Electromagnetic resonance phenomena and equipments to study the relation between solar activity and the magnetoplasma (ionosphere)

Abstract

A more exact definition of the lower ionosphere is given. By the lower ionosphere we mean that part of the upper atmosphere between 50–60 km and 150 km where the phenomenon of ionic diffusion is small, and which influences the propagation of radio waves and atmospheric noises, and which is the seat of the system of currents that produce the variations of the geomagnetic field. The plasma exhibits a particular non-linear behaviour and has a layer (Bailey's layer) where considerable effects of electromagnetic resonance occur. After describing briefly the most recent work on the chemical, ionic and electric structure of the D region, the reason for the non-linear behaviour of the plasma in this region is explained. Using V. A. Bailey's theory of interaction and the Ginzburg-Gurevich kinetic theory the concept of the ‘plasma field’ is given and it is shown how the latter theory allows greater precision in calculating G _eff and v _eff and that among other things G _eff depends on the temperature. To have a more complete idea of the non-linearity of a plasma, the theories of D. Graffi and M. Marziani are given in which the expression for a general non-linear equation of an electromagnetic field. Among the non-linear effects known up to date those studied in Italy, that is gyro-interaction, self-modulation, self-demodulation, detection effect, are considered. After defining the effect of the gyro-interaction (Luxemburg effect with resonance) the essential features of the Bailey theory are given. A brief description is given of the experiments carried out in Italy which showed the existence of resonance in the Luxemburg effect during both day and night, both with pulses and C.W., and the possibility of having a dromedary curve and a bactrian curve; also how in function of the penetration of the wanted wave in Bailey's layer we can pass from one resonance curve to another. From a discussion of the phenomenon of self-modulation one can deduce that there are two different phenomena, that is, a self-demodulation that obviously is not dependent on the power of the wave and must be a linear phenomenon, and a self-modulation that clearly depends on the power and is a non-linear effect which can be explained by the theory of interaction. Since it is not yet clear if the self-demodulation around the gyro-frequency depends on the power or not, the essential lines of the linear theory of Bailey, developed by Sodha and Kaw, and the non-linear theory of Sant Ram and Kaw are given. Finally the concept of ‘ionospheric detection effect’ is given and we also give, with some explanations, the theories proposed by G. Lanza in 1966 and whose conclusions seem to agree with experiments and by A. Gurevich in 1969. In the second part of the present work the methods of measurement and the technique used to demonstrate the above phenomena are given. The new technique of ‘modulated pulses’ that consists in the emission of pulses at MF and VHF modulated with low frequencies or with the gyro-frequency is of particular importance. In fact, if the VHF pulses are modulated at the gyro-frequency it is possible to generate ‘the detection effect’. If, instead, the HF pulses are modulated at audio-frequencies it is possible to show how the self-demodulation and the self-modulation do not depend at all on selective fading. There follows a description of the essential equipment built by the electronics laboratory of the Astronomical Observatory of Rome and located near Rome and at Camerino. The last section of the present work describes briefly the experiments made to date with the above-mentioned equipment and the principal results obtained.