Characteristics of the Ambipolar Electric Field during Ion Upflow Events

A significant amount of magnetospheric plasma originates from the ionosphere and has been transported to the magnetosphere through the ion outflow process. Cold ions in the magnetosphere predominantly originate from the polar cap ionosphere, where an ambipolar electric field, driven by pressure gradients, is one of the main factors initiating the upward flow of ions. In this study, the EISCAT Svalbard radar database from 1999-2021 was used to investigate the characteristics of the ambipolar electric field. The ambipolar electric field was calculated for each identified upflow event in the database, and the relationship between the ambipolar electric field and various factors such as solar activity, geomagnetic activity, diurnal and seasonal variations examined. Our results showed that 27% of the upflow events identified were accompanied by an enhanced ambipolar electric field when the threshold for an enhanced ambipolar electric field was set to 1.5 μV/m. No clear dependence on geomagnetic activity was found, however, a higher percentage of events are associated with enhanced ambipolar electric field during day and dusk than during night and dawn. With respect to solar activity, with the exception of daytime events, there are slightly higher percentages of events associated with an enhanced ambipolar electric field during low solar activity than during high. Seasonal difference were found to be small, with the exception of summer during high solar activity where less events were detected.