Investigation of the structure of traveling ionospheric disturbances on the basis of tomographic experiments

A tomographic reconstruction of the ionosphere at latitudes of 40 degrees - 60 degrees for the daytime conditions of December 1993, is demonstrated. Th e two-dimensional sections show the high-latitude structure of the ionosphe re with quasi-periodical variations in the electron density along a given l atitude with a characteristic scale of similar to 300 km. An analysis of th e spatial structure of the observed inhomogeneities indicates that these st ructures are related to the propagation of medium-scale traveling ionospher ic disturbances (TID) with periods of 15-20 min, It is assumed that the TID considered are caused by internal gravity waves (IGW) with amplitudes of t he velocity meridional components of similar to 100 m/s propagating in the thermosphere at altitudes of the ionospheric F2 layer An analysis of the st ructure of TID and corresponding IGW indicates that, under the given geophy sical conditions, the terminator may be a possible source of the disturbanc es observed.