The solar-activity and latitudinal characteristics of the seasonal, non-seasonal and semi-annual variations in the peak electron densities of the F2-layer at noon and at midnight in middle and low latitudes

Abstract

The F2-layer peak electron density variation is expressed, for a fixed local time, as a superposition of the seasonal, non-seasonal and semi-annual components plus the annual mean and its observed values at noon and at midnight in middle and low latitudes (including the equatorial zone) have been analysed to reveal the solar-activity and latitudinal characteristics of the three components. The seasonal variation shows entirely different behaviour at noon and at midnight. At noon its amplitude is very much enhanced at latitudes higher than 40° geomagnetic latitude when solar activity is high and it has a maximum in winter; in the equatorial zone, however, a maximum occurs in summer with a rather small amplitude. At midnight it reaches a maximum near the summer solstice and is generally predominant over other components except in low latitudes when solar activity is rather high and in the equatorial zone; it has a latitudinal peak at about 25° geomagnetic latitude. The non-seasonal variation is unexpectedly large and, in particular at noon, is generally the most important component in low and equatorial latitudes and/or when solar activity is low; its importance, however, decreases with increasing solar activity. Its amplitude has a latitudinal peak at about 15° ~ 20° geomagnetic latitude. The approximate coincidence of the times of occurrence of a maximum at about the early part of January is noted between the non-seasonal variation for low solar activity and the annual component of the upper atmospheric temperature variation. The semi-annual variation at noon takes a maximum value between about the middle and the end of April (October), which time interval contains the time of occurrence of a maximum in the semi-annual component of the upper atmospheric temperature variation. The amplitude has a latitudinal peak at about 15° latitude both at noon and at midnight and, in the former case at least, maintains roughly a constant proportion to the annual mean, irrespective of solar activity.