Solar Tidal Wind Structures and the E-Region Dynamo

Abstract

Recent theoretical models predict complicated tidal structures in the lower thermosphere which deviate from the classical Hough mode structures commonly used in E-region dynamo calculations. The present paper investigates the electrodynamic effects of such a theoretical wind field, and examines its consistency with measured magnetic effects on the ground. The model of tidal structure is constructed by synthesizing diurnal and semidiurnal contributions excited in-situ and propagating upwards from the mesosphere and below. The individual tidal structures, which are inseparable in their latitude and height dependence, are each determined by solving the linearized tidal equations for a spherical, rotating, viscous atmosphere with anisotropic ion drag. The amplitudes and phases of the individual tidal components are calibrated with incoherent scatter and satellite measurements. The dynamo computations are generally in good agreement in amplitude and phase with the diurnal and semidiurnal harmonics of the observed ground variations at minimum and maximum levels of solar activity. There are, however, real discrepancies on the order of 20% in amplitude and 1 to 2hr in phase which require explanation. In interpreting our theoretical simulations, we attempt to point out the structural features of the E-region tidal winds and conductivities which are most critical to establishing such a consistency between theory and experiment, and to evaluate the status of dynamo theory with particular regard to the structure and variability of the solar tidal winds.