James Faith

Numerical comparison of two schemes for the generation of ELF and VLF waves in the HF heater‐modulated polar electrojet

Generation of ELF and VLF waves in the HF heater modulated polar electrojet is numerically studied. Illuminated by an amplitude modulated HF heater, the electron temperature of the electrojet is modulated accordingly. This, in turn, causes the modulation of the conductivity and thus the current of the electrojet. Emissions are then produced at the modulation frequency and its harmonics. The present work extends the previous one on a thermal instability to its nonlinear saturation regime. Two heater modulation schemes are considered. One modulates the heater by a rectangular periodic pulse. The other one uses two overlapping heater waves (beat wave scheme) having a frequency difference equal to the desired modulation frequency. It is essentially equivalent to a sinusoidal amplitude modulation. The nonlinear evolutions of the generated ELF and VLF waves are determined numerically. Their spectra are also evaluated. The results show that the signal quality of the second (beat wave) scheme is better. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency, consistent with the Tromso observations.

Electron precipitation caused by chaotic motion in the magnetosphere due to large‐amplitude whistler waves

In the magnetosphere, energetic electrons in the radiation belts are trapped by Earths magnetic field and undergo bounce motion about the geomagnetic equator. When a large-amplitude whistler wave is present, the motion of the electrons becomes perturbed. It is shown that the nonlinear interaction due to the spatial dependence of the field quantities causes the motion of some of the trapped particles to become chaotic. Contrary to considering a gyroresonant interaction, this chaotic scattering does not have a directional preference and may therefore offer a plausible explanation of the simultaneous observation of the electron precipitation into the upper atmosphere at geomagnetically conjugate regions due to a single lightning flash [Burgess and Inan, 1990]. After simplifying the dipole configuration of the geomagnetic field, a Hamiltonian formulation is used to study the dynamics of a single, trapped electron on the L = 3 shell, subjected to a large amplitude 13.7 kHz whistler wave. A canonical transformation is introduced to remove the time dependence from the test electrons Hamiltonian. The chaotic behavior of the electron motion is investigated with surface of section and Lyapunov exponent techniques. To show that this chaotic behavior can lead to particle precipitation, the temporal evolution of the equatorial pitch angle of the electron is computed. Considering electrons with an initial pitch angle of 88°, the results are found to be qualitatively independent of the bounce frequency. They show that the equatorial pitch angle of a chaotic electron varies wildly and often dips below 25°, the minimum loss cone angle one would expect to find for a charged particle in the magnetosphere. Therefore the electrons may escape the geomagnetic trap and be precipitated into the upper atmosphere.

Precipitation of magnetospheric electrons caused by relativistic effect-enhanced chaotic motion in the whistler wave fields

In the magnetosphere, energetic electrons in the radiation belts are trapped by the Earths dipole magnetic field and undergo bounce motion about the geomagnetic equator. It is shown that the trajectories of some of the trapped particles can in the presence of a whistler wave become chaotic and wander into the loss cone. Comparing the surface of section plots obtained from the both relativistic equations of motion and from the nonrelativistic ones, the effect of the relativistic correction to the electron motion are shown. The threshold field for the commencement of chaos in the trajectories of electrons with energies of a few hundred keV is found to be lowered by the inclusion of relativistic effects by about an order of magnitude. Waves with these smaller magnetic field amplitudes (about 1% of the geomagnetic field) have been observed propagating between hemispheres. Since this chaotic scattering process does not have a directional preference, it offers a plausible explanation for the simultaneous observation of electron precipitation into the upper atmosphere at geomagnetically conjugate regions because of a single lightning flash [Burgess and Inan, 1990].

Chaotic precipitation of relativistic electrons driven by large amplitude whistler waves

Summary form only given. It has for some time been known that electrons trapped by Earths magnetic field can precipitate along the geomagnetic field lines into the polar regions. Here the secondary ionizations caused by the electrons impacting into the ionosphere can perturb VLF signals propagating in the Earth-ionosphere waveguide. The correspondence of these precipitation events with the presence magnetospheric whistler waves has led to much study of the interaction of whistler waves with the magnetospheric plasma. Most theories have considered Doppler shifted gyroresonant interactions. However, due to the directional nature of these interactions, they are unable to explain the simultaneous observation of precipitation events in geomagnetically conjugate regions in both hemispheres. Therefore we will investigate the interaction on a single test electron with a large amplitude nonresonant wave. We show that the interaction of a whistler wave with a trapped bouncing electron can lead to chaos in the electron trajectory. The dynamics of the electron trajectory depend strongly on both particle energy and wave amplitude. In order to induce chaos with physically realistic wave amplitudes, the particle energy must be quite large, and thus relativistic effects are important. Inclusion of relativistic effects also has the effect of decreasing the required threshold wave field for the onset of chaos for a wide range of particle energies. The end result of chaotic behavior is to enhance the electrons axial kinetic energy, which decreases the particles pitch angel allowing it to be scattered into the mirror field loss cone.

Experimental and numerical study of electromagnetic wave trapping in a time-varying periodic plasma

Summary form only given. It is well known that as an electromagnetic wave propagates through a rapidly growing plasma it will have its frequency spectrum up-shifted. If the plasma additionally has a periodic spatial configuration, the incident wave can excite many Floquet modes of the structure. Many of these modes often elude experimental detection as the multiple scattering processes required to excite higher order modes does not provide for efficient wave-plasma interaction. A way to achieve more efficient interaction is to use a periodic plasma several free space wavelengths long to trap the incident electromagnetic wave. Considering such a structure, as the plasma density grows from zero, the incident wave initially sees a small plasma-free space discontinuity. This provides for a large transmission (and small reflection) coefficient into (from) the structure. In the time it takes the wave propagates to the far end of the structure the plasma continues to grow. At the boundary between the final plasma layer and free space, the plasma density has increased and the reflection coefficient at this boundary is greater than the one encountered at the beginning of the structure. Thus some of the wave energy is trapped within the structure, where it can effectively interact with the plasma, and alter its spectral content. The trapping process is expected to be more effective for the down-shifted Floquet modes. An experiment exhibiting this phenomena confirms that the amplitude of frequency altered pulses is vastly enhanced over the case of a single plasma slab. Experimental results, along with related numerical simulations, showing the large down-shifted lines are presented.

Chaotic particle motion in large amplitude whistler wave in the magnetosphere

Summary form only given, as follows. The motion of it single charged particle in the large amplitude wave field is investigated. In the magnetosphere, energetic charged particles in the radiation belts are trapped by Earths magnetic field. In the equatorial region where a symmetric mirror field may be assumed, these particles undergo bounce motion along the lines of force. When a large amplitude quasi-electrostatic whistler wave is present, the motion of the particles is expected to be perturbed, particularly when the wave frequency comes close to a harmonic or sub harmonic of the bounce frequency. The nonlinear interaction due to the spatial dependence of field quantities is expected to cause the motions of some of the trapped particles to become chaotic. We initially restrict our attentions to the case of one dimensional motion along the magnetic field lines. The equations of motion for a single particle are integrated numerically, and the results investigated in a three dimensional phase space (z,p/sub z/,t), where z and p/sub z/ are the components of position and canonical momentum along the geomagnetic field. This analysis lends itself well to the surface of section technique, which provides information on the location of fixed points, regions of regular orbits, and chaotic regions. In the chaotic regions the particle motion resembles that of a random walk. This can lead to particle diffusion into the loss cone, and thus particle precipitation along the magnetic field lines into the high latitude regions.

Experimental study of the conversion of DC electric fields to microwave radiation by an ionization front created by successive discharges

Summary form only given, as follows. Most current high power microwave sources use either free electrons in the form of beams, or photon emission as the radiation source. Recently, however, a new method involving the direct conversion of DC electric fields to radiation fields has come to light. This new method uses an ionization front moving through a gas filled electrode array to produce an approximately sinusoidal static electric field. As the front passes an electrode pair, a burst of current, and thus a half cycle of radiation is produced. All the pulses then add coherently, giving a radiation field whose energy is derived directly from the DC electric field. Our experiment consists of an 3 band rectangular waveguide, in which several (/spl ges/5) pairs of oppositely placed holes are drilled. Through these holes are placed pairs of electrodes, between which a Marx capacitor bank is discharged. By adjusting the spacing of the electrodes, the first pair can be made to fire first. The resulting ultraviolet radiation from the spark preionizes the gas between the remaining electrodes, causing them to fire in sequence, and assuring that the microwave radiation pulses add coherently.

Experimental study of frequency shift of an electromagnetic wave propagating through a rapidly created periodic plasma

Summary form only given, as follows. Conversion of a monochromatic CW microwave into a frequency up-shifted and chirped pulse through its interaction with a rapidly created plasma has received considerable attention recently. The present work attempts to improve upon the results obtained by our groups previous frequency shift experiments. The latest experiment replaces the previously studied single plasma layer in our vacuum chamber by a periodic discharge, obtained by placing Plexiglas strips across the conductors. These strips selectively prevent gas breakdown when a Marx capacitor bank is discharged between the electrodes. By properly arranging the Plexiglas strips, we can force the plasma to be rapidly created in a periodic structure. Numerical simulations lead us to believe that by proper selection of the periodic structures spacing and periodicity we can exercise some control over the spectral characteristics of the resulting broadband frequency shifted and chirped pulse train. We present a detailed description of the experiment, as well as comparisons between the simulation and experiment.

Numerical simulation for high harmonic inverted cusptron devices

Summary form only given. A particle simulation code has been developed to study the interaction between electrons of the e-layer and the mode fields of the inverted cusptron structure. It has been used to determine the optimum design parameters, such as the dimensions of the tube, electron beam energy, and magnetic field in order to obtain the maximum efficiency of the device for the TE/sub 02/-2/spl pi/ mode. Three harmonics (No = 6, 10, 16) have been considered. The results have been compared with the corresponding ones with the conventional cusptron geometry.