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
Drift‐resonance wave‐particle interaction is a fundamental collisionless plasma process studied extensively in theory. Using cross‐spectral analysis of electric field, magnetic field, and ion flux ...data from the Van Allen Probe (Radiation Belt Storm Probes) spacecraft, we present direct evidence identifying the generation of a fundamental mode standing poloidal wave through drift‐resonance interactions in the inner magnetosphere. Intense azimuthal electric field (Eφ) oscillations as large as 10mV/m are observed, associated with radial magnetic field (Br) oscillations in the dawn‐noon sector near but south of the magnetic equator at L∼5. The observed wave period, Eφ/Br ratio and the 90° phase lag between Br and Eφ are all consistent with fundamental mode standing Poloidal waves. Phase shifts between particle fluxes and wave electric fields clearly demonstrate a drift resonance with ∼90 keV ring current ions. The estimated earthward gradient of ion phase space density provides a free energy source for wave generation through the drift‐resonance instability. A similar drift‐resonance process should occur ubiquitously in collisionless plasma systems. One specific example is the “fishbone” instability in fusion plasma devices. In addition, our observations have important implications for the long‐standing mysterious origin of Giant Pulsations.
Key Points
Unambiguous identification of drift‐resonance in magnetosphere
Broad implications for ring current and ground observations
Drift‐resonance similar to fishbone instability in Tokamak
We present initial dual spacecraft observations that for the first time both constrain the spatial scale size and provide spectral properties at medium energies of electron microbursts. We explore ...individual microburst events that occurred on 2 February 2015 using simultaneous observations made by the twin CubeSats which comprise the National Science Foundation (NSF) Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II). During these microburst events, the two identically instrumented FIREBIRD II CubeSats were separated by as little as 11 km while traversing electron precipitation regions in low‐Earth orbit. These coincident microburst events map to size scales >120 km at the equator. Given the prevalence of coincident and noncoincident events we conclude that this is of the same order of magnitude as that of the spatial scale size of electron microburst, an unknown property that is critical for quantifying their overall role in radiation belt dynamics. Finally, we present measurements of electron microbursts showing that precipitation often occurs simultaneously across a broad energy range spanning 200 keV to 1 MeV, a new form of empirical evidence that provides additional insights into the physics of microburst generation mechanisms.
Key Points
We present estimates of the size of individual microbursts from simultaneous observations of microbursts at 10 km spatial separation
Microbursts can occur over the entire energy range for 200 keV to 1 MeV in energy simultaneously
We present descriptions of the scientific capabilities of the FIREBIRD II CubeSat mission
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
Ultralow frequency (ULF) electromagnetic waves in Earth's magnetosphere can accelerate charged particles via a process called drift resonance. In the conventional drift resonance theory, a default ...assumption is that the wave growth rate is time independent, positive, and extremely small. However, this is not the case for ULF waves in the real magnetosphere. The ULF waves must have experienced an earlier growth stage when their energy was taken from external and/or internal sources, and as time proceeds the waves have to be damped with a negative growth rate. Therefore, a more generalized theory on particle behavior during different stages of ULF wave evolution is required. In this paper, we introduce a time‐dependent imaginary wave frequency to accommodate the growth and damping of the waves in the drift resonance theory, so that the wave‐particle interactions during the entire wave lifespan can be studied. We then predict from the generalized theory particle signatures during different stages of the wave evolution, which are consistent with observations from Van Allen Probes. The more generalized theory, therefore, provides new insights into ULF wave evolution and wave‐particle interactions in the magnetosphere.
Key Points
The effects of wave growth and damping are considered to generalize the conventional ULF wave‐particle drift resonance theory
Particle signatures are predicted to be very different from the characteristic 180 degree phase shift of particle fluxes across energies
Newly predicted particle signatures are consistent with Van Allen Probe observations, which validate the generalized theory
Thirty years ago Paulikas and Blake (1979) showed a remarkable correlation between geosynchronous relativistic electron fluxes and solar wind speed (Vsw). This seminal result has been a foundation of ...radiation belt studies, space weather forecasting, and current understanding of solar wind radiation belt coupling. We have repeated their analysis with a considerably longer‐running data set (1989–2010) from the Los Alamos National Laboratory energetic particle instruments with several surprising results. Rather than the roughly linear correlation between Vsw and log (flux), our results show a triangle‐shaped distribution in which fluxes have a distinct velocity‐dependent lower limit but a velocity‐independent upper limit. The highest‐electron fluxes can occur for any value of Vsw with no indication of a Vsw threshold. We also find a distinct solar cycle dependence with the triangle‐shaped distribution evident in 2 declining phase years dominated by high‐speed streams but essentially no correlation in 2 solar maximum years. For time periods that do show a triangle‐shaped distribution we consider whether it can be explained by scatter due to other parameters. We examine the role of time dependence and time lag in producing the observed distribution. We also look at the same statistical relationship but at energies ≪1 MeV. We conclude that the relationship between radiation belt electron fluxes and solar wind velocity is substantially more complex than suggested by previous statistical studies. We find that there are important ways in which the “conventional wisdom” stating that high‐velocity wind drives high‐MeV electron fluxes is, in general, either misleading or unsupported.
Using measurements from the Van Allen Probes and the Balloon Array for RBSP Relativistic Electron Losses (BARREL), we perform a case study of electromagnetic ion cyclotron (EMIC) waves and associated ...relativistic electron precipitation (REP) observed on 25–26 January 2013. Among all the EMIC wave and REP events from the two missions, the pair of the events is the closest both in space and time. The Van Allen Probe‐B detected significant EMIC waves at L = 2.1–3.9 and magnetic local time (MLT) = 21.0–23.4 for 53.5 min from 2353:00 UT, 25 January 2013. Meanwhile, BARREL‐1T observed clear precipitation of relativistic electrons at L = 4.2–4.3 and MLT = 20.7–20.8 for 10.0 min from 2358 UT, 25 January 2013. Local plasma and field conditions for the excitation of the EMIC waves, wave properties, electron minimum resonant energy Emin, and electron pitch angle diffusion coefficient Dαα of a sample EMIC wave packet are examined along with solar wind plasma and interplanetary magnetic field parameters, geomagnetic activity, and results from the spectral analysis of the BARREL balloon observations to investigate the two types of events. The events occurred in the early main phase of a moderate storm (min. Dst* = −51.0 nT). The EMIC wave event consists of two parts. Unlike the first part, the second part of the EMIC wave event was locally generated and still in its source region. It is found that the REP event is likely associated with the EMIC wave event.
Key Points
RBSP‐B observed EMIC waves and BARREL‐1T detected REP close by on 25‐26 January 2013
The pair of the events is the closest both in space and time among all the events from the two missions
The REP is likely associated with the EMIC wave activity
Relativistic electrons in the Earth's radiation belts are highly dynamic on a variety of timescales during the geomagnetic storm. Using Van Allen Probe spacecraft data, we investigate rapid ...enhancements of relativistic electrons in the outer radiation belt during a corotating interaction region (CIR) driven storm. Successive dipolarizations associated with 100keV‐MeV electron injections are identified. The evolution of energetic electrons is analyzed in the space of adiabatic invariants (μ, K and L*). Within less than a few hours, the phase space density (PSD) of the relativistic electrons promptly increases corresponding to injections of MeV electrons. The PSD of MeV electrons cumulatively increases by a factor of 4–10 at L* = 4.5–5.8 which is likely due to successive groups of dipolarizations and injections. Both near‐equatorial (small K) and off‐equatorial (large K) energetic electrons increase significantly. The increases in the near‐equatorial electrons are still dominant, suggesting the operation of betatron acceleration. The event study shows that successive dipolarizations associated with the CIR‐driven storm may rapidly affect relativistic electrons of the outer radiation belt over a wide range in the phase space.
Key Points
Relativistic electrons increase significantly through a cumulative effect of consecutive dipolarizations associated with a corotating interaction region‐driven storm
The phase space density of MeV electrons increases rapidly over a wide range of adiabatic invariants
The increase of off‐equatorial electrons is nearly half of that of the near‐equatorial electrons
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
We present the first evidence of electron microbursts observed near the equatorial plane in Earth's outer radiation belt. We observed the microbursts on 31 March 2017 with the Magnetic Electron Ion ...Spectrometer and Radiation Belt Storm Probes Ion Composition Experiment on the Van Allen Probes. Microburst electrons with kinetic energies of 29–92 keV were scattered over a substantial range of pitch angles, and over time intervals of 150–500 ms. Furthermore, the microbursts arrived without dispersion in energy, indicating that they were recently scattered near the spacecraft. We have applied the relativistic theory of wave‐particle resonant diffusion to the calculated phase space density, revealing that the observed transport of microburst electrons is not consistent with the hypothesized quasi‐linear approximation.
Plain Language Summary
Microbursts are a subsecond impulsive increase of electron precipitation from the outer Van Allen radiation belt into the atmosphere, believed to be an important loss process of radiation belt electrons. One possible source of microbursts is scattering of trapped radiation belt electrons by a plasma wave called chorus. Diffusion models show that chorus can both accelerate and scatter electrons into the atmosphere. Since microbursts have been previously observed by high‐altitude balloons and low Earth orbiting spacecraft, there has been little evidence that directly link the chorus wave and the microburst that it generated. We show evidence of microbursts and their progenitor waves observed deep inside the outer radiation belt by the Van Allen Probes spacecraft. The Van Allen Probes are configured to extensively study the wave and particle environment in the magnetosphere, which allows us to understand the microbursts' energy dependence, angular extent, and the scattering mechanism. This unique perspective enables us to understand how these electrons were transported by the chorus wave, and compare it to a hypothesized quasi‐linear diffusion model. Our results indicate that the observed transport of microburst electrons was not consistent with the hypothesized diffusion model.
Key Points
The first report of direct observation of microbursts at high altitude, near the equatorial plane
Microbursts' duration, flux enhancement, and energy spectra are similar to prior observations in LEO
Microburst generation is not consistent with a single quasi‐linear gyroresonant interaction with chorus waves
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