Properties of Ion Upflows in the Low-Altitude Polar Ionosphere During CIR- and CME- Driven Magnetic Storms Based on Long-Term EISCAT Observations

We have investigated the effects of Corotating Interaction Region- (CIR-) and Coronal Mass Ejection- (CME-) driven magnetic storms on the characteristics and mechanisms of ion upflow in the low-altitude ionosphere (250–350 km). Our analysis was based on observations from the European Incoherent Scatter (EISCAT) radars at Tromsø (ILAT = ∼66$°$N) and Svalbard (∼75$°$N) from 1996 to 2015. The ion upflows (defined by the averaged upward ion velocity of >10 m/s) in the low-altitude ionosphere were mainly detected in the MLT sectors of 03–09 (dawnside) and 21-03 (nightside) at Tromsø during both CIR- and CME-driven magnetic storms. The ion upflows at Svalbard were not enhanced on the nightside but were remarkable on the dawnside during CIR-driven storms and were also observed on the nightside during CME-driven large storms. On the duskside (15–21 MLT), the low-altitude ion upflows were only detected at Tromsø during CME-driven large storms. Dayside (09–15 MLT) ion upflows were not detected in the low-altitude ionosphere. To investigate the generation mechanisms of low-altitude ion upflows, we compared the convection electric field, ion temperature, and electron temperature between the pre-storm time and after storm onset. The results indicate that frictional heating (Type 1 ion upflow) is dominant at Tromsø during CME-driven large storms in all MLT sectors except the dayside. Conversely, particle precipitation (Type 2 ion upflow) is dominant at Tromsø on the nightside during all magnetic storms except CME-driven large storms. Both mechanisms contribute to ion upflow during CIR-driven small storms on the dawnside of Svalbard.