Latitudinal variation of quasi-biennial oscillation modulation of the semidiurnal tide in the lower thermosphere

Perturbations in the geomagnetic field at the Earth's surface can result from variations in the neutral wind field in the lower thermosphere through dynamo action in the interfused ionospheric E region. Thus spectral analysis of the Sq variations in ground-based geomagnetic data extending continuously over many years, and from several sites worldwide, has been carried out to investigate the variation with latitude of quasi-biennial oscillation (QBO) effects in the semidiurnal tide in the lower thermosphere. This QBO modulation of the tide appears to be greatest around ∼60° latitude. The phase of the oscillation is largely latitude independent and implies QBO modulation of the upward propagating semidiurnal tide close to its source region. The semidiurnal tide appears to propagate upward more easily during the easterly phase of the tropical stratospheric QBO at 30 mbar than during the westerly phase