High-resolution Poynting Flux Statistics from the Swarm Mission: How
Much is Being Underestimated at Larger Scales?
Underestimation of the transfer of energy between the magnetosphere and
ionosphere, the Poynting flux, is a persistent issue in space weather
studies and the high-latitude ionospheric models. Thought to be due to
the inability to resolve small-scale fluctuations of the ionospheric
electric field, this underestimation could lead to significant further
underestimations in parameters such as the thermospheric mass density
and consequential satellite drag. Utilising 16Hz ion velocity and
magnetic field measurements from the Swarm satellite mission, we examine
the observed Poynting flux due to electric field fluctuations on very
small spatial scales (~1km), and then artificially
smooth the data to increase the observed scale. We quantify the decrease
of integrated Poynting flux, poleward of 60/-60 degrees geomagnetic
latitude, with increasing spatial scale. The decrease can be
underestimated by as much as 15% by increasing scale from 1km to only
8.6km, or 16Hz to 2Hz equivalent, with upward Poynting flux decreasing
significantly faster. Our results thus point to a significant Alfvén
wave driven component of the Poynting flux on kilometre scales.
Additionally, we observe a northern hemisphere preference for increased
Poynting flux, of which we examine its dependence on scale size and
interplanetary magnetic field.