We survey 3 years (2013–2015) of data from the Van Allen Probes related to plasmaspheric plume crossing events. We detect 194 plume crossing events, and we find that 97% of the plumes are accompanied ...by very low frequency hiss emissions. The plumes are mainly detected on the duskside or dayside. Careful examination of the hiss spectra reveals that all hiss emissions consist of obvious fine structure. Application of a band‐pass filter reveals that the fine structure is consistent with the occurrence of discrete wave packets. The hiss data display high coherency. The events are classified by location. Duskside hiss and nightside hiss tend to have extremely high polarization with no chorus at the high‐frequency end of the dynamic spectrum. The duskside hiss has a distinct upper frequency limit. On the other hand, the dawnside hiss has strong chorus elements at the upper hiss frequency, which makes the upper frequency limit ambiguous. We show that the structure of whistler mode hiss is different from artificial random noise. Although noise also has fine spectral characteristics, the polarization and waveform data are totally different from the hiss cases. Our results strongly suggest that whistler mode hiss in plasmaspheric plumes universally possesses fine structure.
Key Points
We examine 194 plume crossing events observed by the Van Allen Probes
We observe hiss emissions in 97% of the plume events
All hiss emissions have obvious fine structure
On 23 February 2014, Van Allen Probes sensors observed quite strong electromagnetic ion cyclotron (EMIC) waves in the outer dayside magnetosphere. The maximum amplitude was more than 14 nT, ...comparable to 7% of the magnitude of the ambient magnetic field. The EMIC waves consisted of a series of coherent rising tone emissions. Rising tones are excited sporadically by energetic protons. At the same time, the probes detected drastic fluctuations in fluxes of MeV electrons. It was found that the electron fluxes decreased by more than 30% during the 1 min following the observation of each EMIC rising tone emissions. Furthermore, it is concluded that the flux reduction is a nonadiabatic (irreversible) process since holes in the particle flux levels appear as drift echoes with energy dispersion. We examine the process of electron pitch angle scattering by nonlinear wave trapping due to anomalous cyclotron resonance with EMIC rising tone emissions. The energy range of precipitated electrons agrees with the presumed energy for the threshold amplitude for nonlinear wave trapping. This is the first report of rapid precipitation (<1 min) of relativistic electrons by EMIC rising tone emissions and their drift echoes in time observed by spacecraft.
Key Points
Relativistic electrons disappear following intense EMIC rising tone in a few minutes
Drift echoes of localized electron depletion are detected
This is the direct observation of rapid loss process by the nonlinear EMIC scattering
We conduct test particle simulations to study the perturbations in a hot electron velocity distribution caused by a rising chorus element propagating parallel to the ambient magnetic field in the ...Earth's outer radiation belt. The wavefield is constructed from the nonlinear growth theory of chorus emissions of Omura (2021, https://doi.org/10.1186/s40623-021-01380-w), with additional considerations about saturation and propagation of the transverse resonant current being applied to model the subpacket structure. Using Liouville's theorem, we trace electrons back in time to reconstruct the evolution of electron velocity distribution at the magnetic equator. The electromagnetic hole created by nonlinear trapping and transport effects appears as a depression in the velocity distribution, aligned with the resonance velocity curve. We analyze the decrease of particle flux in this depression and estimate the energy resolution, pitch angle resolution, time resolution and geometric factor of particle analyzers needed to observe the perturbation. We conclude that particle detectors on current or recently operating spacecraft are always lacking in at least one of these parameters, which explains the missing direct observations of sharp phase space density depressions during chorus‐electron nonlinear resonant interaction. However, with a dedicated experiment and appropriate measurement strategy, such observations are within the possibilities of the current technology. Similarity of the simulated density perturbation and a step function mathematical model is used to draw an analogy between the backward wave oscillator regime of chorus generation and the nonlinear growth theory.
Plain Language Summary
The plasma environment in the Earth's magnetosphere supports natural growth of various electromagnetic waves, including the whistler‐mode chorus emissions, which consist of nonlinear chirping tones. These emissions can reach large amplitudes and play a major role in energization of radiation belt electrons. Nonlinear theories of chorus generation imply microscopic perturbations in resonant electron populations. A long‐standing problem is that these predictions were never directly confirmed by experimental observations. Here, we analyze perturbations of electron distribution functions numerically, taking into account spacecraft measurements of short subpackets within each chirping element. We reveal distinct perturbations, which are just below the measurability limits of existing spacecraft instruments. We thus explain the current absence of direct measurements of nonlinear effects of chorus on the electron distribution functions. We also suggest measurement strategies for future spacecraft instruments that can increase the number of detected interaction events.
Key Points
We analyze perturbations in a hot electron distribution caused by nonlinear interactions with a model chorus element with fine structure
A stripe structure of phase space density depletions and elevations are observed, associated with individual subpackets
Resolution of spacecraft instruments required to observe the leading most prominent stripe is estimated
Intense lower band chorus waves are ubiquitous in the inner magnetosphere. Their properties have been modeled by various codes and investigated using measurements of many spacecraft missions. This ...study aims to compare simulated and observed properties of chorus waves. We present detailed comparisons between results from four different codes of nonlinear chorus wave generation and statistical observations from satellites, focusing on the fine structure of such chorus waves. We show that simulations performed with these different codes well reproduce the observed wave packet characteristics, although in somewhat complementary parameter domains as concerns wave packets sizes, amplitudes, and frequency sweep rates. In particular, simulations generate both the frequently observed short wave packets with high positive and negative frequency sweep rates, and the more rare long and intense packets with mainly rising tones. Moreover, simulations reproduce quantitatively both the increase of the size of the observed chorus wave packets with their peak amplitude, and the fast decrease of their frequency sweep rate as their size increases. This confirms the reliability of the main existing codes for accurately modeling chorus wave generation, although we find that initial conditions should be carefully selected to reproduce a given parameter range.
Key Points
Four different codes of chorus wave nonlinear generation well reproduce the observed wave packet characteristics
The simulations recover complementary domains of observed wave packets sizes, amplitudes, and frequency sweep rates
The simulations reproduce the observed short wave packets with high positive and negative frequency sweep rates
Prompt recovery of MeV (millions of electron Volts) electron populations in the poststorm core of the outer terrestrial radiation belt involves local acceleration of a seed population of energetic ...electrons in interactions with VLF chorus waves. Electron interactions during the generation of VLF rising tones are strongly nonlinear, such that a fraction of the relativistic electrons at resonant energies are trapped by waves, leading to significant nonadiabatic energy exchange. Through detailed examination of VLF chorus and electron fluxes observed by Van Allen Probes, we investigate the efficiency of nonlinear processes for acceleration of electrons to MeV energies. We find through subpacket analysis of chorus waveforms that electrons with initial energy of hundreds of keV to 3 MeV can be accelerated by 50 keV–200 keV in resonant interactions with a single VLF rising tone on a time scale of 10–100 ms.
Key Points
Prompt (<1 h) MeV electron acceleration in the inner magnetosphere accompanies substorm injections and strong VLF chorus rising tones
Highly coherent subpacket structure in strong VLF rising tones accounts for significant nonlinear acceleration efficiency
MeV seed electrons can be accelerated by 50 keV–200 keV in 10–100 ms in nonlinear interactions with a single VLF chorus rising tone
Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume ...region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large‐amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p‐p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground‐based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA‐POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field‐aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC‐induced loss processes for this event.
Key Points
Compression‐induced EMIC waves were observed across 12 h of local time
EMIC‐triggered emissions appeared during the strongest compression
Intense EMIC waves outside the plasmasphere depleted the radiation belts
We present an observation of rapid flux depressions in relativistic electrons, which is referred to as “EMIC‐induced drifting electron holes (EDEHs).” The Arase, Van Allen Probes, and THEMIS detected ...simultaneously electron flux fluctuations. The time variation of flux shows depressions of 1‐min scale with energy dispersion, which appear only in the relativistic energy range and small pitch angles. These characteristics of the flux depression indicate that electromagnetic ion cyclotron waves caused pitch angle scattering on a short time scale in a longitudinally limited region. The Arase satellite detected the local depression of the phase space density of 1,000 MeV/G electron, indicating that EMIC waves cause the true loss of electrons. Tracing the energy dispersion profile of EDEHs, we show that EDEHs are formed at localized region in the dusk side. Multisatellite observations demonstrate that a series of EDEHs eventually cause a substantial depression of the radiation belt on 1‐hr time scale.
Plain Language Summary
This study focuses on the electrons with energies ranging from hundreds of keV to several MeV in the Earth's radiation belt. This study reports the observation of “EMIC‐induced drifting electron holes (EDEHs)” in electron flux near‐simultaneously detected by four satellites: Arase, Van Allen Probes A and B, and THEMIS‐A. EDEHs were caused by localized electron precipitation induced by structured electromagnetic ion cyclotron waves. The observation of EDEHs clearly shows the energy range of the loss process, the magnitude of the loss, and the longitudinal range. Our results highlight that the detection of EDEHs provides a way of monitoring the macroscopic dynamics of the magnetosphere through particle acceleration, loss, and transport.
Key Points
“EMIC‐induced drifting electron hole (EDEH)” was simultaneously detected by four satellites in the radiation belt
The radial profiles of phase space density indicate the local losses of relativistic electrons caused by EMIC waves
Multisatellite observations reveal that the flux at L* = 5.2–5.4 gradually decreases in a few tens of minutes
In the magnetosheath, intense whistler mode waves, called “Lion roars,” are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four ...Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field‐aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of ~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure.
Key Points
Whistler mode waves are generated near local minima of magnetic field intensity and propagate along field lines on both sides
Pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation
The waves consist of many small coherent wave packets of tens of electron gyroradii perpendicular to magnetic fields
We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector (FD). The TALE air FD is also sensitive to the Cherenkov ...light produced by shower particles. Low-energy cosmic rays, in the PeV energy range, are detectable by TALE as Cherenkov events. Using these events, we measure the energy spectrum from a low energy of ∼2 PeV to an energy greater than 100 PeV. Above 100 PeV, TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we describe the detector, explain the technique, and present results from a measurement of the spectrum using ∼1000 hr of observation. The observed spectrum shows a clear steepening near 1017.1 eV, along with an ankle-like structure at 1016.2 eV. These features present important constraints on the origin of galactic cosmic rays and on propagation models. The feature at 1017.1 eV may also mark the end of the galactic cosmic ray flux and the start of the transition to extragalactic sources.
We perform an electromagnetic full particle simulation to study the generation mechanism of VLF whistler‐mode chorus emissions in the equatorial region of the magnetosphere. Parabolic variation of ...the static magnetic field is assumed as a model for the dipole magnetic field in the vicinity of the equator. We have cold thermal electrons and relatively low anisotropic hot electrons as plasma particles. In the initial phase, the amplitude growth of the incoherent whistler‐mode waves is determined by the linear growth rate. When the wave amplitude reaches a certain level, it begins to grow more rapidly with a series of rising tone emissions consisting of coherent phase structures in the vicinity of the magnetic equator, and their wave packets propagate away form the magnetic equator. The frequency sweep rates of the excited rising tone elements decrease gradually. We find a distinct threshold for such a nonlinear wave growth generating the rising chorus‐like elements. The relation between the wave amplitude and the frequency sweep rate of each element found in the simulation fully supports the nonlinear wave growth theory of chorus emissions.
PDF
