The mid-latitude ionosphere can play host to several space weather effects such as scintillation and large-scale electron density gradients due to the redistribution of plasma from low to higher latitudes by way of electric fields, neutral wind transport, or a combination thereof. One of the largest sources of mid-latitude space weather effects is storm enhanced density (SED). SED is thought to be the result of sunward streaming plasma along the equatorward edge of the sub-auroral polarization electric field known as the sub-auroral polarization stream (SAPS). SED events are observed in global maps of GPS total electron content (TEC) as spatially narrow plumes of enhanced TEC spanning low, mid, and high latitudes. These plumes are associated with significant gradients in TEC (> 50 TECU/ degree), which have the ability to initiate small-scale irregularities such as those typically associated with scintillation. These gradients have been known to limit the availability of the Federal Aviation Administration's (FAA) Wide Area Augmentation System (WAAS) used for commercial air travel. Understanding the basic characteristics of SED and associated gradients, their ionospheric signatures, and effect on GPS signal strength is important for nowcasting/ forecasting of these features and assessing their impact on commercial communications and navigation systems.

This paper examines the impact of SED and its associated electron density gradients on fundamental ionospheric parameters and GPS satellite signal strength. We present examples of SED events observed in global maps of GPS TEC during active and highly disturbed levels of geomagnetic activity. An example of an ambient mid-latitude ionosphere free of large-scale electron density structures during magnetically quiet conditions is given as a baseline. A comparison between SED occurrences and coincident ionosonde measurements of peak electron density (NmF2), the height of the electron density peak (hmF2), and the critical frequency (foF2) is presented. The results are examined for persistent patterns, which may provide insight into the physical contributors to plasma transport into the SED source region. Electron density profiles versus time, constructed from ionosonde measurements, are used to examine the vertical structure of SED and associated density gradients. A power spectrum analysis of received GPS satellite signals whose paths intersect an SED event is presented. The results are compared with the vertical profiles and GPS TEC measurements.