Horizontal structure of the geomagnetic 2 day variation

Principal component analysis was applied to 30 year geomagnetic field data of six observatories in low latitudes to extract the longitudinal structure of the geomagnetic 2 day variation. It was revealed that characteristics of the 2 day variation agree well with those of the quasi 2 day wave observed in the middle atmosphere; it propagates westward with a zonal wave number 3, and the monthly mean amplitude of the variation is largest in January and July. In addition, an investigation into the latitudinal profile of the variation clarified that the amplitude of the 2 day variation is largest in the equatorial region, although it extends to the middle latitudes. A previous result of spherical harmonics analysis of geomagnetic field data was reanalyzed to identify the global horizontal pattern of overhead currents responsible for the geomagnetic 2 day variation. The results showed that the 2 day variation is attributed to westward propagating current vortices, the foci of which are located along the equator. All these features can be explained by a scenario in which there exists a 2 day wave in the ionosphere whose horizontal structure and temporal variation resemble those in the middle atmosphere, and the wind drives electric fields and currents in the ionosphere through dynamo action.