Reflection properties of the lower polar ionosphere, and features of the excitation and propagation of superlong waves (SLW) at high latitudes (review)

Abstract

REFLECTION PROPERTIES OF THE LOWER POLAR IONOSPHERE, AND FEATURES OF THE EXCITATION AND PROPAGATION OF SUPERLONG WAVES (SLW) AT HIGH LATITUDES (REVIEW) G. F. Remenets and M. I. Beloglazov UDC 550.38+621.37 At the present time, SLW radio waves are used to solve a wide range of problems: inves- tigation of the lower ionosphere and magnetosphere, use in global systems of communications, navigation, and in the transmission of standard frequencies. In this regard, there is special interest currently in the study of the properties of SLW propagation in high latitude regions, where a whole complex of specific ionospheric phenomena exist: these phenomena affect significantly the structure of the SLW field in the waveguide which is formed by the Earth and the ionosphere. Since the latest reviews of high latitude propagation of SLW were published [I, 2], a number of new and rather interesting data have appeared. These new results refer in part to improvements in analysis techniques, enhancements in the precision and resolving power of the methods of measurement, and the accumulation of new quantitative results which merit special consideration. It is well known that SLW propagation processes in the near-earth waveguide are extremely sensitive to the conditions in the lower ionosphere. In the present case, by the term "lower ionosphere" we mean the D-layer and the lower part of the E-layer (i.e., heights of 50-90 km, where electron densities are N e = 10-103 cm-3). We will therefore discuss first of all briefly those papers of the last 5-10 years where new or more precise results have been obtained concerning the free electron content in the lower ionosphere at high latitudes. I. SOURCES OF IONIZATION AND~METHODS OF INVESTIGATING THE ELECTRON CONTENT IN THE LOWER POLAR IONOSPHERE It is well know~ that the regular sources 0f ioniZation in the lower ionosphere which are independent of latitude are the radiation from the sun (photons) and cosmic rays of galactic origin. As has been shown in [3], the principal role in quiet daytime conditions is played by ultraviolet radiation from the sun. The major difference between the high latitude ionosphere and the ionosphere at mid- latitudes is its extraordinary variability, due to the constant incursion of corpuscular fluxes: the latter are extremely irregular in character. I.l. Brief Characterization of the Sources of Anomalous Ionization in the Lower Ionosphere at High Latitudes I.l.l. Intrusion of solar cosmic rays (mainly protons with energies of i-i00 MeV) leads to the formation of significant ionization at anomalously low heights and to the occurrence of a whole complex of effects known by the overall name of PCA (polar cap ab- sorption). The electron density may rise to values of hundreds and thousands of electrons per cubic centimeter at heights of 40-50 km. Each PCA event may last for up to 10-15 days. The region of anomalous ionization which develops occupies the region near the poles: on the average, it is bounded by the geomagnetic latitudes ~' = 65-70 ~ . The average duration of proton anomalies amounts to roughly 30-40 days per year [4]. During years when the sun is quiet, this figure falls to 15-30 days, while in years when the sun is active, it increases to 40-60 days. I.I.2. During magnetic substorms, fluxes of electrons with energies of tens of kilo- electron-volts precipitate into the earthfs atmosphere. These lead to the appearance of auroral ionization, which makes its presence felt in the form of enhanced absorption of cosmic radio noise. The statistically averaged region of auroral fonization (the auroral Leningrad State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 28, No. 12, pp, 1491-1504, December, 1985. Original article submitted February 5, 1985. 0033-8443/85/2812-1029509.50 9 1986 Plenum Publishing Corporation 1029