High-latitude ionospheric irregularities: differences between ground- and space-based GPS measurements during the 2015 St. Patrick's Day storm

We present an analysis of ionospheric irregularities at high latitudes during the 2015 St. Patrick's Day storm. Our study used measurements from similar to 2700 ground-based GPS stations and GPS receivers onboard five low earth orbit (LEO) satellites-Swarm A, B and C, GRACE and TerraSAR-X-that had close orbit altitudes of similar to 500 km, and the Swarm in situ plasma densities. An analysis of the rate of TEC index (ROTI) derived from LEO-GPS data, together with Swarm in situ plasma probe data, allowed us to examine the topside ionospheric irregularities and to compare them to the main ionospheric storm effects observed in ground-based GPS data. We observed strong ionospheric irregularities in the topside ionosphere during the storm's main phase that were associated with storm-enhanced density (SED) formation at mid-latitudes and further evolution of the SED plume to the polar tongue of ionization (TOI). Daily ROTI maps derived from ground-based and LEO-GPS measurements show the pattern of irregularities oriented in the local noon-midnight direction, which is a signature of SED/TOI development across the polar cap region. Analysis of the Swarm in situ plasma measurements revealed that, during the storm's main phase, all events with extremely enhanced plasma densities (> 10(6) el/cm(3)) in the polar cap were observed in the Southern Hemisphere. When Swarm satellites crossed these enhancements, degradation of GPS performance was observed, with a sudden decrease in the number of GPS satellites tracked. Our findings indicate that polar patches and TOI structures in the topside ionosphere were predominantly observed in the Southern Hemisphere, which had much higher plasma densities than the Northern Hemisphere, where SED/TOI structures have already been reported earlier. LEO-GPS data (ROTI and topside TEC) were consistent with these results.