Theory for stimulated scattering of electromagnetic waves

In order to describe the nonlinear coupling of a large amplitude electromagnetic pump wave with the low-frequency modes in the ionosphere, we introduce new systems of equations that are related to the conventional Zakharov equations. In particular, we consider the excitation of ion-acoustic, electron gyro, and collisional gradient drift modes.     

Production of electromagnetic field disturbances due to the interaction between acoustic gravity waves and the ionospheric plasma

The problem of electromagnetic field disturbances produced by the interaction between winds of acoustic gravity waves (AGW) origin and the ionospheric plasma has been considered. It is shown that, when not allowing the electrostatic approach, electromagnetic field disturbances represent shear Alfvén and compressional modes modified by ionospheric Pedersen and Hall conductivities. It is further shown that the quasielectrostatic Alfvén type disturbances give the main contribution to electric field perturbations. Magnetic field perturbations due to Alfvén and compressional modes have the same order of magnitude. Two numerical models for simulation of the problem under consideration have been developed. The first model is intended for the simulation of Alfvén type disturbance production and transmission into the magnetosphere, taking into account the dipole geometry of the geomagnetic field, but a mutual transformation of Alfvén and compressional modes is ignored. The second model is constructed for the simulation of both electromagnetic field disturbance production and their mutual transformation in the ionosphere. The results of numerical simulations with these models show that there is an opportunity for AGW activity monitoring in the lower thermosphere by ground-and satellite-based recordings of magnetic and electric field variations.     

A simple theoretical model of the diffraction of VLF electromagnetic waves by the Antarctic icecap

The Antarctic continent has been modelled as a spherical cap whose pole is coincident with that of the South Pole, which totally absorbs VLF radio waves attempting to propagate over it. The propagation of Omega navigation signals around this model icecap has then been computed using Kirchhoff diffraction theory. Spherical caps extending to 66.5 and 75.5°S have been found to accurately model the signals from Omega La Reunion and Argentina, respectively, received on flights between Christchurch, New Zealand and Scott Base in Antarctica, up to the boundary of the theoretical icecap. These model icecaps were found to be good fits to the boundary of the Antarctic continent, when measured at the 1–1.5 km contour of ice thickness, in the region where the VLF waves diffracted around the icecap. The good agreement obtained between the experimental field strength data and those computed theoretically, using only simple diffraction theory, suggests that coastal refraction plays at most only a secondary role in circumpolar propagation.     

Reflection of VLF radio waves from distant mountain ranges

A computer cross-correlation technique is being used to determine the group delays and directions of arrival of man-made subionospheric VLF signals which have reached the receiver by paths other than the direct great circle path. The 200 baud MSK signals transmitted by NWC, NPM and NLK allow time resolution to at least 5 ms and, with 15 min of integration, the sensitivity can be as low as about 0.1 μV m⁻¹ in quiet conditions. Reflections from the Andes, the Rockies and the mountains of S.E. Asia have now been identified at Dunedin, New Zealand. Round-the-world and round-the-world-the-other-way signals have also been observed.     

Simultaneous generation of electron temperature and density fluctuations by high power HF radio waves

The parametric interaction between right-hand circularly polarized electron cyclotron waves as well as non-resonant density and temperature perturbations is considered by taking into account the radiation pressure and the differential Joule heating nonlinearities. A nonlinear dispersion relation, which admits a new class of thermal parametric instabilities for the case in which Joule heating nonlinearity far exceeds the radiation pressure, is derived. It is found that the temperature and density fluctuations are rapidly driven when the pump frequency is close to the electron gyrofrequency. The relevance of our investigation to enhanced density and temperature fluctuations due to the action of high power HF radio waves in the Earth's ionosphere is pointed out.     

The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.     

Ground and in situ excitation of waves in the ionospheric plasma

A review is presented of seven papers the contents of which range from ULF to VLF wave excitation in the ionospheric plasma by ground-based radio wave and acoustic wave sources, to in situ plasma wave excitation by satellite- and rocket-borne radio transmitters.     

Multiple electromagnetic scattering by a linear array of electrified raindrops

The Discrete Sources Method is used to construct a computational algorithm for the problem of multiple electromagnetic scattering by a linear array of axisymmetric particles. Numerical implementation is made for radiophysical and radio location characteristics using models for raindrops of different shapes, depending on the orientation of the particles and the polarization of the incident field.     

EISCAT observations of large scale electron temperture and electron density perturbations caused by high power HF radio waves

In this paper EISCAT observations of the effect of artificial modification on the F-region electron temperature and electron density during several heating experiments at Tromsø are reported. During O-mode heating at full power (ERP = 240 MW) the electron temperature is increased by up to 55% of its ambient value at altitudes close to the heater interaction height. Measurements of the electron density have revealed both enhancements and depletions in the vicinity of the heater reflection height. These differences are indicative of variations in the balance between the transport and chemical effects. These results are compared with a time dependent numerical model developed from the perturbation equations of Vas'kov and Gurevich [(1975) Geomagn. Aeron.15, 51]. The results of numerical modelling of the electron temperature are in good agreement with the EISCAT observations, whereas there is less good agreement with regard to electron density.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Determination of the wave-vector spectrum for plasma waves and turbulence observed in space plasmas

Several techniques exist for determining the distribution in wave vectors of an electromagnetic space plasma turbulence, based on the simultaneous measurement of several field components at several points in space. They all assume that the field is homogeneous in space and stationary in time. The main concepts used are reviewed (Wave Distribution Function, Field Distribution Function, Wave Telescope, Field Energy Distribution). A distinction is made between the ones that require a knowledge of a dispersion relation and those that do not require any hypothesis concerning the existence of a relationship between the frequency and the wave vector. The inversion techniques used are discussed.     

Theory of equatorial electrojet plasma waves: new developments and current status

There have been several important theoretical breakthroughs in the last few years that have substantially improved our understanding of the electrojet plasma instabilities. We now understand(1) the linear and nonlinear processes that control the longest wavelengths and probably also affect the electrojet current strength,(2) quite a bit about the two-dimensional turbulent cascade process that generates both the type 2 irregularities seen by radar and waves propagating even in the vertical direction, and(3) the anomalous diffusion process that limits the growth of the directly excited short wavelength type 1 waves and explains many of their properties. We finally really understand why type 1 and type 2 waves are different. In this paper we first briefly summarize the observed characteristics of the electrojet irregularities and then discuss the theory, devoting most of our attention to recent developments.     

Standing waves in VLF transmissions produced by geographical discontinuities

It has generally been accepted that the fields constituting the waveguide modes reflected from natural discontinuities in the lower boundary of the earth-ionosphere waveguide have a negligible amplitude compared to the fields constituting the incident modes at VLF (Kirchoff's approximation). Recent measurements, in the region of the South Island of New Zealand, show cases where such reflections play a significant role in the determination of VLF field strength and phase.     

Some aspects of ULF electromagnetic wave relations in a stratified ionosphere by the method of boundary value problem

Formulation and boundary conditions are developed to solve for the electromagnetic waves in a stratified ionosphere and atmosphere as a two-point boundary value problem. In the general case there are up-going and down-going Alfvén modes, up-going and down-going fast modes. It is shown that while large horizontal structure of the perturbation can be attributed to both the fast wave and the Alfvén mode, small horizontal structure can only be attributed to the Alfvén mode. The ratios of the electric field to magnetic field are given for various altitudes, frequencies and horizontal scale sizes. The results show that the magnetic field leads the electric field for the Alfvén mode and the electric field leads the magnetic field for the fast mode. The results also show that the ratio of the electric field to the magnetic field is varied and is not the Alfvén speed of the local medium. Analytical solutions are presented as tests against the numerical ones.     

Parametric excitation of whistler waves by HF heater

Possible generation of whistler waves by Tromso HF heater is investigated. It is shown that the HF heater wave can parametrically decay into a whistler wave and a Langmuir wave. Since whistler waves may have a broad range of frequency, the simultaneously excited Langmuir waves can have a much broader frequency bandwidth than those excited by the parametric decay instability.     

Measurements of gradients in ionospheric parameters with a new nine-position experiment at Millstone Hill

A new nine-position experiment is now routinely carried out with the Millstone Hill incoherent scatter radars which allows estimation of spatial gradients in the measured ionospheric scalar parameters N_e, T_e, and T_i, and in the components of the ion velocity vector v_i. Use of this technique results in improved estimates of basic and derived parameters from incoherent scatter data at times of significant gradients. We detail the data analysis method and present the first results from this new experiment. The gradients in N_e and in the components of vi are used to compute the motion term in the ionospheric F region continuity equation ▿ · (Nv), which is then combined with ∂N/∂t to estimate the O⁺ recombination rate β at night. Meridional neutral winds U_mer are computed from the field-aligned ion velocity v_∥ and a calculation of the O⁺ diffusion velocity v_d, and it is found that horizontal gradients in the ion velocity field at times significantly affect the calculation of the neutral winds.     

Large-amplitude standing planetary waves induced in the troposphere by the Sun

Investigations have been made of the global morphologies of two different Sun-weather relationships: the effect of the 27.5-day solar rotations and the effect of the 11-year sunspot cycles on tropospheric pressure. In both cases solar-induced perturbations occur in the form of large-amplitude standing planetary waves (or some combination of waves) of the (0, n), (1, n) and (2, n) types. Certain spatial features of Sun-weather relationships which have hitherto appeared puzzling are merely manifestations of these planetary waves.The range of the solar-induced 27.5-day oscillations of the height of the 500-mbar level reaches 14 Dm (decametres) in certain parts of the World. Such oscillations are of considerable practical importance; for example, the average pressure gradient responsible for driving the westerlies at upper-middle latitudes in winter varies by as much as a factor of two during the solar rotations. It appears that other short-term changes of solar radiation may also evoke a response in the lower atmosphere; the implications for theoretical studies of tropospheric dynamics are considerable.