The investigators will study the probable causes of midlatitude spread-F events observed by a topside sounder aboard the Japanese ISS-B satellite. The investigation will have two main thrusts. First, the investigators will correlate the occurrence of spreading at midlatitude sites around the globe with geophysically significant planetary indices and with atmospheric weather-front data. This will provide an estimate of the percentage of topside F-region spreading events that are driven by tropospheric weather systems as opposed to magnetospheric, auroral, or IMF-related forcing. This global study will also quantify temporal, latitudinal, and seasonal variations in midlatitude spread F observed by the topside sounder. The second objective is to modify a ray-tracing high frequency (HF) propagation code and use it in conjunction with a published model that simulates F-region perturbations due to gravity waves to study the effects of synoptic gravity waves on topside ionograms. The plasma density distributions generated as the output of the model will be used as the input ionospheric specification for the ray-tracing code. The ray tracing code will then be run at discrete frequencies, and the calculated resonant echoes from the density structures will be binned by their propagation delay times and assimilated to produce simulated ionograms. This approach will allow the investigation of the effects of large-scale gravity-wave induced plasma density perturbations on topside ionograms, such as those from ISS-B. Together these two objectives will comprise a significant step forward in understanding of spread F at midlatitudes. The proposed effort is synergistic with a NASA funded sounding rocket activity to study midlatitude spread F that will occur in 2006. The rocket study will investigate the small-scale structures and potential instabilities involved in midlatitude spread F, and this study will investigate the occurrence probabilities of the phenomenon, and more generally the effects of gravity wave induced perturbations on topside ionograms.
