Statistical Characterization of Storm Enhanced Density based on GPS TEC Observations

Storm enhanced density (SED) involves the redistribution of plasmas in the ionosphere and magnetosphere driven by disturbance electric fields. Associated with SED events are large scale gradients in the total electron density (TEC) over relatively short distances. These TEC gradients can have direct impact on navigation and communication users. For example, marine users have horizontal positioning requirements of 2-5 meters at a 95 percent confidence level for safety of navigation in inland waterways. TEC gradients observed during 2003 SED events resulted in positioning errors larger than 20 meters on differential GPS baselines as short as 200 km. Large TEC gradients associated with SED events (greater than 100 TEC units per degree) have been observed near many large airports in the Northeast and Northwest continental US. Better SED characterization has been needed to improve current ionospheric models and to further our understanding of this phenomenon. The Madrigal database at MIT Haystack Observatory now contains TEC data with an unprecedented combination of global spatial coverage and high temporal resolution. Data from more than 2000 receivers are being incorporated into the daily 2005 TEC maps. This database has allowed for long term statistical studies of the presence of SED. In this paper, we present statistics of SED plumes observed during multiple storms during the 2000-2005 time period. The location, size of gradients, and time evolution of multiple SED events has been statistically characterized using an automated gradient analysis tool. Examples of magnetically conjugate SED plumes over northern Europe and the American longitude sectors will be discussed. Inter-hemispheric comparisons of the TEC magnitude of the observed SED events at the base, as well as within the plume, will be presented. The results also include observations of magnetically conjugate sub-auroral polarization streams (SAPS) which accompany the SED events using DMSP ion drift measurements.