Diurnal, seasonal, and geomagnetic variations of large field-aligned ion upflows in the high-latitude ionospheric F region

A statistical analysis is carried out by using both the European Incoherent Scatter mainland radar (called EISCAT in the following) and the EISCAT Svalbard radar (ESR) data to investigate the morphology of large field-aligned ion upflows in the high-latitude F region ionosphere, emphasizing its dependence on geomagnetic activity and latitude. By binning data with respect to geomagnetic conditions, significant differences are found between quiet and disturbed periods in the morphology of field-aligned upflows obtained by EISCAT around solar maximum: (1) the occurrence frequency on the nightside is more than 3 times higher during disturbed times than during quiet times; (2) the starting altitude of upflows lowers from 350-400 km under quiet conditions to 200-250 kin under disturbed conditions; (3) the magnetic local time (MLT) distribution of the occurrence frequency of upflows exhibits a dawn-dusk asymmetry above 400 kin altitude, and it favors the dusk sector under quiet conditions but favors the dawn sector under disturbed conditions; (4) the disturbed-time upflow occurrence frequency exhibits a semiannual variation above 400 kin, with higher values around equinoxes. ESR observations during the rising phase of the solar cycle reveal some preliminary features of upflows in comparison to EISCAT observations: (1) the occurrence frequency of upflows on the dayside becomes significantly noticeable at about 200 kin altitude, which is lower than that at EISCAT; (2) the upflow occurrence frequency above 400 kin exhibits an obvious MLT distribution, with higher values during 0400-1500 MLT, in contrast to 1800-0200 MLT at EISCAT; (3) the upflow occurrence frequency above ESR is higher than that above EISCAT during 0700-1500 MLT but is lower during 2000-0200 MLT. Possible clues on the driving mechanisms of upflows are drawn from these observations.