Whistler Mode Waves Below Lower Hybrid Resonance Frequency: Generation and Spectral Features

Growth of electrostatic waves around the lower hybrid resonance (LHR) frequency has been discussed. Expressions for growth rate of the excited electrostatic wave around the LHR frequency have been derived and evaluated for ionospheric plasma parameters. The results are in good agreement with the experimental observations reported by earlier workers.

Geophysical investigation of some aeromagnetic lineaments in North Finland

Radial density profiles are measured with various frequencies, powers, pressures, and gas masses in m=0 helicon wave plasmas. The radial density profiles show clear mode changes for the E mode (capacitive coupling), the H mode (inductive coupling), and the W mode (wave coupling) as the rf (radio frequency) power increases. The radial density profile hardly changes as the frequency varies in the E and the H modes but changes substantially in the W mode; it changes from a central peak profile to a hollow density profile near the lower hybrid resonance frequency. The change of radial density profile near the lower hybrid resonance frequency occurs only at pressures below 20 mTorr, and the resonance effects become very weak at higher pressures. The change of the radial density profile may be explained by the density enhancement near the lower hybrid resonance layer. The experiments with various magnetic fields and gas masses show a close correlation between the lower hybrid frequencies and the density profile changes.

Waves near the lower hybrid resonance frequency are electrostatically generated with plates just outside a cylindrical plasma column.These waves penetrate radially and, above a threshold power, are observed to undergo a parametric decay into two lower frequency waves.

The parametric excitation of the ordinary mode and the extraordinary mode propagating perpendicular to the magnetostatic field is investigated for the case of a cold uniform plasma. The pump wave is assumed to be another ordinary mode having its electric field oriented parallel to the magnetostatic field and its frequency exceeding the plasma frequency of the electrons. The extraordinary mode has three branches, namely, a high-frequency branch, an intermediate-frequency branch having a resonance at the upper hybrid resonant frequency, and a low-frequency branch which in the regime of hydromagnetic frequencies is the Alfvén wave and which also has a resonance at the lower hybrid resonant frequency. The ordinary mode does not parametrically couple with the high-frequency branch of the extraordinary mode but such a coupling takes place with the other two branches. This parametric coupling leads to instabilities of the extraordinary mode. Illustrative numerical results are presented for the parametric instabilities of the extraordinary mode near the upper hybrid resonant frequency, the lower hybrid resonant frequency, and the Alfvén wave frequency. Compared to the intermediate-frequency branch, the requirements for the parametric interaction are found to be met easily for the low-frequency branch. The dependence of the maximum growth rates of the parametric instabilities on the strength of the magnetostatic field is also examined.

VLF wave-injection experiments from Siple Station, Antarctica

Using a one-dimensional electromagnetic particle code which simulates self-consistently the full ion and electron dynamics, we study instabilities driven by energetic ions injected continuously in a plasma where the density and the lower-hybrid resonance frequency ωLH increase with time. The simulation shows that the ion cyclotron harmonic wave with ω ≃ lΩi, where l is an integer and Ωi is the ion cyclotron frequency, is excited when ωLH becomes close to lΩi. When ωLH is greater than lΩi, this wave couples with the ion Bernstein mode that has the dispersion curve connecting to ωLH. It is also found that as a result of the instabilities and the wave-wave coupling, the stair-like frequency chirping with the riser Ωi appears in the magnetic fluctuations in the frequency range of ωLH. The frequency chirping has characteristics similar to the frequency chirping observed in the RF radiations at the plasma start-up phase of the LHD experiments.

The derivation and the accuracy of an expression relating the fractional concentration of H+, α1 = n(H+)/ne, to the crossover frequency ω12 for a VLF proton whistler are discussed. To an accuracy of ±3½% it is found that α1 = (264/255) (1 - Λ122), where Λ12 = ω12/Ω1 and Ω1 is the proton gyrofrequency. Values of α1 have been deduced from measurement of proton whistler spectrograms for some of the satellite Injun 3 VLF data and plotted against altitude (400–2600 km) and against invariant latitude (20°–63°) for summer daytime and winter nighttime of 1963. The fractional concentration of H+ is found to be higher for winter nighttime than for summer daytime at all altitudes and latitudes; the ratio was approximately 3:1 at 1000 km. At 1000 km the value of α1 dropped from 0.43 in the 20°–30° invariant latitude range to 0.27 in the 40°–45° range for summer daytime. For winter nighttime α1 was nearly constant at 0.82 for 1000 km from 30° to 50° latitude, but dropped to 0.75 in the 50°–63° latitude range. Near 2400 km for summer daytime, α1 drops from 0.65 at 46° latitude to 0.20 at 56° latitude. This same tendency is observed for winter nighttime, apparently being due to auroral-zone heating. These observations are consistent with the assumption that the heavier ions tend to predominate with increasing latitude for a given altitude. Comparison of these VLF proton whistler results for α1 is made with reasonably good agreement to rocket ion mass spectrometer results of NASA 8.23 and to Alouette 1 VLF results deduced from lower hybrid resonance frequency. This general agreement establishes the VLF radio technique as an independent method for determining ion concentrations in the ionosphere. The general equations for uniquely determining the concentration of O+, He+, and H+ from observation of the critical frequencies (crossovers, hybrid resonances, and cutoffs) due to the presence of ions are presented, and their usage is discussed.

Temporal and spatial development of short gravity waves in a linear wind-wave tank has been measured for wind speeds up to 15 m/s using microwave Doppler spectrometry. Surface waves of wavelength 4·1 cm, 9·8 cm, 16·5 cm and 36 cm were observed as a function of fetch, wind speed and wind duration. The waves grew exponentially from inception until they were about 10 dB smaller than their maximum height, and the temporal growth and spectral transport (spatial growth) rates were about equal when the wave amplitude was sufficiently small. The amplitude of a short gravity wave of fixed wavelength was found to decrease substantially at winds, fetches or durations greater than those at which the short gravity wave was approximately the dominant wave; such phenomena are sometimes referred to as overshoot. The dominant short gravity wave was observed to reach a maximum amplitude which depended only on wavelength, showing that wave breaking induced by an augmented wind drift cannot be the primary limitation to the wave height. Waves travelling against the wind were observed for wavelengths of 9·8 cm, 16·5 cm and 36 cm and were shown to be generated by the air flow at low wind speeds.Measured initial growth rates for 16·5 cm and 36 cm waves were greater than expected, suggesting the existence of a growth mechanism in addition to direct transfer from the wind via linear instability of the boundary-layer flow. Initial temporal growth rates and spectral transport rates were compared to yield an experimental determination of the magnitude of the sum of nonlinear interactions and dissipation in short gravity waves. If the steady-state energy input in the neighbourhood of the dominant wave occurs at the measured initial temporal growth rates, then most of the energy input is locally dissipated; relatively little is advected away. Calculated gravitycapillary nonlinear energy transfer rates match those determined from initial growth rates for 9·8 cm waves and the gravity–capillary wave interaction continues to be significant for waves as long as 16·5 cm. For longer waves the gravity–capillary interaction is too small to bring the short gravity wave to a steady state when it is the dominant wave of the wind-wave system.

The parametric decay of finite-size Alfvén waves in nonperiodic low-beta plasmas is investigated using one-dimensional (1D) hybrid simulations. Compared with the usual small periodic system, a wave packet in a large system under the absorption boundary condition shows different decay dynamics, including reduced energy transfer, localized density cavitation, and ion heating. The resulting Alfvén wave dynamics are influenced by several factors relating to this instability, including the growth rate, central wave frequency, and unstable bandwidth. A final steady state of the wave packet may be achieved when the instability does not have enough time to develop within the residual packet, and the packet size shows well-defined scaling dependencies on the growth rate, wave amplitude, and plasma beta. Under the proper conditions, enhanced secondary decay can also be excited in the form of a narrow, amplified wave packet. These results may help to interpret laboratory and spacecraft observations of Alfvén waves, and to refine our understanding of the associated energy transport and ion heating.

Ray path theory is an asymptotic approximation to the wave equations. It represents efficiently gravity wave propagation in nonuniform background flows so that it is useful to develop schemes of gravity wave effects in general circulation models. One of the main limitations of ray path theory to be applied in realistic flows is in caustics where rays intersect and the ray solution has a singularity. Gaussian beam approximation is a higher-order asymptotic ray path approximation that considers neighboring rays to the central one, and thus, it is free of the singularities produced by caustics. A previous implementation of the Gaussian beam approximation assumes a horizontally uniform flow. In this work, we extend the Gaussian beam approximation to include horizontally nonuniform flows. Under these conditions, the wave packet can undergo horizontal wave refraction producing changes in the horizontal wavenumber, which affects the ray path as well as the ray tube cross-sectional area and so the wave amplitude via wave action conservation. As an evaluation of the Gaussian beam approximation in horizontally nonuniform flows, a series of proof-of-concept experiments is conducted comparing the approximation with the linear wave solution given by the WRF Model. A very good agreement in the wave field is found. An evaluation is conducted with conditions that mimic the Antarctic polar vortex and the orography of the southern flank of South America. The Gaussian beam approximation nicely reproduces the expected asymmetry of the wave field. A much stronger disturbance propagates toward higher latitudes (polar vortex) compared to lower latitudes.

The research presented in this thesis focuses on boundary layer separation and, more specifically, on the related physical mechanisms, such as laminar to turbulent transition, with a vision towards effective separation control. The work is divided in three parts, each corresponding to a different separation scenario. The supporting data is experimental, obtained by means of particle image velocimetry. The first and major part regards the phenomenon of laminar separation bubbles. The spatial and temporal response characteristics of a flat plate laminar separation bubble to impulsive forcing are first investigated in order to shed light on the processes of flapping and bursting. The impulsive disturbance is introduced twodimensionally with a dielectric barrier discharge plasma actuator. The disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding strength in the aft portion of the bubble is reduced. Duration of the recovery of the bubble to its unperturbed state is independent of the forcing amplitude. At the same time, linear stability analysis shows that the growth rate and the frequency of the most unstable mode decreases for increasing forcing amplitude. Throughout recovery, growth rates are directly proportional to the shape factor, indicating that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. It is found that the stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop.

We investigate the propagation, growth, and decay of fast magnetosonic waves in the Earth's magnetosphere which are believed to contribute to proton heating up to energies of a few hundred eV near the magnetic equator. We construct a model of the proton and electron distribution functions from spacecraft data and use the HOTRAY code to calculate the path‐integrated growth and decay of the waves over a range of L shells from L = 2 to L = 7. Instability calculations show that the waves are excited at very large angles of propagation with respect to the magnetic field, ψ ≈ 89°, at the harmonics of the proton gyrofrequency ΩH+ up to the lower hybrid resonance frequency ωLHR by a proton ring distribution at energies of the order of 10 keV. As a “rule of thumb”, we find that growth is possible for ω > 30ΩH+ when the ring velocity exceeds the Alfvén speed vR > vA, and for ω < 30ΩH+ when vR > 2vA. For propagation in the meridian plane, waves generated just outside the plasmapause grow with large amplification as they propagate away from the Earth but eventually lose energy to plasma sheet electrons at energies of a few keV by Landau damping. The waves grow to large amplification at frequencies just below ωLHR. For inward propagation we find that waves generated just outside the plasmapause can propagate to L ≈ 2 with very little attenuation, suggesting that waves observed well inside the plasmasphere could originate from a source region just outside the plasmapause. Strong wave growth only occurs for large angles of propagation, and thus the waves are confined to within a few degrees of the magnetic equator. Waves generated near geostationary orbit and which propagate toward the Earth are absorbed by Doppler‐shifted cyclotron resonance when they propagate into a region where vR < vA. Cyclotron resonant absorption causes pitch angle scattering and heating transverse to the ambient magnetic field. The amount of absorption, and hence transverse proton heating, increases significantly as the thermal proton temperature is increased up to 100 eV, suggesting a feedback process. Ray tracing shows that transverse heating of the thermal proton distribution is most likely to occur just outside the plasmapause where vA is large. Since proton ring distributions are formed during magnetic storms at ring current energies, we suggest that fast magnetosonic waves provide an additional energy loss process for ring current decay.

Cometary plasma wave measurements in the spectral range from the lower hybrid resonance frequency to the electron plasma frequency are presented for the Comets Giacobini-Zinner and Halley. The results show the significant role played by plasma physics in controlling the solar wind interaction with a comet. A variety of very strong wave-particle interactions are found to develop in the entire region where strong mass loading occurs, and additional important plasma instabilities are found in the vast region where newly-born ions are picked up by the solar wind. The difficulties involved in making plasma wave measurements in the presence of large fluxes of dust particles impacting the spacecraft at high speed are discussed.

Trapping of lower hybrid waves in elongated plasma depletions in the Earth's ionosphere