Chorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards to their potential effects on electron dynamics. They can efficiently accelerate or ...precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross calibration in the overlap domain. We used 4 years (2012–2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20°, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45° in the chorus frequency range 0.1fce–0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different magnetic local time sectors and for two ranges of L shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range.
Key Points
The lower and upper band chorus model based on Van Allen Probes and Cluster VLF measurements is developed
The modeled parameters are chorus amplitude and wave normal angle distribution, wave frequency on L, MLT, MLat, Kp
The diffusion rates are estimated and compared with previous model results
We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave ...power at harmonics is due to a nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to the whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.
Resonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low ...geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating only <150 keV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.
Key Points
Near‐equatorial and ground‐based measurements of whistler‐mode waves are accompanied by relativistic electron precipitation
In the presence (absence) of ducted wave propagation, as monitored by propagation to the ground, the precipitating electron energies are above (below) 150 keV
Ducted whistler‐mode waves may play a key role in relativistic electron loss in the inner magnetosphere
Abstract
Whistler mode waves are among the most intense electromagnetic emissions and play an important role in the energy redistribution between electron populations in the Earth inner magnetosphere ...through wave‐particle resonant interactions. Usually generated by transversely anisotropic plasma sheet electron populations (
10–30 keV) through cyclotron resonance, whistler mode waves can effectively accelerate a small fraction of the seed population of energetic electrons (
100 keV) up to relativistic energies. However, these waves can be efficiently damped through simultaneous interactions with much more numerous suprathermal electrons (
0.1–1 keV) via Landau resonance. Recent observations indeed show that electron distributions accompanied by intense whistler mode emissions often contain a plateau‐like electron population at energies close to the energy of Landau resonance with the waves. However, simultaneous observations of these waves and of the related plateau population does not prove a causal relationship. Here, we test the hypothesis that such a plateau population may have been formed by whistler mode waves generated earlier, or by other types of waves. Combining analytical estimates and spacecraft observations, we show that this plateau population is often unlikely to be formed by whistler mode waves alone. We suggest three alternative scenarios that can lead to the formation of plateau populations and test these scenarios based on spacecraft observations. We show that a plateau population can be formed by ultralow frequency electric fields (carried by kinetic Alfven waves or time domain structures) often accompanying injections of plasma sheet electrons—the energy source for whistler mode waves. We also discuss the possible role of ionospheric secondary electrons.
Key Points
Plateau electron populations are often unlikely to be formed by acceleration through Landau resonance with whistler mode waves alone
Field‐aligned electric fields carried by kinetic Alfven waves and electrostatic noise are good candidates for electron plateau formation
A weak damping of whistlers in Landau resonance with preexisting electron plateau populations likely determines their predominant observations
Whistler mode chorus waves are responsible for electron acceleration in Earth's radiation belts. It is unclear, however, whether the observed acceleration is still well described by quasi‐linear ...theory, or if this acceleration is due to intense waves that require nonlinear treatment. Here, we perform a comprehensive statistical analysis of intense lower‐band chorus wave packets to investigate the relationships between wave frequency variations, packet length, and wave amplitude, and their temporal variability. We find that 15% of the wave power is carried by long packets, with low frequency sweep rates (linear trend in time) that agree with the nonlinear theory of chorus wave growth. Eighty‐five percent of the wave power, however, comes from short packets with large frequency variations around the linear trend. The kappa‐like probability distribution of these variations is consistent with random superposition of different waves that could result in a destruction of nonlinear resonant interaction.
Key Points
We investigate the relationships between chorus wave frequency variations, packet length, and wave amplitude
Fifteen percent of the wave power is carried by long packets with low frequency sweep, as predicted by nonlinear chorus wave generation theory
Eighty‐five percent of the wave power comes from packets with large frequency variations that increase as packets become shorter
Electron resonant interaction with whistler mode waves is traditionally considered as one of the main drivers of radiation belt dynamics. The two main theoretical concepts available for its ...description are quasi‐linear theory of electron scattering by low‐amplitude waves and nonlinear theory of electron resonant trapping and phase bunching by intense waves. Both concepts successfully describe some aspects of wave‐particle interactions but predict significantly different timescales of relativistic electron acceleration. In this study, we investigate effects that can reduce the efficiency of nonlinear interactions and bridge the gap between the predictions of these two types of models. We examine the effects of random wave phase and frequency variations observed inside whistler mode wave packets on nonlinear interactions. Our results show that phase coherence and frequency fluctuations should be taken into account to accurately model electron nonlinear resonant acceleration and that, along with wave amplitude modulation, they may reduce acceleration rates to realistic, moderate levels.
Key Points
Chorus wave phase randomly fluctuates between subpackets inside longwave packets
Wave phase coherence significantly influences the efficiency of electron acceleration via nonlinear resonant interaction
Fluctuations of wave frequency within longwave packets similarly reduce the efficiency of nonlinear resonant electron acceleration
During whistler wave excitation, electron energy is transferred from electrons to waves. There is little observational evidence that such a process operates effectively in the magnetotail, however. ...Using a large observational database from the Time History of Events and Macroscale Interactions during Substorms mission, we investigate whistler wave excitation within and around dipolarizing flux bundles (DFBs), critical energy transporters in the magnetotail, and the evacuation of perpendicular electron energy by whistler wave Poynting flux. We find that perpendicular anisotropy of suprathermal (~10 keV) electrons is the major free energy source for whistler wave excitation near DFBs. During earthward transport of electrons by DFBs, 3.7% of the suprathermal electron energy flux is evacuated in the form of whistler wave Poynting flux. This suggests that whistler waves play an important role in electron thermodynamics in the magnetotail by significantly modifying the otherwise adiabatically shaped electron distributions.
Key Points
Perpendicular anisotropy of suprathermal (~10 keV) electrons is the major free energy source for whistler wave excitation near DFBs
Whistler waves can occasionally radiate up to several percent of the energy flux of the suprathermal electron population near DFBs
Substorm growth phase in the magnetotail is characterized by formation of a thin current sheet (CS) becomes unstable due to external or internal drivers. Such instability results in magnetic field ...line reconnection, the substorm onset. The CS thinning, as a key process of substorm dynamics, has been included into many global and local simulations of the magnetotail magnetic reconnection. However, recent observations indicate that the evolution of plasma characteristics and magnetic field configuration during the CS thinning can differ from predictions of the classical adiabatic scenario. In this study, we combine two most extensive datasets of the CS evolution, as observed by Cluster and THEMIS missions for 2001–2009 and 2015, respectively. We show that for a wide range of downtail distances and dawn‐dusk direction there are quite similar quantitative characteristics of the thinning: the magnetic field line stretching (north‐south magnetic field decrease), the intensification of the current density, and the evolution of plasma temperatures and densities. We confirm that the process cannot be directly associated with increase of the lobe magnetic pressure. Using advantages of multispacecraft measurements and CS flapping motion, we demonstrate that the thinning is usually result in the equatorial density increase and plasma temperature decrease. We discuss the revealed evolution features in the context of the thermodynamical CS characteristics for contemporary thinning models.
Key Points
Current sheet (CS) thinning characteristics in the magnetotail are surveyed
Simultaneous plasma cooling and density growth during CS thinning is demonstrated
Evolution of ion and electron temperature anisotropy in thinning CS is shown
Abstract
We present statistical analysis of 11,200 proton kinetic-scale current sheets (CS) observed by the Parker Solar Probe during 10 days around the first perihelion. The CS thickness
λ
is in the ...range from a few to 200 km with the typical value around 30 km, while current densities are in the range from 0.1 to 10
μ
A m
−2
with the typical value around 0.7
μ
A m
−2
. These CSs are resolved thanks to magnetic field measurements at 73–290 samples s
−1
resolution. In terms of proton inertial length
λ
p
, the CS thickness
λ
is in the range from about 0.1 to 10
λ
p
with the typical value around 2
λ
p
. The magnetic field magnitude does not substantially vary across the CSs, and accordingly the current density is dominated by the magnetic-field-aligned component. The CSs are typically asymmetric with statistically different magnetic field magnitudes at the CS boundaries. The current density is larger for smaller-scale CSs,
J
0
≈ 0.15 × (
λ
/100 km)
−0.76
μ
A m
−2
, but does not statistically exceed the Alfvén current density
J
A
corresponding to the ion-electron drift of the local Alfvén speed. The CSs exhibit remarkable scale-dependent current density and magnetic shear angles,
J
0
/
J
A
≈
0.17
×
(
λ
/
λ
p
)
−
0.67
and
Δ
θ
≈
21
°
×
(
λ
/
λ
p
)
0.32
. Based on these observations and comparison to recent studies at 1 au, we conclude that proton kinetic-scale CSs in the near-Sun solar wind are produced by turbulence cascade, and they are automatically in the parameter range, where reconnection is not suppressed by the diamagnetic mechanism, due to their geometry dictated by turbulence cascade.
Simultaneous observations of electron velocity distributions and chorus waves by the Van Allen Probe B are analyzed to identify long‐lasting (more than 6 h) signatures of electron Landau resonant ...interactions with oblique chorus waves in the outer radiation belt. Such Landau resonant interactions result in the trapping of ∼1–10 keV electrons and their acceleration up to 100–300 keV. This kind of process becomes important for oblique whistler mode waves having a significant electric field component along the background magnetic field. In the inhomogeneous geomagnetic field, such resonant interactions then lead to the formation of a plateau in the parallel (with respect to the geomagnetic field) velocity distribution due to trapping of electrons into the wave effective potential. We demonstrate that the electron energy corresponding to the observed plateau remains in very good agreement with the energy required for Landau resonant interaction with the simultaneously measured oblique chorus waves over 6 h and a wide range of L shells (from 4 to 6) in the outer belt. The efficient parallel acceleration modifies electron pitch angle distributions at energies ∼50–200 keV, allowing us to distinguish the energized population. The observed energy range and the density of accelerated electrons are in reasonable agreement with test particle numerical simulations.
Key Points
Oblique large‐amplitude chorus (<100 mV/m) were observed during more than 6 h
Electrons are accelerated from 2 to 10 keV to 50 to 200 keV through the Landau trapping
The proposed acceleration mechanism can produce locally the ∼100 keV seed population
PDF
