Abstract

Sporadic-E (Es) are characterized by denser layers whose formation could be related to wave activity in the lower thermosphere, such as planetary waves, tides, and gravity waves. The first aim of this work was to analyze the Es layer occurrence under different geophysical conditions to establish the importance of these waves. Thus, we used ionosonde observations of four different locations at mid-latitudes that are King George Island (62.2 degrees S, 58.8 degrees W), Argentine Islands (65.3 degrees S; 64.3 degrees W), on the Antarctic Peninsula longitude sector, and Canberra (35.3 degrees S; 149.0 degrees E) and Hobart (42.9 degrees S; 147.3 degrees E), on the Australian sector. The data are divided for winter, equinox, and summer during low (1986-87) and high (1989-90) solar activity levels. Seasonal means of the Es layer frequency and height parameters have been computed. The results show that the diurnal variations of the Es layer occurrence and their frequency parameters are similar for all seasons and solar activity levels over the two longitude sectors. These results do not support the expectations that gravity-wave momentum-fluxes are significant in this case study. The height of the Es layers over Antarctica is about 10 km higher than over Australia. Also, the Es occurrence is higher at night over Antarctica. Computed blanketing Es layer frequency (fbEs) using known formulae are almost identical to those observed for all locations and seasons during low and high solar activity. The Es frequency dependence on solar activity level is also different from what has been shown elsewhere.