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