Net ionospheric currents closing field-aligned currents in the auroral region: CHAMP results

Abstract

By utilizing the high-resolution and precise vector magnetic field measurements from CHAMP during 2001–2005, the characteristics of the net auroral currents calculated by Ampère's integral law are comprehensively investigated. It is found that the net currents deduced from noon-midnight (dawn-dusk) orbits are directed duskward (antisunward). The intensities of the net currents increase linearly when the merging electric field (Em) is growing, exhibiting maximum values of about 2 (1) MA for the net duskward (antisunward) currents when Em exceeds 4 mV/m. For the first time the seasonal variations of the different net currents are shown. The net currents deduced from full orbits show only little seasonal dependence due to a compensation of the effects between the hemispheres. Conversely, the net currents deduced separately for the two hemispheres exhibit prominent seasonal dependences. For the net duskward currents the amplitudes and slopes of Em dependence are both larger by a factor of about 2 in summer than in winter. The related cross-polar cap Pedersen currents are higher in the sunlit hemisphere due to enhanced conductivity. The summer-time duskward currents are larger in the Northern Hemisphere than in the Southern Hemisphere by a factor of 1.5. Conversely, the net antisunward currents show an opposite seasonal dependence. The ratio of summer to winter intensity amounts to about 0.7. In this case the currents are stronger in the Southern Hemisphere.