The impact of randomly distributed field‐aligned density irregularities on whistler‐mode wave propagation is investigated using full‐wave simulations and multipoint spacecraft observations. The irregularities are modeled as randomized density perturbations between 1% and 10% of the nominal background density value with scales of ∼10–60 km transverse and ∼50–500 km along the background magnetic field. The density irregularities affect whistler wave propagation and lead to spatial modulation of wave average power density accompanied by spreading of the wave normal angle distribution. Wave power variation is shown to statistically increase with the depth of density irregularities. The simulation results are in good agreement with the observed correlations of chorus power and variation of the plasma density from multipoint observations by the four Magnetosphere MultiScale spacecraft. The change in fundamental wave properties from scattering from these irregularities affects the efficiency of wave‐particle interactions in the radiation belts and needs to be incorporated into large‐scale energetic‐particle flux models.

Controlled experiments involving injection of 0.5 Hz-8 kHz electromagnetic waves into the Earth's magnetosphere have played an important role in discovering and elucidating wave-particle interactions in near-Earth space. Due to the significant engineering challenges of efficiently radiating in the ELF/VLF: 300 Hz-30 kHz band, few experiments have been able to provide sustained transmissions of sufficient power to excite observable effects for scientific studies. Two noteworthy facilities that were successful in generating a large database of pioneering and repeatable observations were the Siple Station Transmitter in Antarctica and the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Both facilities were able to excite Doppler shifted cyclotron resonance interactions leading to linear and non-linear wave amplification, triggering of free running emissions, and pitch angle scattering of energetic electrons. Amplified and triggered waves were primarily observed on the ground in the geomagnetic conjugate region after traversal of the magnetosphere along geomagnetic field aligned propagation paths or in the vicinity of the transmitter following two traversals of the magnetosphere. In several cases, spacecraft observations of the amplified and triggered signals were also made. The observations show the amplifying wave particle interaction to be dynamically sensitive to specific frequency and also specific frequency-time format of the transmitted wave. Transmission of multiple coherent waves closely spaced in frequency showed that the wave particle interaction requires a minimum level of coherency to enter the non-linear regime. Theory and numerical simulations point to cyclotron resonance with counter streaming particles in the 10-100 keV range as the dominant process. A key feature of the non-linear interaction is the phase-trapping of resonant particles by the wave that is believed to drive non-linear wave amplification and the triggering of free-running emissions. Observations and modeling of controlled wave injections have important implications for naturally occurring whistler mode emissions of hiss and chorus and the broader phenomena of radiation belt dynamics. A review of observational, theoretical, and numerical results is presented and suggestions for future studies are made.

Modeling of gyroresonant wave-particle interactions in the radiation belts requires solving the Vlasov-Maxwell system of equations in an inhomogenous background geomagnetic field. Previous works have employed particle-in-cell methods or Eulerian solvers (such as the Vlasov Hybrid Simulation code) to provide numerical solutions to the problem. In this report, we provide an alternative numerical approach by utilizing a first order finite difference upwind scheme. When coupled to the narrowband Maxwell's equations, the model reproduces linear as well as nonlinear wave growth of coherent signals. Wave growth is nonlinear growth when the wave amplitude exceeds the minimum value for phase trapping of counterstreaming particles and is linear otherwise. The model also demonstrates free-running frequency variation for a case with a high linear growth rate. In addition, the model confirms the theoretical prediction of a stable "phase-space hole" during the nonlinear growth process. The plasma parameters and L shell used in this study are typical of those associated with the Siple Station wave injection experiment.

Controlled experiments with dedicated ground-based ELF/VLF (0.3-30 kHz) transmitters are invaluable in investigating nonlinear whistler mode wave-particle interactions in the Earth's magnetosphere.The most productive such experiment operated between 1973 and 1988 near L = 4 at Siple Station, Antarctica. A major effort has been undertaken to digitize and preserve a significant portion of the historical data set from the original magnetic tapes, and we describe here the data set and the processing techniques used to remove artifacts introduced during recording and playback. We analyze a commonly transmitted diagnostic format from 1986 and present statistics on the occurrence and properties of amplified ELF/VLF waves received by a ground-based receiver at the geomagnetic conjugate location to Siple at Lake Mistissini, Quebec. For the interval examined, only 11% of Siple transmissions are successfully received in the conjugate hemisphere with quiet geomagnetic conditions being significantly more conducive to successful reception. The total growth for the events examined is estimated to be 5-40 dB, and nonlinear growth rates are in the range of 20-350 dB/s. The observations show that as the nonlinear growth rate increases, the duration of nonlinear growth decreases. Significant linear correlation is found between the noise floor and the saturation level, with higher noise floors resulting from increases in natural magnetospheric emissions. Finally, we find a lack of correlation between the nonlinear growth rate and the noise, threshold, and saturation levels.

Observations of magnetospheric chorus being triggered by lightning‐induced whistlers are rare but provide a unique opportunity to remotely diagnose wave‐particle interactions in the Earth's radiation belts. The observations presented herein are unique in that whistlers, originating from lightning, are seen to trigger upper band chorus repeatedly over the course of 2 hr. Each whistler exhibits a distinct upper frequency cutoff that is used to estimate the anisotropy of the hot plasma distribution. Resulting anisotropy estimates are in good agreement with previous in situ measurements. While the anisotropy determines wave growth in the linear regime, access to the nonlinear regime requires the in situ wave amplitude to exceed the threshold for phase trapping of energetic electrons. The results suggest that while upper band chorus is less favorable to be spontaneously generated, the conditions in this band are more conducive for triggering of the chorus instability by an external input wave.