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.

Global characteristics of auroral Hall currents derived from the Swarm constellation: dependences on season and IMF orientation

“…There is no sign of saturation observed for high solar wind input. Similar results have been reported by Zhou and Lühr (2017) for cross-polar cap ionospheric net currents. From Fig.…”

“…7, we compare them with the results of the dedicated study on auroral net currents by Zhou and Lühr (2017). In their Fig.…”

“…However, in the case of conductivity gradients, part of the Hall current is diverted by FACs into space. In a recent study (Zhou and Lühr, 2017) quantified the amount of Hall current that is not closed in the ionosphere. Here we address whether we have recovered the full Hall current or only the source-free part of it.…”

“…Here we address whether we have recovered the full Hall current or only the source-free part of it. In order to validate Hall current estimations, we have tried to determine with our approach the net current in anti-sunward direction (which is not closed in the ionosphere) as was done by Zhou and Lühr (2017) but using a different approach. For calculating the expected anti-sunward current the derived current densities are summed up along each Swarm high-latitude arc on dawn-dusk (05:00-07:00 or 17:00-19:00 MLT) orbits.…”

“…When both electrojets reach the poleward boundary of the nightside auroral oval, a conductivity discontinuity drives the electrojet currents upward along the field lines. Today we know that such a current configuration occurs primarily in the winter hemisphere, where the polar region is in darkness (Zhou and Lühr, 2017).…”