Small-scale structure of the F region of the ionosphere

Abstract

The fine structure of diffuse reflections is analyzed in the paper on the basis of data of continuous recording of theeffective height in vertical sounding at a fixed frequency. The hypothesis is advanced that local regions of generation of small-scale inhomogeneities are present in the ionosphere. Certain sections of the recordings can be interpreted as a reflection from splitting inhomogencities. It has been established experimentally that in ionospheric sounding the reflected signal is diffuse most of the time. The nature of the diffuse component of the ionospheric radio channel is still inadequately studied at present. The low resolving power of the rectangular radio pulses with a duration on the order of I00 ~sec usually used to investigate the ionosphere prevents one from revealing the internal structure of the diffuse signal. The purpose of this article is to describe the morphological features of the fine structure of diffuse reflections and make a preliminary interpretation of new experimental results involving the small-scale part of the spectrum of ionospheric inhomogeneities responsible for the diffuse signal. The results presented here were obtained in vertical sounding of the ionosphere with equipment operating with a complex signal [i, 2], which made it possible to increase the accuracy of the height measurement and the resolving power in height to 1.5 km, an order of magnitude better than with the typical equipment used for ionospheric measurements. The research was carried out in Irkutsk by the method of continuous recording of the effective height in vertical sounding [the h'(t) method] at a fixed frequency in the 3-4 MHz range, which corresponds to reflection from the lower part of the F region. The experimental material consists of a photographic record on movie film of the time distributions of positive maxima of the correlation function for one quadrature component of the response of the ionosphere. The track on the film, representing the h'(t) dependence in the reception of diffuse radio reflection, becomes multivalued, in contrast to a one-beam signal. Out of the entire variety of chaotic and more or less orderly variations of the track in the recording of diffuse signals one can distinguish sections of the recordings in which the group delay undergoes quasiperiodic variations. Such recording sections (henceforth we shall call them "flashes") can have different durations and differ in other features. An example of a flash is presented in Fig. I, showing the time dependence of the group delay of the reflected signal. it is seen from Fig. i that before the marking t o the reflected signal was a one-beam signal with a delay ~i- From the time t o the group delay of the signal starts to vary rapidly: Two periods of an abrupt decrease in the delay and its gradual increase are observed, and then a rapid decrease to the value T2, above which repeated reflections with a negative increment in the group delay are also observed over a certain time interval. After