Storm-Time Subauroral Ionospheric Plasma Density Irregularities and the Substorm Current Wedge

We report on the development of plasma density irregularities in the subauroral ionosphere over the North American sector during the 17 March 2015 Saint Patrick's Day geomagnetic storm. Data from network of ground-based observation instruments including the National Oceanic and Atmospheric Administration Continuously Operating Reference Station global navigation satellite system receivers and Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers, as well as in situ measurements from the Defense Meteorological Satellite Program (DMSP) and the Van Allen Radiation Belt Storm Probes (RBSP) spacecrafts, were examined to characterize the spatial and temporal development of subauroral irregularities. The auroral electrojet (AE) index was used as primary indicator of substorm occurrences, aided by some cross-referencing with the SYM-H index and SYM-H time derivative. Special attention was given to substorms that happened at the beginning of this geomagnetic storm. Analysis of Global Positioning System (GPS) rate-of-total electron content index (ROTI) data along an east-west cut line near the US-Canada border indicates that subauroral ionospheric irregularities may start to form as early as a few minutes after a substantial increase in the AE index. Contemporaneous DMSP and RBSP observations confirmed the presence of subauroral polarization streams wave structures when the aforementioned enhancement in AE index and GPS ROTI occurred. An equatorward expansion of bright auroral arc and subsequent auroral breakups were also seen in the THEMIS all-sky imager observation data. The relatively short time scales for irregularities to develop suggest possible roles played by the penetration of magnetotail plasma flow bursts into the plasmasphere and the substorm current wedge development in their formation in the subauroral ionosphere.