Simultaneous EISCAT Svalbard radar and DMSP observations of ion upflow in the dayside polar ionosphere

Regions where dayside field‐aligned (FA) ion upflows occur are identified, and the relative occurrences and characteristics are compared. The study is based on ∼170 simultaneous events observed with the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and spacecraft from the DMSP. We found that ion upflows occur not only in the cusp and cleft (the low‐altitude portion of the low‐latitude boundary layer), which traditionally have been regarded as regions of ion upflow, but also in the region connected to the mantle. Ion upflows are less frequently seen in the boundary plasma sheet (BPS) and are very rarely seen in the central plasma sheet at high latitude on the dayside. Almost all of the events in which the average FA ion velocity is >100 m s −1 are associated with relatively high soft electron precipitation (differential energy flux of electrons at 100 eV > 10 7 eV cm −2 s −1 sr −1 eV −1 ), although soft electron precipitation with similarly high flux exists also in the BPS, where the ion velocities are mostly <100 m s −1 . These results indicate that soft particle precipitation is the predominant energy source driving ion upflow in the topside ionosphere, but it triggers ion upflow effectively not in the BPS, only in the other high‐latitude regions on the dayside.