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

We present a statistical investigation of the seasonal effect on hemispheric asymmetry in the auroral currents during low (Kp $<$ 2) and high (Kp $\geq$ 2) geomagnetic activity. Five years of magnetic data from the Swarm satellites has been analysed by applying the spherical elementary current system (SECS) method. Bootstrap resampling has been used to remove the seasonal differences between the hemispheres in the dataset. In general, the currents are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Asymmetry is larger during low than high Kp, and during winter and autumn than summer and spring. The NH/SH ratio for FACs in winter, autumn, spring and summer are 1.17 $\pm$ 0.05, 1.14 $\pm$ 0.05, 1.07 $\pm$ 0.04 and 1.02 $\pm$ 0.04, respectively. The largest asymmetry is observed during low Kp winter, when the excess in the NH currents is 21$\pm$5\% in FAC, 14 $\pm$ 3\% in curl-free (CF), and 10$\pm$3\% in divergence-free (DF) current. We also find that evening sector (13-24 MLT) contributes more to the high NH/SH ratio than the morning (01-12 MLT) sector. The physical mechanisms producing the hemispheric asymmetry are not presently understood. We calculated the background ionospheric conductances during low Kp conditions from the IRI, NRLMSISE and CHAOS models. The results indicate that only a small part of the hemispheric asymmetry can be explained by variations in the solar induced conductances.