Two current systems in the preliminary phase of sudden commencements in the magnetosphere

Abstract The preliminary impulse of the sudden commencement is simply explained by generation compressional wave due to compression dayside magnetopause and mode conversion from Alfvén in magnetosphere. However, this simple model cannot explain a time delay peak displacement longer duration higher latitudes pre-noon post-noon sectors polar region. Based on global magnetohydrodynamic simulation magnetosphere–ionosphere system reveals that peculiar behavior geomagnetic variation associated with temporal deformation ionospheric field-aligned current distribution into crescent shape; its lower-latitude edge extends toward anti-sunward direction, higher-latitude almost stays same longitude near noon. Numerical simulations revealed derived different behaviors two systems impulse. first consists connected lower latitude side ionosphere, cross-magnetopause current, magnetosheath (type L system). inertia generated acceleration front solar wind magnetosheath. Thus, longitudinal speed type ionosphere projected ionosphere. In contrast, at H system) upward/downward pre-noon/post-noon sector, respectively, dawn-to-dusk field-perpendicular along magnetopause. moves outer magnetosphere over time. converted convergence return heading sunward direction magnetosphere; driven magnetospheric plasma flow behind region accelerated wind. spreads concentrically subsolar point. Consequently, as system, it does not move much because latitudes. Therefore, high-latitude only slightly. Finally, we clarified between mainly occurs where starts increase Earth. A steep gradient like plasmapause always necessary for current. We also suggest possible structure standing may occur phase. Graphical