| The Earth's ionosphere, a region of the upper atmosphere spanning altitudes from approximately 100 to 1000 km, contains a complex pattern of electron densities produced by solar emissions, atmospheric chemistry and dynamical processes. In this dissertation, a plasma disturbance effect is identified in long-term observations, characterized statistically, and analyzed using numerical modeling. Results drawn from the model are subjected to verification using a dedicated observational campaign.; The distinctive feature treated is a pattern of diurnal double maxima (DDM) in total electron content (TEC) observations. The observed DDM events have a clear relationship with geomagnetic disturbances known as substorms. A time-dependent ionospheric model is used to simulate observed DDM events over a latitudinal range of {dollar}pm{dollar}38{dollar}spcirc{dollar} (dip latitude), and in two longitude sectors (75{dollar}spcirc{dollar}W and 7{dollar}spcirc{dollar}E). Modeling results show that TEC DDM patterns can be created by a combined effect of ionospheric F region plasma vertical drifts and highly altitude-dependent chemical loss mechanisms. Modeling studies explore two possible substorm-related dynamical sources for these perturbations: magnetospheric electric field penetration and overshielding effects, or traveling disturbances in the neutral atmosphere. Local time, latitudinal, and longitudinal characteristics of these dynamical perturbations are investigated in order to define global-scale signatures of the ionosphere's response to substorms.; An observational campaign was formulated and conducted to verify model predictions. The techniques included (1) magnetometers in the auroral zone for indications of substorm activity, (2) incoherent scatter radars, from high to low latitudes near 75{dollar}spcirc{dollar}W longitude, to measure ionospheric electron densities, plasma drifts, and meridional neutral winds, and (3) all sky CCD cameras and a Fabry-Perot interferometer for 6300A airglow and neutral winds at a sub-auroral site. The results of the campaign give clear support for the DDM modeling results of TEC variations associated with plasma vertical drifts. The ionospheric observations made under substorm conditions also allowed for a more general discussion of substorm effects, such as perturbations of electric fields and neutral winds, dynamo actions, and interplays between them. |
