Disturbance dynamo electric fields in response to geomagnetic storms occurring at different universal times

Perturbed electric fields in the Earth's ionosphere during storm activities may result from the penetration electric fields from high latitudes and/or from the dynamo mechanism driven by the neutral disturbances, depending on the storm phases. In general, the identification of the penetration electric fields is easier than that of the dynamo electric fields. At times, the latter becomes unperceivable or difficult to identify. This is an interesting problem that motivates a model study to investigate the possible reasons. Model runs made from the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model will be presented. Theoretical studies of ionospheric responses to geomagnetic storms with model simulations indicate that the intensities of disturbance dynamo electric fields are highly dependent on various parameters, for example, solar activities, seasonal effects and universal times, etc. When geomagnetic activities commence at 01 ~ 07 UT in summer solstices with low solar fluxes, the disturbance dynamo electric fields become very small. Compared with the general daily variations, they seem to be unperceivable. This phenomenon can be explained by the model results, which show that the positive charge accumulation at low latitudes will be weakened when the equatorward neutral disturbances penetrate the opposite hemisphere in the storm time. For other cases, the magnitudes of the dynamo electric fields are relatively larger under the same geomagnetic activities.