Nonlinear disturbances in the ionosphere due to acoustic gravity waves

The influence of the higher harmonics of an internal gravity wave on the formation of nonlinear quasi-periodic disturbances in the F-region of the Earth's ionosphere is considered. It is shown that the Boussinesq approximation cannot be used in describing a plane nonlinear gravity wave as nonlinearities associated with the compressibility of the atmosphere have to be taken into account.     

Tropospheric gravity waves and their possible association with medium-scale travelling ionospheric disturbances

Analysis of pressure fluctuations observed over a period of several days using an array of microbarographs has shown the existence of long trains of gravity waves with two or more waves often present simultaneously. Meteorological data from radiosonde ascents indicates that many of the waves have a velocity which matches that of the background wind at some level within the troposphere. Generally this height corresponds to that of a frontal zone marking the transition between air masses and it is suggested that the waves may have been generated by shear flow instability within the frontal layer. Theoretical considerations, based on a three-layer model troposphere, show that some of the observed waves could have been ducted in or near the frontal zone. Some evidence is found to indicate that a non-linear wave-wave interaction between pairs of waves occurring simultaneously in the frontal zone could yield secondary waves with the characteristics of the gravity waves which had been observed in the thermosphere at appropriate times and whose group paths were traced to source regions in the troposphere in the general vicinity of the microbarograph array.     

The role of acoustic gravity waves in the generation of spread-F and ionospheric scintillation

In the aggregate, acoustic gravity waves in the F-region constitute a spectrum of geophysical noise extending from the frequencies involved in diurnal variations up to the Brunt-Väisälä buoyancy frequency. They drive a roughly uniform power spectrum of travelling ionospheric disturbances (TIDs) with vertical scales of the order of the atmospheric scale height H and with horizontal scales extending from the radius of the Earth down to H. It has been known since the 1950s that this permits multiple normals onto the F-region from an ionosonde, thereby creating the multiple-trace type of spread F on ionograms. At shorter scales the spectrum of TIDs decreases in strength and, below the mean free path of the neutral atmosphere, creates a spectrum of plasma turbulence aligned along the Earth's magnetic field. Progressively shorter scales are responsible for phase scintillation, for amplitude scintillation and for blur-type spread F on ionograms. A weak extension of the spectrum to scales less than the ion gyroradius is responsible for spread F and transequatorial propagation in the VHF band. Under evening conditions in equatorial regions a band of TIDs with wavelengths of the order of 600 km can, at times, have a phase velocity that matches the drift velocity of the plasma (Röttger 1978). This band of TIDs is then amplified until it breaks (Klostermeyer 1978). The associated explosive increase in plasma turbulence creates the plume phenomenon discovered by Woodmn and La Hoz (1976).     

Weak nonlinear theory of the ionospheric response to atmospheric gravity waves in the F-region

The nonlinear ionospheric response to atmospheric gravity waves is studied in an approximate fashion using a new approach. The concept of nonlinear travelling ionospheric disturbances (TIDs) is outlined, and the nonlinear behaviour of atmospheric gravity waves is calculated. A principal result is that harmonics are generated which cause the wave velocity perturbation to deform. The ionospheric response is investigated by solving the continuity equation for ionization in the F-region. The distortion of the TIDs waveform produced by the nonlinear interactions is depicted. The nonlinear TIDs depart seriously from a cosinusoidal wave described by previous linear TID theory. The distorted TIDs appear as ‘sharp peak’ and ‘sawtooth’ waveform shapes. The ‘peaks’ can be upward or downward, and the ‘sawteeth’ forward or backward, depending on the wave parameters. The nonlinearly distorted TIDs show a good agreement with various observed ionospheric irregularities produced by atmospheric gravity waves.     

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.     

Calculation of ionospheric effects due to acoustic radiation from an underground nuclear explosion

Within the framework of the ionospheric detection of underground nuclear tests, we have developed an analytic computing technique for the acoustic effect of a confined nuclear explosion on upper layers of the Earth's atmosphere. The relationship (32) is obtained, which relates the nuclear test parameters (depth, explosion yield, and mechanical properties of the rock) to the vertical displacement of the ionosphere produced by the shock wave over the explosion's epicentre. It is also shown that most of the acoustic energy produced by a confined underground nuclear explosion escapes upward, with only a small fraction being captured by the atmospheric waveguide.     

A study of gravity waves in ionospheric electron content at L = 4

Using satellite radio beacon transmissions, travelling ionospheric disturbances have been observed in the electron content at L = 4. Waves are a common feature at this latitude, present for at least 98% of all daylight hours. The amplitude is usually 1–4% of the mean electron content and periods range between 15 and 90 minutes. Simultaneous observation of two satellite beacons, giving an effective east-west separation of 350 km, indicated apparent east-to-west velocities of 200–700 m/s.A search was made for a likely source of the waves, using data from magnetometers and riometers, from incoherent scatter radar measurements of Joule heating, and from orbiting satellite measurements of electron influx, but no definite source could be established.It is also shown that travelling disturbances are closely related to occurrences of spread-F on ionograms at high latitudes.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

Interactions between diurnal tides and gravity waves in the lower thermosphere

Recent progress on interactions between breaking gravity waves and the diurnal tide in the upper mesosphere and lower thermosphere is reviewed, mainly based on the recent results of our numerical models.     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

Reflection of electromagnetic waves by density gradients in a magnetised plasma

The reflection of electromagnetic waves by density gradients in a plasma with a magnetic field derived from a scalar potential is studied. Conditions are found for reflection at grazing incidence at a surface S containing field lines. No restriction is placed on the angle between the wave-normal and the field, and account is taken of the curvature of the surface and the anisotropy of the medium. Particular reference is made to whistler-mode waves in the magnetosphere. The minimum electron density enhancement needed for reflection is calculated. Guidance by small-scale irregularities and ‘super whistlers’ (whistlers with upper cut-off above one-half the equatorial electron cyclotron frequency) are briefly discussed.     

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

Temporal variations of ionospheric waves in the D- and F-regions

It is shown that mesoscale ionospheric wave disturbances in the D- and F-regions regularly occur at all times of day, night and season (characteristic periods ~100, 24, 12.6 min) and are a characteristic property of dynamic processes in the ionosphere.     

Horizontal velocity dispersion of medium-scale travelling ionospheric disturbances in the F-region

Daytime observations of the horizontal velocity dispersion of medium-scale travelling ionospheric disturbances (TID) near the F2 peak height have been carried out using an array of HF Doppler sounders in central Japan. Cross-correlation analysis of sample records has shown that the horizontal trace velocity is a decreasing function of the period of fluctuations in the range 13.3–40 min. The theoretical dispersion of the atmospheric gravity waves is also calculated using Klostermeyer's (1974) method. Comparison between the observed and the calculated results suggests the possibility that the components of the lower period of the observed velocity dispersion may be a remnant of the quasi-evanescent mode pertinent to lower-height levels.     

Multiple beam observations of mid-latitude ionospheric disturbances by the MU radar

The pulse-to-pulse beam steerability of the M U radar of Kyoto University enables us to observe multiple beam positions simultaneously. Based on 560 h of this type of data, we present two typical patterns of mid-latitude ionospheric disturbances and their horizontal traveling characteristics. Wavy structures have not been found in large-scale disturbances. Isolated disturbances travel primarily southward (equatorward) in disturbed conditions, while no preferred direction is observed in quiet conditions.     

Generation of stationary irregularities due to heating of the high-latitude ionospheric E-region

The nonlinear equations describing the generation of artificial irregularities in the E-region of the high-latitude ionosphere due to Joule electron heating under the action of high-power radio waves are derived, including the low-frequency nonlinearities, and investigated in the three-wave approximation. Expressions for the stationary spectra of the short wavelength two-stream and long wavelength gradient-drift and current-convective irregularities are presented. The typical saturation amplitudes of the density fluctuations are obtained.     

The classification of distorted-mirror reflections and its ionospheric modification due to the underlying ionization

A systematic classification of reflections from a sinusoidally distorted-mirror has led to the identification of three classes of rays: direct simple rays which hit the mirror once on reflection; direct complex rays which hit the mirror several times before reflection; indirect rays which leave from and return to the source at different angles. The conditions on the distortion amplitude, wavelength and height for the different classes to exist are obtained. A comparison of results is made with those from a Chapman layer with a sinusoidally varying height of peak density, in the absence of any magnetic field. The broad classification of solutions is preserved, but direct and indirect rays are no longer independent and at large scale heights the presence of underlying ionization removes some solutions. The construction of ionograms of these profiles shows that indirect and complex direct rays contribute significantly to the traces observed.     

Theoretical study of propagation of short periodic gravity waves within the thermosphere. Application to the plasma line measurement and to a Faraday rotation experiment

The harmonic analysis of the plasma frequency fluctuations obtained with the plasma line experiment at St. Santin shows the existence of a noise of fluctuations for periods below 20 min. The main feature of this noise is to present a cut off low period which is essentially variable. A theoretical study of the propagation of short periodic gravity waves within a realistic thermosphere shows that the main parameter governing the position of the cut off frequency is the wind profile. The same theoretical study shows that the gravity waves of period between ¦16, 20¦min can propagate essentially during the day and the waves of period between ¦10, 13¦min propagate during the night. This conclusion is in agreement with the results of a Faraday rotation experiment.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

The influence of high speed plasma streams on the ionospheric plasma

As shown by statistical investigations, high speed plasma streams (HSPS) in the solar wind cause direct ionospheric effects in the D- and Es-layers at auroral and subauroral latitudes due to increasing precipitation of high energetic particles as well as indirect effects in the F2-region at high, middle and equatorial latitudes caused by auroral heating processes. The ionospheric effects increase with the strength of the HSPS and are most pronounced for HSPS during IMF pro sectors (sectors with negative B_z-component). Seasonal differences of the ionospheric response to solar velocity changes are caused by the IMF influence (maximum effect at equinoxes) as well as internal atmospheric reasons (enhanced variability during winter).