Whistler Mode Research Articles

Alpha Transmitter Signal Reflection and Triggered Emissions

AbstractRussian Alpha radio navigation system (RSDN‐20) emits F1 = 11.9 kHz signals into the magnetosphere which propagate as whistler mode waves. Observed by waveform continuous burst mode from Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on Van Allen Probes, a case is presented and featured with ducted propagation, multiple reflections, and triggered emissions. Both risers and fallers appear in the triggered emissions. We use a ray tracing method to demonstrate ducted propagation, which has a similar wave normal angle near 150° as the observation. The arrival time of reflected signals is estimated using propagation analysis and compared with the observed signal arrival time. To test the nonlinear cyclotron resonance theory, the interaction region scale and the order of chirping rate in triggered emission are estimated. The estimated interaction region scale of MLAT = −3° is smaller than the observed MLAT = −6°. The discrepancy may be caused by the parallel propagation assumption and background field model.

Wave‐Particle Interactions Associated With Io's Auroral Footprint: Evidence of Alfvén, Ion Cyclotron, and Whistler Modes

AbstractThe electrodynamic coupling between Io and Jupiter gives rise to wave‐particle interactions across multiple spatial scales. Here we report observations during Juno's 12th perijove (PJ) high‐latitude northern crossing of the flux tube connected to Io's auroral footprint. We focus on plasma wave measurements, clearly differentiating between magnetohydrodynamic (MHD), ion, and electron scales. We find (i) evidence of Alfvén waves undergoing a turbulent cascade, suggesting Alfvénic acceleration processes together with observations of bi‐directional, broadband electrons; (ii) intense ion cyclotron waves with an estimated heating rate that is consistent with the generation of ion conics reported by Clark et al. (2020, https://doi.org/10.1029/2020GL090839); and (iii) whistler‐mode auroral hiss radiation excited by field‐aligned electrons. Such high‐resolution wave and particle measurements provide an insight into satellite interactions in unprecedented detail. We further anticipate that these spatially well‐constrained results can be more broadly applied to better understand processes of Jupiter's main auroral oval.

Low-frequency whistler waves excited by relativistic laser pulses.

It is shown by multidimensional particle-in-cell simulations that intense secondary whistler waves with special vortexlike field topology can be excited by a relativistic laser pulse in the highly magnetized, near-critical density plasma. Such whistler waves with lower frequencies obliquely propagate on both sides of the laser propagation axis. The energy conversion rate from laser to whistler waves can exceed 15%. Their dispersion relations and field polarization properties can be well explained by the linear cold-plasma model. The present work presents a new excitation mechanism of whistler modes extending to the relativistic regime and could also be applied in magnetically assisted fast ignition.

Phase Decoherence Within Intense Chorus Wave Packets Constrains the Efficiency of Nonlinear Resonant Electron Acceleration

AbstractElectron 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.

Observations and Modeling of Whistler Mode Waves in the Magnetospheric Density Ducts

AbstractThis paper presents results from a modeling study of localized packages of very low frequency (VLF) whistler mode waves observed by the NASA Van Allen Probes (also known as the Radiation Belt Storm Probes [RBSP]) satellites in the equatorial magnetosphere. The waves are detected inside enhancements of the plasma density, which we interpret as high‐density ducts. The quality of the ducting depends on the wave and duct parameters. The goal of this study is to investigate relations between these parameters. This study is based on the analysis and simulations of the electron MHD model. The results demonstrate that (1) the high‐density ducts with the magnitudes and perpendicular sizes observed by the RBSP satellites can trap VLF waves with the observed frequency and guide them over a significant distance with little attenuation. (2) It is possible to identify the range of magnitudes of the parallel and perpendicular wavelengths using the information about frequency of the waves and parameters of the duct.

Suprathermal Electron Evolution Under the Competition Between Plasmaspheric Plume Hiss Wave Heating and Collisional Cooling

AbstractSuprathermal electrons are a major heat source of ionospheric plasma. How the suprathermal electrons evolve during their bounces inside the plasmasphere is a fundamental question for the magnetosphere‐ionosphere coupling. On the basis of Van Allen Probes observations and quasi‐linear simulations, we present here the first quantitative study on the evolution of suprathermal electrons under the competition between Landau heating by whistler mode hiss waves and Coulomb collisional cooling by background plasma inside a plasmaspheric plume. We show that the Landau heating can prevail over the collisional cooling for >50 eV electrons and cause the field‐aligned suprathermal electron fluxes to increase by up to 1 order of magnitude within 1.5 hr. Our results imply that the plasmaspheric plume hiss waves could mediate energy from the ring current electrons to the ionospheric plasma.

How whistler mode hiss waves and the plasmasphere drive the quiet decay of radiation belts electrons following a geomagnetic storm

We show how an extended period of quiet solar wind conditions contributes to a quiet state of the plasmasphere that expands up to L ∼ 5.5, which creates the perfect conditions for wave-particle interactions between the radiation belt electrons and whistler-mode hiss waves. The correlation between the hiss waves and the plasma density is direct with hiss wave power increasing with plasma density, while it was generally assumed that these quantities can be specified independently. Whistler-mode hiss waves pitch angle diffuse and ultimately scatter freshly injected electrons into the atmosphere until the slot region is formed between the inner and outer belt and the outer belt is drastically reduced. In this study, we use and combine Van Allen Probes observations and Fokker-Planck numerical simulations. The Fokker-Planck model uses consistent event-driven pitch angle diffusion coefficients from whistler-mode hiss waves. Observations and simulations allow us to reach a global understanding of the variations in the trapped electron population with time, space, energy, and pitch angle that is based on the existing theory of quasi-linear wave-particle interactions. We show, for instance, the outer belt is pitch-angle homogeneous, which is explained by the event-driven diffusion coefficients that are roughly constant for equatorial pitch angle α 0∼<60°, E>100 keV, 3.5<L<Lpp∼6. The impact of this work is to bring an improved understanding of the belt evolution based on the integration of high quality and highly temporally and spatially resolved measurements that are integrated in modern computations. We also propose the event-driven method as an accurate method (within ×2) to predict the electron flux decay after storms.

Ionosphere Feedback to Electron Scattering by Equatorial Whistler Mode Waves

AbstractPlasma sheet electron precipitation, much of it driven by whistler mode waves, is critical for magnetosphere‐ionosphere coupling. This precipitation leads to a secondary electron population at low altitudes, which moves upward along magnetic field lines to the equatorial plasma sheet. We investigate observational evidence for such electron precipitation and the ionospheric feedback provided by the secondary electron outflows. Using the near‐equatorial Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we show that whistler mode wave bursts are accompanied by enhanced field‐aligned electrons. The properties of these electrons are not related to the whistler mode waves, as would have been expected from local wave‐particle interactions. Thus, it is unlikely that the field‐aligned electrons are formed by electron Landau resonance with whistler mode waves. This population can be the secondary electron outflows resulting from plasma sheet electron precipitation to the ionosphere. Combining THEMIS, Cluster, and Fast Auroral Snapshot Explorer (FAST) measurements, we show that this field‐aligned electron population is also observed at low altitudes, where it is associated with high‐frequency electrostatic waves. These low‐altitude waves are likely generated by the secondary electron outflows and, in turn, scatter these electrons outside of the loss cone, allowing them to be observed by the wide field‐of‐view particle instrument on the near‐equatorial THEMIS spacecraft. We discuss how this secondary electron population can subsequently alter whistler mode wave generation and propagation in the magnetosphere.

Collisional and resonance absorption of electromagnetic waves in a weakly collisional, inhomogeneous magnetoplasma slab

Absorbance of normally incident electromagnetic wave on a cold, weakly collisional, and inhomogeneous magnetoplasma slab is investigated. The plasma density is Budden-like sinusoidal profile, where the inhomogeniety is treated as a multilayered system of homogeneous sub-cells within the transfer matrix technique. For incident wave frequencies much above the ion cyclotron frequency, only right hand circularly polarized waves are relevant for wave propagation parallel to a static magnetic field. Calculations are performed in normalized parameters, that make the results suitable for many applications including atmospheric and laboratory plasmas. The presence of the dc-magnetic field leads to the formation of two absorption bands explained by plasma collisional dissipation and electron cyclotron resonance in the low frequency branch of the $R$-wave below the electron cyclotron frequency. The transmittance shows the emergence of the low frequency electron cyclotron wave, which becomes a Whistler mode at very low frequency. More detailed discussion on the effect of plasma collisionality, inhomogeneity, and dc-magnetic field on the propagation characteristics is given at the relevant place within the body of the manuscript.

Plasma Waves Causing Relativistic Electron Precipitation Events at International Space Station: Lessons From Conjunction Observations With Arase Satellite

AbstractWe report three different types of relativistic electron precipitation (REP) events observed at International Space Station (ISS), associated with electromagnetic ion cyclotron (EMIC) waves or whistler mode waves as observed by the Arase satellite at conjugate locations near the magnetic equator. Three different detectors installed on the ISS were complementarily used; CALET/CHD as the detector of precipitating MeV electrons, MAXI/RBM as the detector of sub‐MeV electrons from horizontal and vertical directions, and SEDA‐AP/SDOM to quantitatively measure the energy spectrum. The REP event on 21 August 2017 shows a quasiperiodic intensity variation at ~1 Hz which corresponds to variations of the EMIC waves at the Arase altitudes. The REP event on 24 April 2017 shows rapid and irregular intensity variation which corresponds to the amplitude variation of chorus waves, while the REP events on 26 October 2017 shows a smooth quasiperiodic time variation at ~0.2 Hz which corresponds to the amplitude variation of “electrostatic” whistler mode waves. This study clearly demonstrates that the time variation of REP events at ISS are caused by various types of plasma waves near the magnetic equator.

Global Distribution of Whistler Mode Waves in Jovian Inner Magnetosphere

AbstractWhistler mode waves are suggested to play a dual role in precipitation loss and acceleration of energetic electrons in Jovian magnetosphere. Using the combined wave data from Juno and Galileo, we constructed global maps of whistler mode waves in the Jovian inner magnetosphere (M‐shell &lt;20). Whistler mode waves are found to be extensively present over M‐shells from 6 to ~13, peaking at ~10, and extend to magnetic latitudes at least up to 50°. Whistler mode wave intensities vary from a few pT to ~100 pT, similar to the intensity level at Earth. The new finding on the latitudinal extension of whistler mode waves indicates their potential effects on electron precipitation over a broad energy range from ~100 eV to several MeV. The newly constructed whistler mode wave spectra are crucial for evaluating the accurate role of whistler mode waves in energetic electron dynamics in the Jovian inner magnetosphere.

Whistler Mode Waves Excited by Anisotropic Hot Electrons With a Drift Velocity in Earth's Magnetosphere: Linear Theory

AbstractWith a linear theoretical model, we have investigated the properties of whistler waves excited by anisotropic hot electrons with a drift velocity parallel to the background magnetic field, which is usually neglected in previous studies. It is found that a finite drift velocity can significantly change the properties of excited whistler waves, resulting in distinct properties for parallel and antiparallel propagating waves. In the high‐beta regime, as the drift velocity increases, the frequency of parallel propagating whistler waves increases, while that of antiparallel propagating waves is found to decline. So parallel and antiparallel propagating whistler waves appear in different frequency bands. However, the growth rate of parallel wave is always smaller than that of antiparallel wave and falls below 10−2Ωe for large drift velocities (vd/vth &gt; 1.5), in which case the parallel wave may be too weak to be observed. Generally, the growth rate of whistler waves in both directions is enhanced with the increasing anisotropy or proportion of hot electrons. In the low‐beta regime, the trends of the frequency and linear growth rate of excited whistler waves are quite similar to those in the high‐beta regime. But with the increase of the drift velocity, the wave normal angle of parallel propagating whistler waves gradually declines until reaching 0, while that of antiparallel propagating waves continues to increase. Our study may be helpful to understand various whistler mode spectra observed in the Earth's magnetosphere.

The Formation of Electron Heat Flux in the Region of Diffuse Aurora

AbstractWhistler and electrostatic electron cyclotron harmonics waves are responsible for scattering and precipitating the energetic plasma sheet electrons that drive the diffuse aurora. These primary electrons with energies in the kiloelectron volt range, simultaneously precipitating in magnetically conjugate regions, produce the secondary electron population and can be reflected by the atmosphere back through the magnetosphere and precipitate into the conjugate region with additional follow‐up atmospheric backscatter. Primary, degraded, and secondary electrons can be trapped back into the magnetosphere as they travel back and forth between the two magnetically conjugate ionospheres and continuously delivering their energy to the cold plasma sheet electrons and form the electron thermal fluxes that deposit this energy at the upper ionospheric altitudes. We consider the formation of these heat fluxes focusing on the magnetosphere‐ionosphere energy interplay of the entire superthermal electron spectra from 1 eV up to 10 keV and discuss the efficiency of the different spectral energy intervals that contribute to the electron plasma heating at the magnetospheric altitudes. Our parametric studies at L = 6.8, with lower and upper band chorus whistler wave amplitudes of 10 pT and electron cyclotron harmonic wave amplitudes of 1 mVm−1, indicate the dominant role of the whistler mode in the formation of the electron heat flux coming from the magnetosphere to the ionosphere.

Energetic Electron Scattering due to Whistler Mode Chorus Waves Using Realistic Magnetic Field and Density Models in Jupiter's Magnetosphere

AbstractWe evaluate energetic electron scattering in pitch angle and energy using realistic magnetic field and density models due to whistler mode chorus waves in Jupiter's magnetosphere and study their dependences on various wave and background parameters. We calculate the bounce‐averaged diffusion coefficients by considering the latitudinal variation of total electron density and ambient magnetic field intensity, using the VIP4 internal magnetic field and CAN current sheet model. The electron phase space density evolution due to chorus waves is simulated at M shell of 10, using the central wave frequency at 0.1fce and wave amplitude of 30 pT. Under the typical values of the ratio between the plasma frequency and electron cyclotron frequency, chorus waves could cause fast pitch angle scattering loss of energetic electrons from tens to several hundred keV in several hours, and gradual acceleration of relativistic electrons at several MeV in several days. The electron pitch angle scattering at ~500 keV and the acceleration at several MeV are both enhanced using the latitudinally varying density and VIP4 + CAN magnetic field model compared to the electron evolution using the constant density and dipole magnetic field model. Our sensitivity study indicates that the electron scattering at higher energy is caused by waves at lower frequencies or in a lower‐density background plasma, and the scattering is faster for waves at smaller wave normal angles. The electron diffusion is mainly caused by waves at lower latitudes, but the waves at higher latitudes (&gt;30°) contribute to the electron loss at higher energies (&gt;2 MeV).

A Model of the Subpacket Structure of Rising Tone Chorus Emissions

AbstractThe nonlinear growth theory of chorus emissions is used to develop a simple model of the subpacket formation. The model assumes that the resonant current, which is released from the source to the upstream region, radiates a new whistler mode wave with a slightly increased frequency, which triggers a new subpacket. Saturation of the growth in amplitude is controlled by the optimum amplitude. Numerical solution of advection equations for each subpacket, with the chorus equations acting as the boundary conditions, produces a chorus element with a subpacket structure. This element features an upstream shift of the source region with time and an irregular growth of frequency, showing small decreases between adjacent subpackets. The influence of input parameters on the number of subpackets, the shift of the source, the frequency sweep rate, and the maximum amplitude is analyzed. The model well captures basic features of instantaneous frequency measurements provided by the Van Allen Probes spacecraft. The modeled wave field can be used in future particle acceleration studies.

Whistler Mode Waves in the Compressional Boundary of Foreshock Transients

AbstractEarth's foreshock is filled with backstreaming particles that can generate a variety of waves and foreshock transients. According to recent studies, these particles can be further accelerated while being scattered by field fluctuations, including waves, inside foreshock transients, contributing to particle acceleration at the parent bow shock. The properties of these waves and how they interact with particles and affect particle acceleration inside foreshock transients are still unclear, however. Here we take the first step to study one important type of these waves, whistler waves. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations and employ multiple case studies to investigate the properties of whistler waves in the compressional boundaries of foreshock transients where THEMIS wave burst mode is triggered. We show that the whistler waves are quasi parallel propagating with bidirectional Poynting vectors, suggesting that they are locally generated. We focus on how they interact with electrons. We show that the diffusion surfaces for these waves in the electron velocity space match the observed electron phase space density distribution contours better when the modeled pitch angle diffusion coefficients from these waves are higher. We also demonstrate that higher‐energy electrons are more likely to be scattered by whistler waves. Our results suggest that whistler waves are important for scattering tens to hundreds of electronvolt electrons inside foreshock transients and elucidate electron dynamics and whistler wave properties in such environments.

Asymmetry of Quadratic Energy Transfer Between Ion Cyclotron and Whistler Modes in Fully Developed Hall Magnetohydrodynamic Turbulence

Asymmetry of Quadratic Energy Transfer Between Ion Cyclotron and Whistler Modes in Fully Developed Hall Magnetohydrodynamic Turbulence

Effects of Nonlinear Resonance Broadening on Interactions Between Electrons and Whistler Mode Waves

AbstractEffects of nonlinear resonance broadening on diffusion coefficients for interactions between parallel propagating whistler mode waves and relativistic electrons in a uniform magnetic field are investigated in this work using test particle simulations. In quasi‐linear theory, variations of pitch angle and energy are calculated along the unperturbed trajectory, resulting in difficulties when the pitch angle is near 90° or when wave amplitudes are large. Using a test particle simulation code, we investigate effects of nonlinear resonance broadening on scattering of electrons and compare the results with a previous nonlinear theory by Karimabadi et al. We demonstrate that Karimabadi et al.'s theory gives better agreement with test particle simulations compared with standard quasi‐linear theory, provided that the resonance broadening width is allowed to vary. Our result suggests further development of the nonlinear theory and should be useful to improve radiation belt modeling.

The Generation of Upward‐Propagating Whistler Mode Waves by Electron Beams in the Jovian Polar Regions

AbstractUpward‐moving energetic electrons with energies of 1 MeV and above were observed over the entire Jovian polar region. The electrons were found to be associated with intense broadband whistler mode waves, similar to terrestrial whistler mode auroral hiss. Upward‐propagating whistler mode hiss at Earth is known to be generated by upward‐moving, magnetic field‐aligned electron beams (from electric field‐aligned potentials), by a beam‐plasma instability at the Landau resonance. Assuming this process at Jupiter, we present a linear stability analysis, showing the electron distribution functions (based on inverted‐V observations made by the Juno Jovian Auroral Distributions Experiment, JADE‐E, instrument) are unstable. The polarization of the modeled waves is consistent with whistler mode hiss (right‐hand circularly polarized). From the results of the linear stability analysis, we find that the calculated growth rates are sufficient to produce the observed whistler mode waves.

Global Model of Whistler Mode Chorus in the Near‐Equatorial Region (|λm|&lt; 18°)

AbstractWe extend our database of whistler mode chorus, based on data from seven satellites, by including ∼3 years of data from Radiation Belt Storm Probes (RBSP)‐A and RBSP‐B and an additional ∼6 years of data from Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A, THEMIS‐D, and THEMIS‐E. The new database allows us to probe the near‐equatorial region in detail, revealing new features. In the equatorial source region, |λm|&lt;6°, strong wave power is most extensive in the 0.1–0.4fce bands in the region 21–11 magnetic local time (MLT) from the plasmapause out to L∗ = 8 and beyond, especially near dawn. At higher frequencies, in the 0.4–0.6fce frequency bands, strong wave power is more tightly confined, typically being restricted to the postmidnight sector in the region 4&lt;L∗&lt;6. The global distribution of strong chorus wave power changes dramatically with increasing magnetic latitude, with strong chorus waves in the region 12&lt;|λm|&lt;18° predominantly observed at frequencies below 0.3fce in the prenoon sector, in the region 5&lt;L∗&lt;8.