Auroral current systems in the high-latitude ionosphere are concentrated to the auroral ovals that circle Earth’s magnetic poles. They consist of field-aligned currents (FAC) that connect Earth’s magnetosphere and ionosphere, their closure currents, and the eastward and westward electrojets at about 110 km altitude. We analyzed five years of magnetic field measurements from the European Space Agency’s Swarm satellites and concluded that while the auroral currents in the Northern and Southern hemispheres are very similar, they are not completely symmetrical: On average, currents in the Northern hemisphere are 10-15% stronger than their southern counterpart (Workayehu, et al., 2019).
In a further study (Workayehu, et al., 2020) we found that there is also large seasonal variation, with the hemispheric asymmetry being largest during local winters and statistically insignificant during local summers. The physical mechanism producing the asymmetry is currently unknown, but our model calculations indicate that differences in the background electrical conductivity, affected by sunlight and magnetic field strength, cannot explain the observations. Other possible reasons, to be investigated in the future, include asymmetries in the convection electric field or auroral conductance enhancements caused by energetic particle precipitation.
Figure: Analysis of Swarm satellite measurements of field-aligned currents (top row) and electroject currents (bottom row) during quiet conditions (planetary activity index Kp<2), when the asymmetry between North (left column) and South (right column) hemisphere is most prominent.
Workayehu, A. B., Vanhamäki, H., & Aikio, A. T. (2019). Field‐aligned and horizontal currents in the Northern and Southern Hemispheres from the Swarm satellite. Journal of Geophysical Research: Space Physics, 124, 7231- 7246. doi: 10.1029/2019JA026835
Workayehu, A. B., Vanhamäki, H., & Aikio, A. T. (2020). Seasonal effect on hemispheric asymmetry in ionospheric horizontal and field‐aligned currents. Journal of Geophysical Research: Space Physics, 125, e2020JA028051. doi: 10.1029/2020JA028051
Viimeksi päivitetty: 1.2.2021