Time and latitude variations of blanketing sporadicEof different intensities

Abstract

An in-depth study of blanketing sporadic E (Es) layers at temperate latitudes has been carried out with the purpose of clearly establishing many pertinent details of their temporal and latitudinal variations. A new method of analysis has been used in which blanketing Es layers have been separated into four groups according to their intensity, which is defined in terms of the ratio (ƒbEs/ƒoE). From an analysis of eight-years' (1958–1965) data of hourly ƒbEs, values at ten stations, it has been possible to establish, for the first time, that (1) Es layers of different intensities show different daytime variations with regard to their occurrence, and (2) there is a clear-cut change from higher to lower midlatitudes in the pattern of these daytime variations. It is shown that the daytime Es occurrence does not go through two maxima at all latitudes and in all seasons. The seasonal and solar cycle variations in the average ƒbEs and in the percentage occurrence of Es layers in each group are also shown. The ƒbEs values of intense Es layers show a marked decrease with decreasing solar activity, and this indicates the existence of solar cycle variation in the neutral wind shears and possibly in other relevant parameters in the E region. On the other hand, the occurrence of ‘intense’ and ‘moderately strong’ Es layers shows an increase with decreasing solar activity. Both features are shown to be consistent with the conclusions of wind-shear theory. A careful analysis of the Es variations with geographic latitude and with H (the horizontal component of geomagnetic field) has produced no unambiguous evidence for the dependence of Es occurrence or ƒbEs on H; and the inability of the present analysis to bring out this theoretically predicted dependence is attributed to the possibility that significant variations of neutral wind shears with latitude and geographic location can easily obscure the dependence on H. It is shown that the abnormally high incidence of ‘intense’ Es layers at Kokubunji is a purely summertime phenomenon. It is pointed out that this high incidence at Kokubunji cannot be explained in terms of the large H value alone, but the occurrence of very strong wind shears at this station has to be assumed. It is emphasized that much self-restraint is needed against making simplistic statements about the observed morphological characteristics of sporadic E.