New Insights Shed on Field -Aligned Currents in Magnetosphere

Aurora driven by field-aligned currents.

Field -aligned electric currents are extremely important in both the solar and terrestrial magnetospheric contexts. New research by Lühr et al  (Geophysical Research Letters, 2015) reveals new aspects of field-aligned currents in the Earth's magnetosphere. Let us recall that field -aligned currents (FACs) are one component designated  (J _|| ) of "dynamo currents" in the magnetosphere with the other the cross-field (J _^) current . In respect of the latter, it is required that the dynamo action send currents to specific regions to provide a Lorentz force: (J ⊥ X  B).

In detailed auroral models it can be shown that the "dynamo currents" in such a process flow earthward on the morning side of the magnetic pole and spaceward on the evening side. The circuit can be visualized completed by connecting the two flows across the polar ionosphere, from the morning side, to the evening side.

In regard to FACs in the magnetosphere, they've been of intense interest to physicists studying space weather phenomena since first discovered in the early 1960s.  Until now, all that's been learned of FACs has been via lone satellites from which the extracted data has been subject to crippling assumptions, i.e. that the currents are static only and have a sheet geometry (current sheet)

To ascertain if those assumptions were warranted Lühr et al used data from the European Space agency's SWARM constellation mission to observe how FACs of varying sizes changed over time. They examined the measurements collected during the first three months after the satellites were launched in November, 2013 - when 3 spacecraft were relatively close together.

The authors found that small-scale currents, extending up to 10 km, are highly variable. Smaller scale FACs remain stable for period of only up to 10 seconds and their spatial structure is indeterminate. Meanwhile, large scale FACs extend up to 150 km and are stable for up to 60 seconds.  The geometry shows these currents are roughly four time longer - along their longitudinal axis - than they are wide, confirming a sheet-like structure.

The new study also showed that large FACs are likely both sheet-like as well as stable. This means that single -satellite estimates of larger FACs at auroral latitudes are relatively accurate. Of course, more research needs to follow to confirm the validity of these early results but there is much confidence in the space physicist community - especially using numerical models as well - that this can be done and expeditiously.