Case studies of drivers of ionospheric upwellings

Electron precipitation is a phenomenon that occurs when highly energetic electrons are ‘‘rained’’ into the
ionosphere through electromagnetic waves energisation, and as a result, ambipolar electric field is set up to accelerate
particles such as O? to higher energy in order to cause an upwelling. Similarly, Joule heating, which points to neutral
atmosphere heating, can also cause the neutral gas to upwell and as a result, pull up ions along the field lines. Analysis of
the EISCAT Svalbard Radar (ESR) data in this work indicates periods when (i) ambipolar electric field is set up to drive ion
upflow, (ii) Joule heating is responsible to the upwelling ions, and (iii) when both drivers combined to cause the ener-
gisation of the ion for upflow. The peak upwelling during the March 15, 2007 ambipolar-electric-field-driven event
indicates enhanced ion flux of up to 1:0 ? 10 14 m?2 s?1 , covering up to the upper E-region. However, the September 11,
2007 event is Joule-heating-driven, and the heating is most effective in the F-region altitude, modifying the plasma
pressure gradient and resulting in field-aligned ion acceleration. When strong electric field drives ion population through
the neutral gas, friction heating occurs and an elevation of the plasma pressure gradient as a result. This is observed in the
September 29, 2007 event, where both ambipolar electric field and the Joule heating are co-drivers, making the upwellings
to cover long duration of up to 5 h. Data from Cooperative UK Twin Located Auroral Sounding System (CUTLASS)
indicates cusp signature for the dayside events.