Seasonally-Averaged Interhemispheric Asymmetry in Poynting Flux in Low-Earth Orbit: Swarm Observations

Abstract:

We recently reported an unexpected seasonally-averaged northern preference for the Poynting flux entering the ionosphere in low-Earth orbit. Using data from the European Space Agency’s Swarm satellite mission, we demonstrated how this seasonally-averaged asymmetry might be explained by a differential reflection of Alfven waves from the northern and southern auroral ovals, likely as a result of the offset of the Earth's dipole from the center of the Earth. Interestingly, this seasonally-averaged asymmetry manifests both on the dayside and on the nightside. As different processes govern magnetosphere-ionosphere coupling dynamics on either side of the terminator, two separate paradigms are brought forward to explain the asymmetry. In both cases, the root cause is the greater offset of the south magnetic pole with respect to the Earth’s rotation axis in the ionosphere. On the nightside, asymmetry in the magnitude of discrete arcs related to ambient conductance and Alfven impedance, and the potential role of the ionospheric feedback instability, are examined in the context of the observed seasonally-averaged interhemispheric energy difference. Meanwhile on the dayside, an inter-hemispherically asymmetric mismatch between the Pedersen and Alfven impedances is investigated as a candidate mechanism to asymmetrically redirect overall Poynting flux from one hemisphere to the other from season to season. Together the dayside and nightside processes conspire to create a significantly energetically asymmetric magnetosphere-ionosphere system. The impacts of this seasonally-averaged asymmetry in energy input may have important implications for both the nature of global magnetosphere-ionosphere-thermosphere coupling and the wider upper atmosphere response to space weather.