Storm-Time Mid-Latitude Dayside TEC Enhancements: Longitudinal Dependence

Large-scale storm-time electron density enhancements have often been observed at Millstone Hill and other locations in the US, and have been less frequently reported from other stations around the globe. This has raised the question as to whether the formation of such enhancements may have a longitudinal dependence, and whether the US might occupy a favored longitude sector for such effects. To answer this question, we work from the premise that the driver for creating the enhancements is an electric field penetrating to mid-latitudes. Previous modeling studies carried out at Utah State University have shown that such an electric field may cause the dayside TEC to be increased by as much as 300 units during a large storm. We incorporate an expanded Volland-type electric field model into the Utah State University Time Dependent Ionospheric Model [TDIM] to simulate the effects of geomagnetic storms at locations throughout the mid-latitude northern hemisphere. We perform simulations at longitudes around the globe, covering a 24 hr range of storm onset times at each longitude. We find that a longitudinal dependence does indeed exist. Any given longitude sector has its own optimal storm onset time, whereby the storm-time density enhancements are maximized. However, by no means are the maximum possible enhancements equal in different longitude sectors. In fact, there are ranges of longitude, particularly in the Asian sector, where a penetrating electric field has very little effectiveness in creating density enhancements; while in other regions, such as the US sector, the same electric field will produce greatly elevated levels of TEC.