Abstract
Abstract. Based on drift velocity measurements of the EDI instruments on Cluster during the years 2001–2006, we have constructed a database of high-latitude ionospheric convection velocities and associated solar wind and magnetospheric activity parameters. In an earlier paper (Haaland et al., 2007), we have described the method, consisting of an improved technique for calculating the propagation delay between the chosen solar wind monitor (ACE) and Earth's magnetosphere, filtering the data for periods of sufficiently stable IMF orientations, and mapping the EDI measurements from their high-altitude positions to ionospheric altitudes. The present paper extends this study, by looking at the spatial pattern of the variances of the convection velocities as a function of IMF orientation, and by performing sortings of the data according to the IMF magnitude in the GSM y-z plane, |ByzIMF|, the estimated reconnection electric field, Er,sw, the solar wind dynamic pressure, Pdyn, the season, and indices characterizing the ring current (Dst) and tail activity (ASYM-H). The variability of the high-latitude convection shows characteristic spatial patterns, which are mirror symmetric between the Northern and Southern Hemispheres with respect to the IMF By component. The latitude range of the highest variability zone varies with IMF Bz similar to the auroral oval extent. The magnitude of convection standard deviations is of the same order as, or even larger than, the convection magnitude itself. Positive correlations of polar cap activity are found with |ByzIMF| and with Er,sw, in particular. The strict linear increase for small magnitudes of Er,sw starts to deviate toward a flattened increase above about 2 mV/m. There is also a weak positive correlation with Pdyn. At very small values of Pdyn, a secondary maximum appears, which is even more pronounced for the correlation with solar wind proton density. Evidence for enhanced nightside convection during high nightside activity is presented.
Highlights
Large spatial and temporal variability is a fundamental property of magnetospheric convection
This is mainly caused by variations in the driving solar wind and interplanetary magnetic field (IMF) conditions together with the complexity of the coupled system
The major part of the transfer of energy and momentum from the solar wind to Earth’s magnetosphere and the basic high-latitude convection patterns are known to be caused by reconnection between the IMF and Earth’s geomagnetic field (Dungey, 1961) and to a minor extent by quasi-viscous interaction processes at the magnetopause (Axford and Hines, 1961)
Summary
Large spatial and temporal variability is a fundamental property of magnetospheric convection. Since the geomagnetic tions of the selected solar wind parameters over the time-span field lines are nearly equipo2te3ntials, and the large scale circulation map to high magnetic latitudes, this plasma circulation and its variability is manifested in the high latitude of our data set: clock angle θ, magnitude of the IMF in the GSM (y,z)-plane |ByIMz F| (sometimes called BT or “transverse IMF magnitude”), the solar wind speed |Vs.w.|, the magnitude ionosphere. To −35 nT under disturbed IMF conditions while the partial This manifests itself, for example, in different seasonal covring current activity index ASYM-H as well as the average erages at the Northern and Southern Hemisphere (see Fig. 5 clock angle distribution of the IMF vectors do not show any in Paper 1, middle and bottom panels). This difference is smaller within the very central part of the polar cap, where the mapped EDI drift data have best coverage
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