Non-migrating tides in the ionosphere-thermosphere: In situ versus tropospheric sources

In this paper we demonstrate how magnetic control of ion-neutral interactions in the ionosphere-thermosphere (IT) system effectively produces source terms for non-migrating solar tides in the neutral momentum equations for the thermosphere. The National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) is utilized to quantify these tides, and to assess their importance relative to those that propagate upward from lower atmospheric regions. The primary diurnal tides excited in situ by the above mechanism include DE1, D0 and DW2, with zonal wind amplitudes on the order of 20 m s⁻¹ (5–10 m s⁻¹) at ∼500 km (∼350 km) under solar maximum (minimum) conditions. Smaller amplitude semidiurnal non-migrating tides, mainly SE1, S0, SW1, and SW3, are also generated under solar maximum conditions. The aggregate effect of these tidal components is to produce extrema ranging from −110 to +140 m s⁻¹ in a typical illustration of latitude versus longitude at a constant local time. The associated wind circulations include vertical wind perturbations that drive temperature perturbations through adiabatic heating and cooling effects. At high latitudes, hydromagnetic coupling effects generate non-migrating tidal components including DE1, D0, DW2, SE1, S0, and SW1, which show interhemispheric differences in both amplitude and latitudinal structure due to interhemispheric differences in the offset between the geographic and geomagnetic poles. Our computational results show that the in situ generated non-migrating tidal components dominate some parts of the tidal spectrum at high levels of solar activity and suggest that in situ generated non-migrating tides must be taken into account in order to reconcile differences in data-model comparisons.