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.     

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.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

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.     

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.     

Schumann resonance effects of electrical conductivity perturbations in an exponential atmospheric/ionospheric profile

Eigenmode solutions are computed for the n = 1 … 3 Schumann resonances in a perturbed, unmagnetized vertical atmospheric conductivity profile σ = 10⁻¹⁶ exp (z/3.1) mho m⁻¹ for z ⩽ 100 km and σ = 10⁻² mho m⁻¹ for z > 100 km. For the unperturbed exponential profile the radial electric field E_r is nearly constant z ≲ 40 km, and decreases rapidly above 50 km. The tangential field E_ϑ > E_r for z ≳ 65 km. The Joule dissipation profile in this case has an absolute maximum at about 50 km and a smaller relative maximum at 90 km with a deep relative minimum at 65 km. The maximum dissipation thus occurs in the middle atmosphere, making the Schumann resonances particularly susceptible to conductivity perturbations in this region. The perturbations of this study comprise Gaussian-shaped enhancements or depressions of FWHM ≈ 10 km impressed on the unperturbed profile. Eigenfrequencies and Q-values are computed for the full range of perturbation amplitudes 10⁻³−10³ and altitudes 30–90 km. The perturbations induce overall eigenfrequency variations of ± 1.0, ±1.5, and ±2.5 Hz in the n = 1, 2, and 3 modes, respectively, and Q-values spanning the range 3.5–11.0. The results of this calculation extend those of previous works investigating the Schumann resonance response to atmospheric conductivity perturbations, and may be useful for interpreting experimental observations in terms of external ionization source intensities of GCR, Lyman-α, or solar cosmic or X-rays, or variations in middle atmospheric chemical constituents.     

The effects of travelling ionospheric disturbances on ionograms

The numerical synthesis of ionograms by ray-tracing in an analytic two-dimensional ionosphere is simplified by a technique which avoids the usual wastage of ray tracings. The technique is particularly suitable for obtaining ionogram sequences corresponding to a moving ionospheric disturbance. It is applied firstly to a moving tilted ionisation increase, which gives the familiar travelling-cusp records. Ionograms do not give a good measure of the vertical distribution of ionisation, and in the case considered lead to an overestimate of the horizontal size of the increase by a factor of about 2. For a travelling ionisation decrease or trough, the ionograms show an additional U-shaped trace overlapping the main trace. Calculated ionograms agree closely with some observations. It is difficult to obtain any measure of the size of the electron-density decrease; critical frequency scalings may give no indication of the passage of an intense disturbance. Similar results are obtained for a wavelike perturbation in an exponential topside ionosphere. Topside ionograms then show multiple ‘nose’ traces, following in general the curve for the unperturbed ionosphere. It is suggested that the multiple echoes frequently observed on topside ionograms may, in some cases, be due to refraction in large disturbances rather than the commonly-assumed ducting mechanism.     

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).     

Computation of the zonally-averaged circulation driven by heating due to radiation and turbulence

In a zonally-averaged dynamic model the steady state circulation of the middle atmosphere between 10 and 110 km altitude has been calculated for solstice. To investigate the combined effect of turbulent heat conduction and dissipation of eddy kinetic energy on the mean circulation, the dissipative heating has been parameterized in terms of the buoyancy term modified by a residual Richardson number. It is shown that turbulence will result in net heating of the mesopause region to be consistent with a zero mass flux through a pressure surface. It is also demonstrated that the combined effect of turbulent heat conduction and dissipation can modify the mean circulation remarkably if the Richardson number is made latitude-dependent.     

Some effects in the middle atmosphere due to lightning

A brief review is given of some of the electrodynamic responses of the middle atmosphere to lightning. Attention is focused on the precipitation of energetic electrons from the magnetosphere, due to whistler mode electromagnetic waves. The secondary ionisation and bremsstrahlung radiation created, and some of the ways in which such effects can be detected, are also considered. Finally, the possibilities of positive feedback mechanisms operating between the atmosphere and the magnetosphere are investigated.     

Reconstruction of horizontally-inhomogeneous ionospheric structure from oblique-incidence backscatter experiments

Fridman and Fridman [(1994) J. atmos. terr. Phys. 56, 115] suggested a method of reconstructing the horizontally-inhomogeneous ionospheric structure using vertical- and oblique-incidence backscatter sounding (OBS) ionograms measured at a single location. In the present paper this technique has been used to analyze experimental data and tested against independent vertical sounding (VS) measurements. By using the OBS and VS ionograms measured at Irkutsk as source data for the method we reconstructed ionization profiles over Tomsk (1050 km to the west of Irkutsk). We found that the reconstructed profiles are in reasonable agreement with the profiles obtained from VS measurements at Tomsk.     

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.     

Some effects of geomagnetic activity and ionospheric height rises on mid-latitude spread-F occurrence

The analysis investigates the base heights of the ionosphere (h′F) when spread-F is recorded at Brisbane, Australia, for 2 separate periods, namely July and August 1966 when spread-F occurs frequently and September and October 1966 when the activity is much lower. It is found that for July and August there is little tendency for spread-F to occur preferentially when the base height is above the average for the period. However, for the September–October period, spread-F occurs more often (by a factor of 2 or 3 depending on the geomagnetic activity) when the base height is above rather than below the average height. Also, this analysis shows that the overall spread-F occurrence (for both periods investigated) decreased to some extent following increased geomagnetic activity. This suppressed activity in the hours following geomagnetic activity is confirmed by superposed-epoch analyses using K indices (for Macquarie Island) as controls. It is suggested that the results of all the analyses might be explained by invoking a transition height in the ionosphere (controlled by the neutral-particle density of the upper atmosphere). Ionospheric off-vertical reflections from above this height would be recorded as spread-F traces in this model. This transition height would be low in July and August when the neutral-particle density is low and higher in September and October. It is further proposed that changes in the neutral-particle density could also be associated with reduced spread-F activity following increased geomagnetic activity, as well as influencing the diurnal, annual and sunspot-cycle variations of spread-F occurrence in mid-latitudes.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.     

The global distribution of ionospheric small-scale irregularities from topside sounding data

This paper examines the global distribution of electron density irregularities with scales of the order of several tens to hundreds of meters in the ionosphere by using topside sounder data from the COSMOS-1809 satellite obtained in May–June and December 1987. The diffuse traces of Z-waves on topside ionograms in a frequency band just below the upper hybrid resonance are used for diagnostics. These traces are attributed to the scattering of sounder-generated ordinary and slow extraordinary mode waves.     

Effects of neutral temperature on meridional winds estimated from ionospheric data

Indirect determination of meridional winds using ground-based ionosonde data from low latitude regions, under the assumption that the thermosphere and the F-region of the ionosphere behave as a closely coupled system, has been critically examined. The significance of neutral temperature and its variations in the above estimates has been demonstrated through individual case studies after duly validating the procedure adopted. Since the measured neutral temperatures have shown large deviations from the existing atmospheric models on many occasions and more so during high solar activity periods, it has been shown that the neutral temperature effects on the F-region heights should be properly accounted for before one attempts to estimate meridional winds. However, it has also been shown that during low solar activity periods, use of atmospheric models may still provide a fairly reasonable average picture. Examples of these effects are presented and discussed.     

Regimes of ionospheric turbulence from fractal analysis of satellite radio signal scintillations

Samples of amplitude scintillations of the radio signal from a geostationary satellite obtained at a midlatitude station near Irkutsk were processed. For calculating the fractal dimensionalities the Grassberger and Procaccia [(1983) Physica D9, 189] algorithm was used. Results of the data processing tend to divide into two groups. One group includes those realizations for which it was possible to obtain reliable estimates of dimensionality. Three of the seven realizations considered were in this group, and the fractal dimensionalities were found to be low (3.12 4.5). The other data fall within the second group; a reliable estimate of dimensionality for them is unobtainable in terms of the method used. We suppose that this is attributable to the high dimensionality of the process. Power spectra of the signals of this group are close to those with an exponent of −2. The spectra of the signals of the first group are markedly steeper. On the basis of the data analyzed it is supposed that there exist two modes of ionospheric turbulence in midlatitudes, namely the mode with low dimensionality typical of localized turbulent processes, and the mode with high dimensionality typical of homogeneous turbulence that covers an extensive region of the ionosphere.     

Nuclear Iran is Not an Option: A new negotiating strategy to prevent Iran from developing nuclear weapons

The negotiations between Iran and the P5+1¹ over Tehran's nuclear enrichment activities have not only failed to reach an agreement but have brought Iran much closer to the threshold of mastering the technology to produce nuclear weapons. There are many factors that precipitated this breakdown, including the West's inability to understand and deal with the Iranian psychological disposition, the failure to present to Iran the severity of the punitive measures that could be inflicted as a consequence of their defiance, and the US administration's misleading policy that gave Iran the room to maneuver. There is an urgent need to adopt a distinctively new strategy toward Iran consisting of three tracks of separate but interconnected negotiations: The first should focus on the current negotiations on Iran's enrichment program and the economic incentive package; the second should concentrate on regional security and the consequences of continued Iranian defiance; and the third track should address Iran's and the United States' grievances against each other. The United States must initiate all three tracks without which future talks will be as elusive as the previous negotiations, except this time the West and Israel will be facing the unsettling prospect of a nuclear Iran.