Relation between the asymmetric ring current effect and the anti-sunward auroral currents, as deduced from CHAMP observations

During magnetically active periods the storm-time disturbance signal on ground develops commonly an azimuthal asymmetry. Negative deflections of the magnetic horizontal (H) component are enhanced in the 18:00 local time sector and smallest in the morning sector. This is commonly attributed to the asymmetric ring current effect. In this study we are investigating the average characteristics of anti-sunward net currents that are not closing in the ionosphere. Their intensity is growing proportionally with the amount of solar wind input to the magnetosphere. There is almost twice as much current flowing in the winter hemisphere as on the summer side. This seasonal dependence is more pronounced on the dusk than on the dawn side. Event studies reveal that anti-sunward currents are closely related to the main phase of a magnetic storm. Since also the asymmetry of storm-time disturbances build up during the main phase, we suggest a relation between these two phenomena. From a statistical study of ground-based disturbance levels during magnetically active periods we obtain support for our suggestion. Observed storm-time disturbance amplitudes are clearly smaller in the summer hemisphere than in the winter part. This difference increases toward higher latitudes. We propose a new 3D current system responsible for the zonally asymmetric storm-time disturbance signal that does not involve the ring current. The high-latitude anti-sunward currents are connected at their noon and midnight ends to field-aligned currents that lead the currents to the outer magnetosphere. The net current branch on the morning side is closed along the dawn flank plasmapause, and the evening side currents along the dusk flank magnetopause. Regardless through which loop the current is flowing, near-Earth storm-time disturbance level will in both cases be reduced in the morning sector and enhanced in the evening.