Atmospheric tides and associated magnetic effects

Abstract

The dynamical theory of atmospheric tides and its relation to observations on the ground and in the upper atmosphere are reviewed briefly. The lunar tidal oscillations of atmospheric pressure observed on the ground are thought to be almost entirely due to gravitational forces, whereas the solar tidal oscillations are believed to be mainly caused by solar heating. Previous theories of resonant, gravitational excitation of the solar tidal oscillations are not consistent with models of the upper atmosphere obtained from rocket measurements. Tidal oscillations at ionospheric heights are inadequately known experimentally and are inadequately explained by present theories, which neglect viscous losses and the ionized nature of the upper atmosphere and whose linear approximations break down at ionospheric height. The dynamo theory attributes the quiet‐day variations of the geomagnetic field to tidal oscillations at ionospheric height. The theory has been used either to derive the magnetic variations from an assumed tidal wind system or to derive the tidal wind system from the observed magnetic variations. The results of both types of calculations lend broad support to the dynamo theory, although neither approach is free of arbitrary assumptions. The additional daily magnetic variations observed during magnetically disturbed days are particularly intense in the auroral zone. These variations are believed to be caused by some form of interaction of the solar wind with the geomagnetic field, but the nature of the interaction is not yet known with certainty. Various proposed theories of these daily disturbance variations are described, and one is discussed in detail. This theory predicts that the generating mechanism responsible for the daily disturbance variations is present, in reduced strength, even on magnetically quiet days. Observations tend to confirm this conclusion.