Ionospheric Electromagnetic Energy Input
Here we examine the electromagnetic energy input into the ionosphere by
assessing the Poynting vector associated with perturbations along the
satellite world-line, calculated using\(S=\ \frac{1}{u_{0}}E\ \times B\) where \(u_{0}\) is the magnetic
constant, and E and B denote the electric and magnetic
field vectors of the perturbation fields, respectively. By applying
band-pass filters it is possible to remove the influence of large-scale
variations of the Earth’s main field as measured along the trajectory of
the moving satellite, and to focus on the Poynting flux contributions
arising from perturbations at various scales of interest. With a single
satellite it is impossible to uniquely separate the impacts arising from
spatial and temporal variations along the satellite world-line. However,
as shown for example by [16][17], analysis of the wave impedance
as a function of frequency in the Swarm frame provides strong evidence
for the importance of Alfvén waves in MIC.
Figure 1 shows the statistical Poynting flux over two separate one
month-long time periods, one in the northern near-summer solstice
conditions (1-31 July, 2016; panels (a) and (b)) and the other in
the northern near-winter solstice conditions (15 November-15 December,
2016; panels (c) and (d)), for both the dayside (left column) and the
nightside (right column) as determined by magnetic local time (MLT).
These two intervals were chosen to reflect periods where the Swarm A
orbits were in similar noon-midnight local time orientations. In this
Figure, the error bars show the median spanned by the upper and lower
quartiles in the statistics, with the scale dependence of the Poynting
flux as a function of frequency derived by the application of a
time-domain Savitzky-Golay low-pass filter of varying width along the
x-axis (see Methods for details). It can be seen that on the dayside
during near-summer solstice, there is a clear statistical preference for
more electromagnetic energy to be driven into the northern hemisphere
than the southern hemisphere at Swarm altitudes. On the dayside during
near-winter solstice, the preferential direction of the energy transfer
does reverse such that there is more Poynting flux directed into the
southern hemisphere. However, and very significantly, the asymmetry in
the interhemispheric energy transfer is much smaller than in the
near-summer solstice period. As a result, there is a clear preference
for more energy transfer into the north. Indeed, if the results from
these two months approximating the near-summer and near-winter solstice
periods are summed, the implied summer-winter seasonally-averaged
Poynting flux will have a clear net northern preference.
On the nightside (panels (b) and (d)), the northern preference for
electromagnetic energy transfer is even more stark. During the
near-winter solstice on the nightside (panel (d)), there is a reduction
in the northern preference, but remarkably the direction of the
electromagnetic energy transfer does not reverse as compared to the
near-summer solstice such that the median direction of energy transfer
remains slightly northwards even in the near-winter solstice at night.
In all cases the error bars plotted in this Figure, and which refer to
the 25% and 75% quartiles in Poynting flux, appear be a significant
fraction of the median. However, we emphasise that this feature should
not be interpreted as a low statistical significance of our result
demonstrating northern preference for electromagnetic energy transfer
seen at Swarm. Instead, the large range of average Poynting flux
magnitudes represented by the quartiles simply reflects the expected
large variability in the magnitudes of energy flux from hour to hour and
day to day in response to non-steady solar wind forcing. Supplementary
Material Figures 1 and 4, for the northern near-winter and near-summer
solstice periods, respectively, show this effect clearly. It can be seen
that during more intense geomagnetic activity, the magnitude of the
Poynting flux increases in both hemispheres. This can be seen
particularly during conjugate observations from adjacent northern and
southern hemisphere passes, where the Poynting flux is seen to increase
and decrease in tandem in both hemispheres in response to varying levels
of driving. In particular for the near-summer solstice period it can be
seen that the northern Poynting flux dominates over the conjugate
southern hemisphere counterpart, both on the dayside and on the
nightside, in the time domain across the whole interval despite it
spanning a wide range of intensities of solar wind driving conditions.
Therefore the northern preference for electromagnetic energy input
persists in the time domain from event to event, and not just when
combined statistically as in Figure 1.
Figure 1 further shows that in general the electromagnetic Poynting flux
observed at Swarm appears to be smaller on the nightside than on the
dayside. This is evident both in the near-summer and near-winter
solstice periods and appears to be a general characteristic of the
magnitude of the observed electromagnetic energy input arising from
electromagnetic fluctuations at this altitude. A likely explanation for
this is that a significant fraction of the incoming electromagnetic
energy is converted to the kinetic energy of downgoing auroral electrons
as a result of coupling at higher altitudes above Swarm in the nightside
auroral acceleration region (AAR) located around 4000-12000 km in
altitude [18]. This inference is consistent with the concept whereby
the ionospheric feedback instability [19] can produce discrete arcs
which convert incoming electromagnetic energy into field-aligned
electron acceleration in the AAR. This feedback process happens
preferentially at night where the background conductivity is low, and
where in the absence of dayside EUV illumination strong conductivity
gradients can be formed [20]. In such a paradigm, the reduction in
nightside Poynting flux observed at Swarm, located below the AAR, is
explained as a result of significant energy removed in association with
discrete arc auroral electron acceleration above.
Interestingly, in the data shown in Figure 1, the interhemispheric
energy fraction appears to be independent of scale. This suggests that
the processes responsible for the observed asymmetry are most likely
self-similarly active at and/or self-similarly impact all transverse
scales considered.