The variability of ionospheric dynamo currents

The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents. (author)