Seasonal and IMF effects on hemispheric asymmetry in ionospheric horizontal and field-aligned currents

Abstract:

We present statistical investigation of the seasonal and interplanetary magnetic field (IMF) effects on hemispheric asymmetry in the high-latitude ionospheric current systems. Five years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system (SECS) method. The ionospheric horizontal and field-aligned currents (FACs) for each auroral oval crossing are calculated. 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. The mean values of FACs as well as the horizontal curl-free (CF) and divergence-free (DF) currents in 1^o magnetic latitude by 1 h magnetic local time grid cells are calculated for each local season and Kp conditions.

Averaging over all activity and IMF conditions, we found larger currents flowing in the NH than in the SH with the NH/SH ratio for FACs: 1.17­±0.05, 1.14­±0.05, 1.07­±0.04 and 1.02 ­± 0.04 in winter, autumn spring and summer, respectively. Comparing different activity and IMF conditions, we find that the hemispheric asymmetry is larger during low than high Kp conditions during local winter and autumn than local spring and summer seasons and during positive IMF Bz than negative IMF Bz. Additionally, the IMF By has a strong influence on the hemispheric asymmetry, especially during northward IMF Bz. We also find that the evening sector (13-24 MLT) contributes more to the high NH/SH ratio than the morning (01-12 MLT) sector. The physical mechanism producing the hemispheric asymmetry are not presently understood, but we are investigating the role of solar-induced background conductivities and the convection electric field.