Seasonal, Kp and IMF dependence of hemispheric asymmetry in ionospheric horizontal and field-aligned currents

We present statistical investigation of the seasonal, geomagnetic activity and interplanetary magnetic field (IMF) dependence of  hemispheric asymmetry in the auroral currents. Magnetic data from the Swarm satellites has been analyzed by applying the spherical elementary current system (SECS) method. Bootstrap resampling has been used to remove the difference in the number of samples as well as activity and IMF conditions between the local seasons and the hemispheres. In general, the currents are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Asymmetry is larger during low (Kp<2) than high Kp (Kp≥2) and during local winter and local autumn than local summer and local spring. Averaging over all Kp and IMF conditions, we find larger currents flowing in the NH than in the SH with the NH/SH ratio for FACs: 17­±5%, 14±5%,7±4% and 2±4% in winter, autumn spring and summer, respectively.  When making the statistical analysis for different IMF directions, we find that the orientation of IMF By has strong influence on the hemispheric asymmetry in the auroral currents, but this influence depends on local season. When IMF By is positive in NH (negative in SH), on average FACs as well as ionospheric horizontal currents are stronger in NH than inSH in most local seasons under both signs of IMF Bz. Conversely, when IMF By is negative in NH (positive in SH), the hemispheric differences of auroral currents during most local seasons are small except in local winter. Overall, comparing the hemispheres for opposite signs of IMF By, we find larger hemispheric asymmetry when IMF By is positive in  NH (negative  in SH) than vice versa.

The factors causing the observed hemispheric asymmetries in the auroral currents are not understood at the moment. Background conductances from the IRI model and cross polar cap potential values from SuperDARN dynamic modelsuggest that solar induced ionospheric conductances and convection electric field cannot explain all the observed features of the hemispheric asymmetry in auroral currents. The role of conductance enhancements due to auroral particle precipitation and possible asymmetries in the energy  flux of precipitating particles need to be investigated in future studies.