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

Abstract

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.

Interplanetary magnetic field control of polar patch velocity

“…This is consistent with earlier reports on patch drifting direction [Heelis, 1984;Heppner and Maynard, 1987;Greenwald et al, 1990;Fukui et al, 1994]. Based on continuous observations over an extended period in the central polar cap, our statistical results also confirmed the results on the IMF B y control of the patch drift direction.…”

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets

“…Unless an additional mechanism is invoked, a single B z¯i p will not reproduce the most salient feature of polar cap patches which is the Horizonal Velocity repetitiveness of the patches when they transit the polar cap. During IMF B z southward conditions, we typically observe, inside the polar cap, a train of several patches and rarely a single patch (Weber et al, 1991;Fukui et al, 1994). In a dierent approach, Rodger et al (1994), and Valladares et al (1994) suggested that¯ow channel events or large plasma jets can produce enhanced losses creating a region of reduced number densities across the TOI.…”

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets

“…The density enhancements are usually a factor of 2 during solar minimum conditions, but can reach a factor of 10 during solar maximum years (Buchau and Reinisch, 1991). The patches move in the antisunward direction (Fukui et al, 1994), lack polar cap particle precipitation (excluding polar rain) (Weber et al, 1986), have sizes between 100 and 1000 km, and possess a patchy signature in the 630.0 nm airglow emissions.…”