Seasonal and Scale-Dependence of Electromagnetic Energy Input
The time periods around the equinoxes also show similar behaviour in terms of the northern preference for electromagnetic energy transport at Swarm altitudes, with the time periods closer to the equinoxes exhibiting behaviour that falls between the dynamics seen near the solstices (see Figure 2). The median and quartile Poynting fluxes at small, medium and large spatial scales shown in Figure 2 continue to show the northern preference, and how this preference evolves with season from the peak interhemispheric asymmetry in the northern near-summer solstice, through the equinoxes, to the northern near-winter solstice. Significantly, the behaviour of the interhemispheric asymmetry in electromagnetic energy flux is self-similar at small, medium and large scales, suggesting that most likely the same physical processes are active in magnetosphere-ionosphere coupling across the entire range of spatial and temporal scales shown in Figures 1 and 2, and during all seasons. It is also possible that energy is transferred between scales within this system via a cascade [22]. It can be seen that the sum totals of Poynting fluxes (north hemisphere flux plus south hemisphere flux) remain relatively similar across the seasons on the dayside, suggesting a relatively constant average total energy input is then re-distributed differentially into the two hemispheres – but with a seasonally averaged northern preference which is especially strong near the summer solstice. On the nightside, the same conclusion applies – except that the northern preference is even stronger than on the dayside. Indeed, on the nightside the northern preference is so pronounced that the median Poynting flux at Swarm altitudes is always much larger in the north, and at all scales, independent of season – even near winter solstice.
Note that in our analysis we have taken care to exclude the possibility that the northern preference we report could occur as a result of sampling bias, for example as a result of the inclination of the Swarm orbit generating impacts from differentially sampling auroral zone crossings at different angles of attack with respect to the auroral oval. For example, the orbits of Swarm A for all four seasonal time periods were chosen to be confined in the noon-midnight meridian (see Supplementary Material Figure 5). Moreover, exploration of limited subsets of the two time periods in Figure 1, focusing on only MLT times one hour before or after 00 or 12 MLT (i.e., more strictly confined to the noon-midnight local time meridian) also produced similar results further confirming the characteristics reported in Figure 1 are real (see e.g., Supplementary Material Figure 6). Note that although the electromagnetic fields are derived in local coordinates, both perpendicular polarisations of E and B fields are used to derive the parallel component of the Poynting flux. As such the reported northern preference for electromagnetic energy transfer appears to be geophysical in origin, and not the result of sampling bias or orbital orientation effects.