Sporadic E-Region Ionization at Watheroo Magnetic Observatory 1938-1944

Characteristics of sporadic E (Es) at the Watheroo Magnetic Observatory, Western Australia, have been determined from continuous ionospheric recordings since June, 1938. Average diurnal curves show most frequent occurrence at night with maximum near midnight, local time, although there is a tendency for the most intense Es to occur during day hours. The seasonal features of Es already well established for the Northern Hemisphere have been confirmed for the Southern Hemisphere with a maximum of Es in local summer months. Annual trends show increasing values from 1938 to 1941 with decreasing values from 1941 through 1944. An upward trend is indicated for 1945, suggesting a minimum in 1944. This annual characteristic is significant in view of an apparent inverse relationship with sunspots in the Northern Hemisphere. Separate analyses were made of Es to determine 40- to 80-megacycle propagation conditions for a 1000-mile path in per cent of time for selected hours of November, 1941, which was the period of greatest Es activity. Results show 40-megacycle signals supported for 15 per cent of time, while 80-megacycle signals dropped to less than 1 per cent of time. As a test for solar origin of Es, the data were examined for recurrence tendencies in successive 27-day solar rotational periods. No pronounced recurrences of Es at 27-day intervals are apparent, from which it may be inferred that Es has no direct relationship with other recurrent solar phenomena such as sunspots and other centers of solar activity.