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.