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.