Effect of finite electron and ion temperature on magnetospheric whistler mode raytracing
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Numerical raytracing is an important technique that is being used to determine the trajectory of whistler mode waves in the magnetosphere. Previous whistler mode raytracing techniques were developed by assuming cold magnetospheric plasma. In this work we analyze the effect of finite electron and ion temperature on the whistler mode wave trajectories.Keywords:
Whistler
Mode (computer interface)
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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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 physical characteristics of whistler ducts in Earth's magnetosphere is inquired theoretically. By using two kinds of model of ducts in which the VLF electromagnetic waves are guided, we obtain the distributions of magnetic field and current in the whistler duets. When the magnetic field deviates from potential or force-free field, the field-aligned current can be produced, and it relates closely with the intensity and lifetime of ducts. On the basis of these results, we may conclude that a stronger field-aligned current obsvered in high-latitute magnetosphere is an important cause of the whistlers and their echoes being received frequently at ground stations of high-latitute regions.
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Ring current
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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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Abstract In the Earth's magnetosheath (MSH), several processes contribute to energy dissipation and plasma heating, one of which is wave‐particle interactions between whistler waves and electrons. However, the overall impact of whistlers on electron dynamics in the MSH remains to be quantified. We analyze 18 hr of burst‐mode measurements from the Magnetospheric Multiscale (MMS) mission, including data from the unbiased magnetosheath campaign during February‐March 2023. We present a statistical study of 34,409 whistler waves found using automatic detection. We compare wave occurrence in the different MSH geometries and find three times higher occurrence in the quasi‐perpendicular MSH compared to the quasi‐parallel case. We also study the wave properties and find that the waves propagate quasi‐parallel to the background magnetic field, have a median frequency of 0.2 times the electron cyclotron frequency, median amplitude of 0.03–0.06 nT (30–60 pT), and median duration of a few tens of wave periods. The whistler waves are preferentially observed in local magnetic dips and density peaks and are not associated with an increased temperature anisotropy. Also, almost no whistlers are observed in regions with parallel electron plasma beta lower than 0.1. Importantly, when estimating pitch‐angle diffusion times we find that the whistler waves cause significant pitch‐angle scattering of electrons in the MSH.
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Magnetosheath
Pitch angle
Plasmasphere
Lower hybrid oscillation
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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.
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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)
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Plasmasphere
Spectrogram
Hiss
Frequency band
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Context. The evolution of the solar wind electron distribution function with heliocentric distance exhibits different features that are still unexplained, in particular, the fast decrease in the electron heat flux and the increase in the Strahl pitch angle width. Wave-particle interactions between electrons and whistler waves are often proposed to explain these phenomena. Aims. We aim to quantify the effect of whistler waves on suprathermal electrons as a function of heliocentric distance. Methods. We first performed a statistical analysis of whistler waves (occurrence and properties) observed by Solar Orbiter and Parker Solar Probe between 0.2 and 1 AU. The wave characteristics were then used to compute the diffusion coefficients for solar wind suprathermal electrons in the framework of quasi-linear theory. These coefficients were integrated to deduce the overall effect of whistler waves on electrons along their propagation. Results. About 110 000 whistler wave packets were detected and characterized in the plasma frame, including their direction of propagation with respect to the background magnetic field and their radial direction of propagation. Most waves are aligned with the magnetic field and only ∼0.5% of them have a propagation angle greater than 45°. Beyond 0.3 AU, it is almost exclusively quasi-parallel waves propagating anti-sunward (some of them are found sunward but are within switchbacks with a change of sign of the radial component of the background magnetic) that are observed. Thus, these waves are found to be Strahl-aligned and not counter-streaming. At 0.2 AU, we find both Strahl-aligned and counter-streaming quasi-parallel whistler waves. Conclusions. Beyond 0.3 AU, the integrated diffusion coefficients show that the observed waves are sufficient to explain the measured Strahl pitch angle evolution and effective in isotropizing the halo. Strahl diffusion is mainly attributed to whistler waves with a propagation angle of θ ∈ [15.45]°, although their origin has not yet been fully determined. Near 0.2 AU, counter-streaming whistler waves are able to diffuse the Strahl electrons more efficiently than the Strahl-aligned waves by two orders of magnitude.
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Orbiter
Pitch angle
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