Abstract:
We have used EISCAT Svalbard Radar data, obtained during the International Polar Year 2007
campaign, to study ionospheric upflow events with fluxes exceeding 1013 m 2 s
1
. In this study, we have
classified the upflow events into low, medium, and high flux upflows, and we report on the incidence and
seasonal distribution of these different classes. It is observed that high upflow fluxes are comparatively rare
and low flux upflow events are a frequent phenomenon. Analysis shows that occurrence peaks around local
noon at 31%, 16%, and 2% for low, medium, and high-flux upflow, respectively, during geomagnetically
disturbed periods. In agreement with previous studies on vertical and field-aligned flows, ion upflow is
observed to take place over a wide range of geomagnetic conditions, with downflow flux occurrence being
lower than upflow occurrence. In contrast to previous observations, however, the upflow occurrence is
greater around noon during highly disturbed geomagnetic conditions than for moderate geomagnetic
conditions. Analysis of the seasonal distribution reveals that, while high-flux upflow has its peak around local
noon in the summer, with its occurrence being driven predominantly by high geomagnetic disturbance,
the occurrence of low-flux upflow is broadly distributed across all seasons, geomagnetic activity conditions,
and times of day. The medium-flux upflow events, although distributed across all seasons, show an
occurrence peak strongly related to high Kp. Furthermore, during highly disturbed conditions, the low-flux
and medium-flux upflow events show a minimum occurrence during the winter, whereas minimum
occurrence for the high-flux upflow events occurs in autumn.
