Implications for Magnetosphere-Ionosphere-Atmosphere Energy
Transfer in the Auroral Zone
The seasonally averaged northern preference for electromagnetic energy
input at Swarm altitudes seems to exist in both the dayside and
nightside auroral zones. On the nightside, it implies stronger average
discrete arc auroral precipitation in the south, as residual Poynting
flux there is lower and as discrete arcs are expected to be the primary
absorber of electromagnetic energy incident from above. This conclusion
is also consistent with some previous conjugate auroral observations at
specific instants in time [8] but which has not been examined
statistically in terms of electromagnetic energy input. Meanwhile on the
dayside, our results appear to complement the statistics of [26] who
used DMSP data to observe auroral electron energy flux as a function of
season and hemisphere (note however that [26] did not explicitly
look at north-south seasonal asymmetries as we do here).
For example, [26] reported that all types of aurora maximised on the
nightside in local winter. Meanwhile, they observed that the electron
fluxes were higher on the dayside in local summer. The former
observation is consistent with our hypothesis that the electromagnetic
energy we observe on the nightside is lower during local winter than
summer, which we propose occurs as a result of preferential discrete
auroral electron acceleration in the AAR above Swarm. The fact that we
also observe higher Poynting fluxes in the dayside summer hemisphere
suggests that both auroral acceleration and the residual Poynting flux
penetrating to Swarm altitudes under the AAR are both higher in local
summer than local winter on the dayside. Based on our results we hence
suggest that dayside discrete aurora might also be more pronounced
during the northern summer than southern summer, an assertion that could
be verified by a future study looking for this potential seasonal
interhemispheric asymmetry using DMSP particle data. Similar asymmetries
in seasonal interhemispheric nightside discrete auroral electron flux
could also be investigated using DMSP data in the same way.
On the dayside we also observe that the sum of northern and southern
Poynting flux appears to be almost constant across seasons. Certainly
there is a preference for more energy input into the summer hemisphere,
but the northern preference means that this is more pronounced in the
northern summer rather than the southern summer on the dayside. Overall,
this suggests that the total dayside electromagnetic energy which
reaches Swarm altitudes might be rather constant with season, but that
ionospheric conductivity effects and asymmetric Alfvén wave reflections
result in a redistribution of the incoming energy flux from one
hemisphere to the other. This favours the summer hemisphere, with a more
pronounced interhemispheric asymmetry occurring during the northern
summer.
As shown in Figures 1 and 2, these same dayside and nightside
interhemispheric seasonal asymmetries occur across a wide range of
spatial scales, and in general the same behaviour is maintained across
small (10-150 km), medium (150-250 km), and large (>250 km)
spatial scales. Previous work has shown that to some degree, similar
behaviour can be inferred in terms of field-aligned currents (FACs)
[12] which must logically be associated with these electromagnetic
perturbations as well – especially since they are related to Alfvén
wave dynamics. For example, previous observational studies have made the
link between Alfvénic disturbances and FACs, and have suggested that
both may be explained as part of the same physical framework which can
explain the characteristics of magnetosphere-ionosphere coupling at
these scales [17][25][27][28]. This study is the first
to demonstrate that, when seasonally averaged, the high-latitude
electromagnetic Poynting flux observed across a wide range of scales has
a definitive northern preference both on the dayside and on the
nightside. The incoming electromagnetic energy, and indeed likely the
incoming discrete auroral electrons, are expected to additionally play a
role in driving ionosphere-thermosphere-atmosphere (I-T-A) coupling
below, perhaps including effects from gravity waves and other
atmospheric phenomena, driven from above (see e.g. [29]). Therefore
the northern preference for electromagnetic energy input which we report
here could also be very important in relation to impacting the dynamics
of the global atmosphere, perhaps having implications for an
interhemispheric asymmetry in the long-term atmospheric climate
response.
Overall, we propose that, for electromagnetic energy input at Swarm
altitudes, northern preference can likely be explained by the relative
displacement of the north and south auroral ovals with respect to the
Earth’s rotation axis, causing effective interhemispheric differential
solar illumination of the two auroral ovals. This effect may also be
present on other magnetized planets or moons where the magnetic dipole
is offset from the planet’s center, asymmetries in MIC occurring as a
result of the impacts of differential ionospheric conductivity.