A modeling study of the longitudinal dependence of storm time midlatitude dayside total electron content enhancements

Large-scale storm time enhancements of total electron content (TEC) have often been observed at Millstone Hill and other locations in the United States and have been less frequently reported from other stations around the globe. This has raised the question of whether the formation of such enhancements may have a longitudinal dependence and whether the North American continent might occupy a favored longitude sector for the appearance of such effects. We examine two mechanisms for producing storm time dayside TEC enhancements. Heelis et al. (2009) showed that a high-latitude electric field that has expanded to midlatitudes can enhance the dayside TEC by as much as 300 units. We use such an electric field as a driver for the TDIM ionospheric model, studying its effect across a range of longitudes, and we find that there is indeed a longitudinal asymmetry that favors the enhancement of TEC in the American sector. Second, we examine the role of the thermospheric wind during storm conditions and find that it has potentially an equally large effect, with a longitudinal dependence of its own that may either enhance or counteract the effect of the expanded electric field. In both cases, the effect is of the order of a 10% to 20% change in TEC.