Ground-based GPS observation of SED-associated irregularities over CONUS

\ It has been known that steep total electron content (TEC) gradients observed at the boundary between the storm-enhanced plasma density (SED) and the low TEC region at subauroral and midlatitude regions are associated with ionospheric irregularities that impact communication and navigation systems. However, the relationship between the SED-associated irregularities and TEC gradients is still not well understood, partly because of the difficulties of resolving small-scale TEC gradients from sparsely distributed TEC observations. In this study, we examine the relationship between the SED-associated irregularities and TEC gradients during the intense geomagnetic storms of 31 March 2001 and 30 October 2003. To explore this relationship, TEC maps over the continental United States (CONUS) were constructed from ground-based GPS TEC observations, using Kalman filter update formulae with a recently developed nonstationary wavelet-based covariance model that enables resolution of TEC structures on both large and finer scales. Our results show that intense TEC gradients and ion drifts are thought to be required conditions for the formation of irregularities on the northeast side of the SED. Additionally, our methodology identified the narrow east-west stretch of TEC enhancement within the midlatitude low TEC region on 30 October 2003, and this TEC enhancement is most likely to be caused by auroral precipitation.