Asymmetric Seasonal Auroral Zone Illumination
Here we advance a paradigm which can explain the observed persistent asymmetry and northern preference for incoming Poynting flux at Swarm altitudes based on the known offset of the magnetic dipole moment from the center of the Earth towards the northwest Pacific [23]. This offset generates different relative effective solar illumination of the auroral ovals in the northern and southern hemispheres arising from the rotation of the Earth. The offset can also introduce asymmetries in the magnetic fields in the auroral zones as well (cf. [10]). A model of the north and south auroral ovals at two particular instants, shown in blue and red, respectively, is shown in the left and right hand panels of Figure 3. In each panel, the two ovals are over-plotted in a two dimensional projection of a polar view of the Earth in the geocentric solar ecliptic (GSE) x-y plane, and where the x=0 line marks the terminator and with the nightside beyond the terminator shaded in grey. As the Earth rotates, the offset of the dipole axis from the rotation axis sweeps the auroral ovals, whose location is defined by the magnetic field, further into and out of the sunlight; the offset of the dipole from the Earths centre making the excursions into and away from the sunlight significantly more pronounced in the southern hemisphere than the north. This also changes with season. To illustrate this effect, the left panel (a) in Figure 3 shows an example from the northern summer solstice, while the right hand panel (b) shows the northern winter solstice, at the same UT.
The offset of the magnetic dipole from the Earths center means that in the south the magnetic pole at the Earth’s surface is further from the Earth’s rotation axis than the north magnetic pole [10][11]. As a result, the southern auroral oval experiences more diurnal variation in its motion both across the terminator into the nightside, and across the terminator into the dayside 12 hours later, than its northern counterpart as a result of the rotation of the Earth. This means that at certain times the southern oval spends fractionally more time in darkness than the northern oval, and at others fractionally more time in daylight. To illustrate the impacts of the Earth’s rotation on the extent of the ovals in the x-y plane through one Earth rotation, the dashed lines show the circles which mark out 65° MLAT while the solid circles show the 75° MLAT. This MLAT range may be taken to be roughly representative of an auroral oval. Meanwhile the bold line traces circles which show the locus of the geomagnetic poles (90° MLAT).
In Figure 3 it can be seen that the maximum area of the northern auroral oval which is in shadow during (northern) summer solstice (left panel) is approximately one quarter of the total oval area, whereas the maximum shadow of the southern oval during winter solstice is larger, reaching as much as approximately one third of its total area. According to the hypothesis described above, discrete auroral acceleration would occur preferentially in dark background ionospheric conductivity conditions. This would make the southern oval more susceptible to losing energy to nightside discrete auroral electron acceleration processes, consistent with the geometrical aspects of shown in Figure 3 and with the nightside reduction in electromagnetic energy transfer observed at Swarm altitudes shown in Figures 1 and 2.
Meanwhile on the dayside the southern oval also similarly experiences more variation in solar illumination than the north, potentially traversing further into and dwelling longer in sunlit dayside regions where there is expected to be a greater mismatch between the Pedersen and Alfvén impedances as a result of increased background Pedersen conductance due to dayside solar EUV illumination. This would be expected to lead to greater average Alfvén wave ionospheric reflection coefficients in the south, as per the ionospherically reflecting Alfvén wave paradigm [16][17][24][25]. In turn this could lead to a stronger reflection of Poynting flux from the southern ionosphere back towards the equator than in the north. This may lead to an overall redirection of a fixed equatorial energy source on the dayside away from the southern hemisphere and into the northern hemisphere, in line with the observations presented in Figure 2 (panels (a), (c), and (e)). These two dayside and nightside Alfvén wave processes may therefore generate a different MIC response as a result of different ambient dayside and nightside ionospheric conductivity conditions. Nonetheless, they could occur in tandem and could collectively be responsible for the observed northern Poynting flux preference both on the dayside and on the nightside. In both cases, the effect would be to create the observed northern preference for incoming Poynting flux when observed at Swarm altitudes.