Hemispheric asymmetry in field-aligned and ionospheric horizontal currents from the Swarm satellite measurements

We present statistical investigation of the high-latitude ionospheric current systems in the Northern hemisphere (NH) and Southern hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly four 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. 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 hemisphere and activity level. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling.

Averaging over all seasons, we found that for the low activity level the currents in the NH are stronger than in the SH by 12 ± 4 % for FAC, 9 ± 2% for the horizontal CF current and 8 ± 2% for the horizontal DF current. During the high activity level, the hemispheric differences are not statistically significant.

When making the statistical analysis for the four seasons separately, we find a seasonal dependence in the hemispheric asymmetry. During low Kp conditions, both FACs and horizontal currents are larger in the NH than SH with the largest difference observed in winter. In winter, the currents in the NH are larger than the SH by 21 ± 5 %  for FAC, 14 ± 3% for the horizontal CF current and 10±3%  for the horizontal DF current. During the high activity level, the asymmetry is smaller compared to the low activity level with the largest and smallest hemispheric differences observed in autumn and summer, respectively. In autumn, the currents in the NH are larger than the SH by 8 ± 5%  for FAC, 9 ± 2%  for the horizontal CF current and 8 ± 3%  for the horizontal DF current. Interestingly, during high Kp conditions, the NH/SH ratio of horizontal current is >1 in autumn and <1 in spring.

The physical mechanism producing the hemispheric asymmetry is not known. One hypothesis is that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation may play a role. We present preliminary results indicating that only a small part of the seasonal dependence in the NH/SH total current ratios can be explained by variations in the background conductances caused by solar irradiance and affected by local hemispheric values of the magnetic field.