High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements

In this study, we investigate statistical, systematic variations of thehigh-latitude convection cell structure during northward IMF.Using 1-min-averages of Cluster/EDI electron drift observations abovethe Northern and Southern polar cap areas for six and a half years(February 2001 till July 2007), and mapping the spatially distributedmeasurements to a common reference plane at ionospheric level in amagnetic latitude/MLT grid, we obtained regular drift patternsaccording to the various IMF conditions.We focus on the particular conditions during northward IMF, wherelobe cells at magnetic latitudes >80°with opposite (sunward) convection over the central polar cap area permanent feature in addition to the main convection cellsat lower latitudes. They are due to reconnection processes at themagnetopause boundary poleward of the cusp regions.Mapped EDI data have a particular good coverage within the central partof the polar cap, so that these patterns and their dependence on varioussolar wind conditions are well verified in a statistical sense.On average, 4-cell convection pattern are shown as regular structuresduring periods of nearly northward IMF with the tendency of asmall shift toward negative clock angles.The positions of these high-latitude convection foci are within 79°to 85° magnetic latitude and 09:00–15:00 MLT. The MLT positions areapproximately symmetric ±2 h about 11:30 MLT, i.e. slightlyoffset from midday toward prenoon hours,while the maximum (minimum) potential of the high-latitude cellsis at higher magnetic latitudes near their maximum potential differenceat ≈−10° to −15° clock angle for the North (South)Hemisphere.With increasing clock angle distances from ≈IMFB_z+,a gradual transition occurs from the 4-cell pattern via a 3-cellto the common 2-cell convection pattern, in the course of whichone of the medium-scale high-latitude dayside cells diminishes and disappearswhile the other intensifies and merges with the opposite main cell of thesame polarity to form the large "round-shaped" convection cell whenapproaching a well-known IMFB_y-dominated configuration.Opposite scenarios with interchanged roles of the respective cells occurfor the opposite turning of the clock angle and at the Southern Hemisphere.The high-latitude dayside cells become more pronounced with increasingmagnitude of the IMF vector