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.