High-latitude plasma convection from Cluster EDI:: variances and solar wind correlations

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, |B-yz(MFI)|, the estimated reconnection electric field, E-r,E-sw, the solar wind dynamic pressure, P-dyn, the season, and indices characterizing the ring current (D-st) 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 B,, component. The latitude range of the highest variability zone varies with IMF B-Z 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 |B-yz(IMF)| and with E-r,E-sw, in particular. The strict linear increase for small magnitudes of E-r,E-sw starts to deviate toward a flattened increase above about 2 mV/m. There is also a weak positive correlation with P-dyn. At very small values of P-dyn, 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.