Kinetic generation of whistler waves in the turbulent magnetosheath
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<p>Whistler waves, right-hand polarized waves with frequencies below the electron cyclotron frequency, are common in many space plasma regions such as the Earth&#8217;s magnetosheath. They can be generated by electron temperature anisotropy, in which case the instability grows through cyclotron resonance. A common way to determine the stability of an electron distribution function is to compare the parallel and perpendicular temperature (with respect to the background magnetic field) to stability thresholds. However, such an approach based on the moments of the distribution function can potentially leave out some properties of the distribution which are important for wave generation.</p><p>In this work, we investigate the features of the electron distribution functions measured by MMS in the turbulent magnetosheath downstream of a quasi-parallel shock. We show that even though statistically whistler waves tend to occur close to the regions where the stability threshold is exceeded, they are also observed in regions predicted to be stable to wave generation. For such waves we observe that the electron pitch angle distribution often has the so-called butterfly shape (with minima in both the parallel and perpendicular directions) and is located in magnetic field minima. Using a linear numerical dispersion solver (WHAMP), we show that the butterfly distribution is unstable to whistler wave generation even though the instability threshold based on the associated moments is not exceeded. Comparison between the numerical results and waves measured by the MMS spacecraft indicate that the observed whistler waves are generated by the butterfly distribution. This phenomenon has previously been observed in mirror modes and large scale magnetic holes. Our findings show that it also occurs on smaller scales (~1 ion inertial length) in more turbulent environments, such as the quasi-parallel magnetosheath.</p>Keywords:
Whistler
Magnetosheath
​​The Earth’s magnetosheath is a turbulent plasma region where the interplay between coherent structures and various plasma waves affect the particle dynamics and energy transfer. The properties of the magnetosheath are controlled by the upstream conditions. Magnetosheath plasma downstream of a quasi-parallel bow shock (the angle between the shock normal and the interplanetary magnetic field being less than 45°) tends to have stronger fluctuations while a quasi-perpendicular shock leads to a more stationary magnetosheath. These two geometries create different environments for processes such as wave generation. One example is whistler waves that can be excited by non-Maxwellian electron velocity distributions formed in local magnetic structures. Whistler waves have been observed throughout the magnetosheath. As previous statistical studies have considered the region as a whole, it is yet unexplored which magnetosheath geometry creates more favorable conditions for whistler wave generation.In this work, we address this issue and investigate how the occurrence and properties of whistler waves depend on the magnetosheath configuration. We detect whistler waves using data from the Magnetospheric Multiscale (MMS) mission. We compare whistler wave occurrence to the shock normal angle estimated from upstream conditions, as well as local conditions which are typically different between the quasi-parallel and quasi-perpendicular geometries. The results give an indication of the conditions needed for the whistler waves to efficiently dissipate energy in the turbulent magnetosheath.
Magnetosheath
Whistler
Pitch angle
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The role of whistler mode during the onset of magnetic reconnection (MR) has been widely suggested, but the manifestations of its highly dispersive and anisotropic propagation properties in reconnection events remain largely unclear. Comparing results from a recently developed theoretical model for reconnection in terms of whistler's dispersive behavior with those reported from laboratory experiments on fast spontaneous magnetic reconnection, we demonstrate that the onset of fast reconnection in electron current layers (ECLs) emits whistler wave packets. The time scale of the explosively fast reconnection events are inversely related to the whistler mode frequencies at the lower end of the whistler frequency band. The wave packets in this frequency band have a characteristic angular dispersion, which marks the initial opening of the reconnection exhaust angle. The multidimensional propagation of the whistler for the reconnection with a strong guide magnetic field is investigated, showing that the measured propagation velocities of the reconnection electric field along the guide field in the Versatile Toroidal Facility at MIT quantitatively compare with the group velocities of the whistler wave packets. The whistler mode dispersive properties measured in laboratory experiments without a guide magnetic field in the magnetic reconnection experiments at Princeton also compare well with the theoretically predicted dispersion of the wave packets depending on the ECL width. Fast normalized reconnection rates extending to ∼0.35 at the MR onset in thin ECLs imply whistler wave propagation away from the onset location. We also present evidences for the whistler wave packets being emitted from reconnection diffusion region as seen in simulations and satellite observations.
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Abstract We present experimental observations and detailed investigation of the variety of proton whistlers that includes transequatorial and ionospherically reflected proton whistlers. The latter have previously been indicated from numerical modeling of spectrograms. The study is based on six‐component ELF wave data from the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite which permits to obtain not only spectrograms displaying the power spectral density but also such wave properties as the polarization, wave normal angle, wave refractive index, and normalized parallel component of the Poynting vector. The explanation of various types of proton whistlers is based on the properties of ion cyclotron wave propagation in a multicomponent magnetoplasma, with special consideration of the effect of ion hybrid resonance reflection. Analysis of experimental data is supplemented by numerical modeling of spectrograms that reproduces the main features of experimental ones. As a self‐contained result, we provide conclusive experimental evidences that the region illuminated by a lightning stroke in the Earth‐ionosphere waveguide may spread over a distance of 4000 km in both hemispheres.
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Spectrogram
Hiss
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Whistler
Waveguide
Transverse wave
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Abstract The solar wind electron velocity distribution function deviates significantly from an equilibrium Maxwellian distribution and is composed of a Maxwellian core, a suprathermal halo, a field-aligned component strahl, and a higher-energy superhalo. Wave–particle interactions associated with whistler wave turbulence are introduced into the kinetic transport equation to describe the interaction between the suprathermal electrons and the whistler waves and to explain the observation that the halo and the strahl relative densities vary in an opposite sense. An efficient numerical method has been developed to solve the Fokker–Planck kinetic transport equation. Application of the numerical method to suprathermal electrons in the solar wind in the presence of whistler waves is presented. Comparison and analysis between the numerical results and observations are made.
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We present a numerical study of the propagation of VLF whistler waves in the magnetospheric plasma. In this study the plasma is considered to be homogeneous in the direction along the ambient magnetic field and strongly inhomogeneous across it. The goal of this investigation is to understand whistler propagation in magnetic‐field‐aligned channels (also called ducts) with either enhanced or depleted plasma density. In particular, the paper is focused on situations where the transverse scale size of the duct is comparable to or smaller than the perpendicular wavelength of the whistler. In this case, classical analysis of the whistler dynamics based on the geometrical optic approximation becomes invalid, and numerical solutions of the full wave equations should be performed. Our simulations extend the earlier analysis based on the ray‐tracing technique and analytical studies of the very low frequency wave equations. We show that high‐density ducts are inherently leaky and this leakage depends on the perpendicular wavelength of the wave inside the duct. We also show that whistler trapping occurs not only at density maxima and minima but also at critical points along a density gradient. This effect can explain whistler guiding along strong transverse plasma density gradients at the plasmapause.
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Plasmasphere
Transverse wave
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Abstract The propagation of whistler waves in a magnetized plasma containing multiple small‐scale (100 m to 1 km) field‐aligned irregularities of enhanced electron density is considered analytically and by means of numerical simulations. Such systems of irregularities can develop in the upper ionosphere during the generation of density ducts by high‐frequency heating facilities and other types of active experiments. The simulation parameters are close to those of an active experiment where a whistler wave of 18 kHz emitted by a ground‐based very low frequency (VLF) transmitter was received onboard the DEMETER satellite at 700 km above the SURA heater. The study reveals a number of remarkable properties of the VLF waves' propagation, including the existence of specific waveguide modes of the small‐scale density structures and of a characteristic transverse size d 0 of the irregularities. Irregularities with small density enhancements around 10–20% and transverse sizes larger than d 0 ∼1 km can serve as separate waveguides for VLF waves. In their turn, single irregularities narrower than d 0 cannot be considered as individual ducting structures. Numerical simulations show that, for the analysis of the electromagnetic whistlers' propagation, a system of closely spaced irregularities with scales narrower than d 0 can be modeled by an equivalent ducting structure with a smoothed density profile. Such equivalent structure has the same ducting properties for whistlers and can be produced by averaging with a sliding window of a scale about d 0 the original density distribution.
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Atmospheric duct
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The origins and properties of large amplitude whistler wave packets in the solar wind are still unclear. In this Letter we utilise single spacecraft electric and magnetic field waveform measurements from the ARTEMIS mission to calculate the plasma frame frequency and wavevector of individual wave packets over multiple intervals. This allows direct comparison of experimental measurements with theoretical dispersion relations to identify the observed waves as whistler waves. The whistlers are right-hand circularly polarised, travel anti-sunward and are aligned with the background magnetic field. Their dispersion is strongly affected by the local electron parallel beta in agreement with linear theory. The properties measured are consistent with the electron heat flux instability acting in the solar wind to generate these waves.
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Wave vector
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Summary form only given. Very-low frequency (VLF) waves in the whistler mode are important for the dynamics of energetic electrons in the Earth's magnetosphere. In particular, whistlers are capable of scattering high-energy radiation belt electrons into the loss cone via cyclotron resonance, and therefore, the controlled injection of kHz VLF wave power can significantly reduce the lifetime of MeV-energy electrons. In this presentation we reports results from two of our recent research projects. One of them uses EMHD simulations to investigate whistler propagation through the ionosphere detected on the DEMETER satellite above HAARP transmitter at Alaska. Our simulations show that detected by the DEMETER satellite wave power enhancements localized above an active heating region are consistent with whistler propagation inside multiple density ducts. The initial concentration of wave power inside the ducts suggests that a density-dependent generation mechanism, such as linear mode conversion from lower hybrid waves, is responsible for generating these whistlers. Magnetic field-aligned density variation eventually causes power to leak from the ducts. This combination of ducting and leakage appears sufficient to explain the characteristics of the DEMETER signal. This event demonstrates that artificially generated density enhancements can act as whistler ducts, trapping the wave and guiding them across considerable distances with a minimal reduction in amplitude. Another project investigates propagation of whistler-mode waves in the magnetized plasma with the transverse gradient in density and in the background magnetic field. This project reveals that the density gradient can support whistler ducting if the background magnetic field has a gradient in the same direction as the density, but with a different rate. We provided a quantitative criterion for parameters of the background media and whistler waves providing ducting and demonstrate relevance of these results to the whistler ducting in the radiation belt.
Whistler
Atmospheric duct
Plasmasphere
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Abstract. VLF-ELF broadband measurements onboard the MAGION 4 and 5 satellites at heights above 1 Re in plasmasphere provide new data on various known phenomena related to ducted and nonducted whistler wave propagation. Two examples are discussed: magnetospherically reflected (MR) whistlers and lower hybrid resonance (LHR) noise band. We present examples of rather complicated MR whistler spectrograms not reported previously and argue the conditions for their generation. Analytical consideration, together with numerical modelling, yield understanding of the main features of those spectrograms. LHR noise band, as well as MR whistlers, is a phenomenon whose source is the energy propagating in the nonducted way. At the plasmaspheric heights, where hydrogen (H+) is the prevailing ion, and electron plasma frequency is much larger than gyrofrequency, the LHR frequency is close to its maximumvalue in a given magnetic field. This frequency is well followed by the observed noise bands. The lower cutoff frequency of this band is somewhat below that maximum value. The reason for this, as well as the possibility of using the LHR noise bands for locating the plasma through position, are discussed.Key words. Magnetospheric physics (plasmasphere; wave propagation)
Whistler
Plasmasphere
Spectrogram
Hiss
Frequency band
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