energies. We explain that initially generated chorus waves quickly isotropize the electron distribution through Landau resonant acceleration, and the isotropization occurs for higher energies at higher latitudes. The isotropized population, after returning to the magnetic equator, leads to a chorus gap typically below 0.5 fce by suppressing wave excitation.
Plain Language Summary
Chorus waves are naturally occurring electromagnetic waves in the Earth’s magnetized space known as the magnetosphere, and they typically have two frequency bands. A previous study proposed that the banded characteristics is produced by two anisotropic electron populations. This study aims to address two basic questions about banded chorus waves: 1) are chorus frequency gaps formed near the equatorial source region or during propagation away from equator? and 2) why are chorus frequency gaps usually located below 0.5 fce (fce: electron gyro‐frequency)? Van Allen Probe spacecraft data reveals that chorus frequency gaps are formed in the equatorial source region where the waves propagate both in the parallel and anti‐parallel directions to the magnetic field. By showing the electron distribution in association with banded chorus waves, we explain that initially generated chorus waves quickly accelerate electrons as they propagate away from equator, and the electrons being accelerated at high latitudes lead to a chorus gap below 0.5 fce when they bounce back to the equatorial source region.
Key Points
Chorus spectral gaps are formed near the equatorial source region where parallel and anti‐parallel emissions are interleaved
Landau resonance causes electron parallel acceleration and phase space density isotropization around half electron Alfven velocity, which increases with latitudes
Chorus spectral gaps are usually found below 0.5 fce due to electron anisotropy reduction caused by Landau resonance at high latitudes
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
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)
Substorms generally inject tens to hundreds of keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeVelectron ...injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the premidnight sector at L ∼ 5.5, Van Allen Probes (Radiation Belt Storm Probes)‐A observed a large dipolarization electric field (50 mV/m) over ∼40 s and a dispersionless injection of electrons up to ∼3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL‐GEO, THEMIS‐D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by 1 order of magnitude in less than 3 h in the outer radiation belt (L > 4.8). Our observations provide evidence that deep injections can supply significant MeV electrons.
Key Points
Unambiguous evidence of deep injections of MeV electrons from multispacecraft
Extremely large electric fields (50 mV/m) associated with the dipolarization
Strong dipolarizations may supply significant MeV electrons to radiation belts
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