Using the 1986 data of Indian observatories, new results are given on 8 landmark parameters that depict the structures of hourly latitudinal profiles of worldwide part of Sq (WSq) current from 0700 to 1700 local time. Diurnal and seasonal features are discussed. Variations of the ratio of peak return to peak forward current intensity demonstrates that, when the current contracts, it intensifies relatively greater at a farther than at a nearer distance to the current centre. Some evidence was found for the possible existence of counter worldwide part of Sq (CWSq) when the WSq current flows counter to its normal direction. About 14 per cent of CWSq profiles are found predominantly in the morning and afternoon hours and their parameters are discussed.
The possibility of the generation of decameter scale ionospheric plasma density irregularities, that must be responsible for dusk scatter, by the plasma gradient drift instability (GDI) at F-region altitudes is considered. It is shown that the dusk scatter could be produced by the ion density perturbations which appear as a result of the development of the GDI produced by the maximum westward plasma drift in the region poleward of the trough minimum. Possible reasons for the appearance of growth of the GDI waves as a result of the development of the trough plasma GDI during or just after sunset in the F-region are discussed. It is shown that, if the GDI begins after sunset, then the influence of the drift velocity shear results in the action of the GDI during 1–2 hours after sunset, which is close to the duration of dusk scatter.
Two numerical one-dimensional and time-dependent models of the topside auroral ionosphere in the altitude range 200–3000 km are presented: they are based on two different numerical schemes to solve the eight-moment approximation of Boltzmann's equation, namely the Flux Corrected Transport (FCT) and the Method of Lines (ML). The transport equations for densities, velocities, temperatures and heat fluxes are simultaneously solved along the magnetic field lines for each constituent of the ionospheric plasma assumed to be composed of electrons and of O + and H + ions. These models, using the MSIS-86 neutral atmosphere model, include solar EUV photoionization, chemical and collisional processes between the various charged and neutral species. Steady-state results for both near-summer and winter conditions as well as for diurnal evolution are presented and compared to experimental data from the European Incoherent SCATter (EISCAT) VHF radar. It is shown that independently of the numerical scheme, the ionospheric structure is very well reproduced, given realistic external sources (solar ionization and heating, magnetospheric energy input). On the basis of the comparisons, the eight-moment approximation is validated up to 3000 km altitude. Furthermore, simulation of a diurnal evolution shows terminator effects on the enhancement of the downward F2 electron heat flow.
Ionospheric modifications produced by powerful high frequency radio waves are studied using the method of field-aligned scattering of diagnostic HF radio signals. Experimental data have been obtained in the course of two ionospheric heating campaigns carried out in November 1983-February 1984 and in May 1993 on the radio paths Kiev-Vasilsursk-St Petersburg and Krasnodar-Vasilsursk-St Petersburg. Observations of HF scattered signals have been made by the Doppler spectrum method with high time resolution. Analysis of the experimental data shows the appearance of quasiperiodic variations in Doppler frequency shift, fd, with periods of 30–60 s during the heating cycles. Powerful HF waves are assumed to excite the Alfvén resonator, generating oscillations of the magnetic field lines in the heated region and giving rise to artificial fd variations and magnetic pulsations. In the case of countinuous action of the powerful HF transmitter, artificial ionospheric waves are sometimes generated with periods corresponding to medium-scale travelling ionospheric disturbances (12–25 min), representative of internal gravity waves.
A new compact formulation is given of the equation of transfer for a scattering system for a magnetized plasma. The formulation is based on the low temperature kinetic model and accounts for scattering from fluctuations in density, magnetic field, electric field and current. It is demonstrated that scattering from several types of fluctuations can be significant and that the relative phases of the fields scattered by these fluctuations must then be taken into account by including cross-correlations between fluctuations. It is shown that significant enhancement or cancellation of scattered power can result from cross-correlations, and that this is of practical consequence to existing microwave scattering experiments on fusion plasmas.
