Periodic disturbances in the ionosphere

Total electron content measurements, at latitudes of 34°S and 42°S, showed that 30–50% of all large-scale irregularities in the ionosphere are periodic in space and in time. The observed wavelengths were generally between 40 and 160 km. The periods ranged from about 60 minutes down to a sharply defined lower limit of 15 minutes. This agrees precisely with the spectrum for gravity waves in the F region. Periodic disturbances did not occur near sunrise and sunset. Daytime disturbances caused periodic changes in total content of a few per cent, and were present for about 5% of the time at the lower latitude in summer. They became more common at the higher latitude and in winter, when the ionosphere was continually disturbed by the passage of large irregularities. Most of these had periods of about 20 minutes, suggesting a thermobaric resonance in the F region. Nighttime disturbances were generally more intense and occurred most frequently at the lower latitude in summer.