M. J. Rycroft

Interrelation of noise‐like and discrete ELF/VLF emissions generated by cyclotron interactions

It is shown that quasi-monochromatic whistler waves (wavelets) can be caused by the strong cyclotron instability, stimulated by hiss emissions. The hiss, generated by the cyclotron instability of an anisotropic smooth energetic electron distribution, creates a step-like deformation of the distribution function at the boundary between resonant and nonresonant electrons. This deformation leads to the strong amplification of the wavelet whose frequency corresponds to that for cyclotron resonance with electrons at the step. Analytical calculations for this amplification have been made using the rigorous theory of the cyclotron instability in an inhomogeneous magnetic field. The wave amplification can be 2 orders of magnitude greater than that for a smooth distribution function. A self-consistent computational analysis of the cyclotron instability is developed on the basis of quasi-linear theory. This confirms both the formation of the step-like deformation of the distribution function and the wavelet generation.

Whistler and Alfvén mode cyclotron masers in space

Preface 1. Introduction 2. Basic theory of cyclotron masers (CMs) 3. Linear theory of the cyclotron instability (CI) 4. Backward wave oscillator (BWO) regime in CMs 5. Nonlinear cyclotron wave-particle interactions for a quasi-monochromatic wave 6. Nonlinear interaction of quasi-monochromatic whistler mode waves with gyroresonant electrons in an in homogeneous plasma 7. Wavelet amplification in an inhomogeneous plasma 8. Quasi-linear theory of cyclotron masers 9. Nonstationary generation regimes, and modulation effects 10. ELF/VLF noise-like emissions and electrons in the Earths radiation belts 11. Generation of discrete ELF/VLF whistler mode emissions 12. Cyclotron instability of the proton radiation belts 13. Cyclotron masers elsewhere in the solar system and in laboratory plasma devices Epilogue Glossary of terms List of acronyms References Index.

Cyclotron acceleration of radiation belt electrons by whistlers

The work consdiers the non linear scattering of energetic electrons in the earths radiation belts due to cyclotron interaction with VLF whistlers. In particular we consdier the acceleration of electrons which may result from trapping in the inhomogeneous medium. It is shown that considerable electron heating may result, and that the very anisotorpic electron distribution functions observed by Bell etal may be explained

Electron acceleration in the magnetosphere by whistler-mode waves of varying frequency

Acceleration of relativistic electrons in an inhomogeneous geomagnetic field during their resonant interaction with longitudinally propagating whistler-mode waves of varying frequency has been considered. Specific features of acceleration of electrons trapped by the wave field have been studied. Previous estimates of the efficiency of such acceleration have been generalized with regard to relativistic effects, and the simple formula for energy gain in a wide range of initial energies has been obtained. It has been indicated that the energy gain during a single interaction between electron and a whistler-mode wave packet, with typical parameters of an element of chorus emissions in the Earth’s magnetosphere, can reach several keV. The conditions of this acceleration mechanism realization are discussed. Specifically, it has been found that, in the case of chorus emissions in the Earth’s magnetosphere, this mechanism can be effective for electrons with perpendicular energies several times as high as such an energy of electrons generating chorus.

Survey of magnetospheric line radiation events observed by the DEMETER spacecraft

[1]xa0Magnetospheric line radiation (MLR) events are electromagnetic waves in the frequency range between about 1 and 8 kHz that, when presented as a frequency-time spectrogram, take the form of nearly parallel and clearly defined lines, which sometimes drift slightly in frequency. They have been observed both by satellites and ground-based instruments, but their origin is still unclear. We present a survey of these MLR waves observed by the DEMETER spacecraft (at an altitude of about 700 km). Three years of VLF Survey mode data were manually searched for MLR events, creating the largest event satellite database of about 650 events, which was then used to investigate the wave properties and geographical occurrence. Finally, the most favorable geomagnetic conditions (Kp and Dst indices) for the occurrence of MLR events have been found. It is shown that MLR events occur mostly at L > 2 (upper limit is given by a limitation of the spacecraft), they occur primarily inside the plasmasphere, and there is a lower number of events occurring over the Atlantic Ocean than elsewhere on the globe. The MLR events occur more often during the day and usually during, or after, periods of higher magnetic activity. Their frequencies usually lay between about 2 and 6 kHz, with the total frequency bandwidth of an observation being below 2 kHz in the majority of cases. Moreover, it is shown that the longitudinal dimensions of the MLR events can be as large as 100° and they can last for up to a few hours. Finally, we discuss a possibility that MLR events may be triggered by power line harmonic radiation (PLHR) and we report an event supporting this hypothesis.

MODELLING ELECTRIC AND MAGNETIC FIELDS DUE TO THUNDERCLOUDS AND LIGHTNING FROM CLOUD-TOPS TO THE IONOSPHERE

Abstract Following some lightning flashes from energetic thunderclouds, blue jets and red sprites are observed in the atmosphere above the cloud and into the ionosphere. In order to understand the physical processes leading to these and associated phenomena, both the temporal and spatial evolution of the electric (and magnetic) fields due to the thundercloud and the lightning discharge are modelled. These numerical simulations are carried out either using a quasi-electrostatic code or an electromagnetic code, with appropriate boundary conditions and grids. The redistribution of electric charge and the electromagnetic pulse due to the lightning, can accelerate electrons, which collide with neutrals and ions, heating them, and also ionising the atmosphere. Runaway electrons and/or electrical breakdown of the atmosphere can also occur. The first and second positive bands of molecular nitrogen are excited appreciably if sufficient energy is produced. The situation is strongly nonlinear. The results (see also Cho and Rycroft, this issue) show the temporal and spatial development of (a) the electric field divided by the neutral gas density, and (b) the energy density of optical emissions (up to 10 13 photons m −3 s −1 ). They show that energy propagates up to the ionosphere in 0.3 ms, at the speed of light. A ring of optical emissions is created, the outer rim of which propagates horizontally in the ionosphere at an altitude ∼ 90 km, reaching a radial distance of 280 km in a further 0.7 ms. At the same time, the intense electric field at > 07 km altitude above the thundercloud creates a much enhanced ( ∼ 10 3 ×) electron density (with a radius up to 25 km) which lasts for several ms. This heated region modifies the amplitude and phase characteristics of VLF radio waves propagating in the Earth-ionosphere waveguide.

Efficiency of electron acceleration in the Earth’s magnetosphere by whistler mode waves

The efficiency of energetic electron cyclotron acceleration in the Earth’s magnetosphere in different regimes of electron resonant interaction with parallel propagating whistler mode waves of variable frequency, specifically, with chorus ELF-VLF emissions, is considered. The regime of stochastic acceleration, typical of the interaction between particles and noise-like emissions, and particle acceleration in the regime of nonlinear trapping by a quasimonochromatic wave field are discussed. The specific feature of the latter regime consists in its non-diffuse character, i.e., the definite sign of the energy variation depending on the frequency variation in the wave packet. The trapped electron energy becomes higher if frequency increases within an element, which is typical of chorus emissions. For the parameters typical of chorus emissions (the amplitude of a wave magnetic field B∼ = 102 nT, the initial frequency ω ∼ 0.3ωH, and the frequency variation &Dω ∼ 0.15ωH, where ωH is the electron gyrofrequency), the energy increase during one act of such an interaction at L = 4−5 exceeds the rms variation in the energy of untrapped electron (during stochastic acceleration) by one-two orders of magnitude. The estimates indicate that a considerable fraction (several tens of percent) of the chorus element energy can be absorbed by electrons accelerated in the trapping regime during a single hop.

A new parametric reflection mechanism for ducted whistlers and an explanation of precursors

Abstract A new reflection mechanism of ducted whistler-mode waves in the magnetosphere, which can be defined as a parametric reflection, is discussed. Unlike the well-known process of backscattering, parametric reflection occurs when the scale length of electron density inhomogeneities is much less than the whistler wave-length; the reflected whistler wave appears simultaneously with the excitation of short-scale lower hybrid resonance (LHR) plasma waves. The efficiency of such a parametric reflection is higher than that of traditional backscattering. Two cases are analysed. The first is when a reflected whistler wave arises via the process of the parametric decay instability of a sufficiently intense whistler wave into two quasielectrostatic short-scale waves, that is a LHR wave and an ionic (ion cyclotron or sound) wave. The second case is when the initial whistler interacts with naturally occurring ionic waves. Quantitative estimates show that both cases can exist in the magnetosphere. In the case of ducted whistler, generated by a lightning discharge, reflected signals at a given frequency are formed near two points along the magnetic field line where this frequency coincides with the local LHR frequency. This leads to appearance on a spectrogram of signals with a hook-like shape, which precede the whistler. It is shown that these signals can be associated with different types of whistler precursors.

SOUNDING THE MAGNETOSPHERE BY SIGNALS FROM VLF RADIO TRANSMITTERS

Abstract A new remote sensing method for the Earths magnetosphere is suggested. The method is based on the parametric reflection of whistler mode waves from the level in the magnetosphere where a whistler wave frequency coincides with the local lower hybrid resonance frequency. In this region a sufficiently intense whistler signal generates two electrostatic plasma waves which can produce a reflected whistler wave. The known localization of the whistler reflection leads us to suggest a new method of electron density measurements in the magnetosphere, using a grid of frequencies from a powerful ground-based VLF transmitter. Parametric reflection of whistler signals in the magnetosphere can appear from naturally occurring short-scale low frequency turbulence (ion sound or ion cyclotron waves). In this case, using this new method we can obtain important information about this turbulence. Some experimental data which demonstrate the appearance of VLF echo signals from the magnetosphere with anomalously small group time delays are discussed from this point of view.

ELF/VLF radio signals caused by ionospheric demodulation of MF/HF radio transmitter signals

Abstract A quantitative theory of the generation of ELF/VLF signals, which are caused by the ionospheric demodulation of modulated signals from broadcasting MF/HF transmitters, is developed. Two cases are considered when the ionospheric current, modulated by broadcasting signals, is a one-dimensional linear current (the polar electrojet), and when it is an extended two-dimensional current sheet. It is shown that the main demodulation effect is caused by the formation of an ionospheric travelling wave antenna. In the case of the polar electrojet, this antenna radiates ELF/VLF waves in two directions, one of which coincides with the line connecting the broadcasting transmitter with the ELF/VLF receiver, and the other one is in a direction which is the mirror image of this line relative to the current direction. In this case the polarisation of the ELF/VLF magnetic field component varies widely. In the case of the extended ionospheric current, the demodulation takes place along the line connecting the MF/HF transmitter with the ELF/VLF receiver, and the horizontal ELF/VLF magnetic field component is normal to this line. Amplitude and phase distortions in the ELF/VLF signal are absent below a frequency ⪅ 1 kHz and appear above ∼ 1 kHz. Quantitative estimates of the amplitude of the ELF/VLF signal give a value ∼0.15pT which is close to the experimentally observed value.