Convection of polar cap patches observed at Qaanaaq, Greenland during the winter of 1989–1990

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During weak Bz or Bz < 0 conditions, antisunward convection dominates the central polar cap as shown by Digisonde drift measurements. During these conditions, polar cap F layer patches are observed routinely to drift antisunward within the overall plasma convection. A study is presented which compares patch motion derived from 630.0‐nm all‐sky intensified camera (ASIC) images taken at Qaanaaq, Greenland (87° CGL) with simultaneously obtained Digisonde drift measurements. During four periods of the winter 1989/1990 43 630.0‐nm patches were identified and followed in their motion across the ASIC field of view. The shape, velocity, and drift direction of the individual patches were determined. The patch motion was then compared with the Digisonde drift measurements. The drift direction is generally antisunward (±25°); both data sets are in excellent agreement, with a typical ±20° scatter around a common central value. The velocity magnitudes from both measurements show considerable variability, but both measurements generally cover the same velocity range. The variations and variability of the velocity magnitude are discussed in the context of simultaneous IMF measurements. Rapid changes in velocity were traced to changes in Bz. Deviations from antisunward of the azimuths of the optical patch drift and the Digisonde drift measurements were controlled by IMF By in agreement with published By control of convection.