The tide produces winds in the ionosphere which drive the conducting ions across the Earth’s magnetic field resulting in a dynamo. The dynamo emf causes electric currents to flow in the ionosphere and the magnetic fields of these currents are detected by magnetometers on the ground. There is a world-wide distribution of these magnetometers so it is possible to analyse their records to deduce the form of a world-wide current system in the ionosphere driven by the atmospheric lunar tide.

Recently it has been possible to simulate this current system using a computer dynamo model with the lunar atmospheric tidal winds as input. A reliable model of these winds has only recently become available, being derived partly from the gravitational effect of the Moon on the atmosphere and partly from the up and down movement of the lower boundary of the atmosphere, particularly the oceans. The latter motions were synthesised from satellite measurements of the ocean surface movements.

The simulation yielded reasonably good agreement with the observations. The form of the tide changes with season and some seasons gave better agreement than others.

Robert Stening