The Aurora, Birkeland Currents and Iridium Data

Iridium dB Data
The data from Iridium that are used to monitor the auroral regions comes from the Attitude Control System (ACS) onboard each Iridium satellite. The ACS has the job of keeping each satellite correctly oriented in space. A critical component of any satellite ACS is the magnetometer which senses the geomagnetic field in the along track, cross track and nadir pointing directions. Magnetic fields are generated by electric currents. The geomagnetic field is generated by electric currents deep within the Earth. In space, there are large electric currents that electrically couple the solar wind momentum and magnetosphere with the underlying ionosphere. At auroral latitudes, these are known as Birkeland currents which flow parallel with the geomagnetic field.

At low Earth orbit (~800 km altitudes), a reasonable approximation for the auroral regions is that the Birkeland currents flow radially. Therefore, the magnetic field due to these currents is in a plane parallel with the Earth surface. A sample of these data from Iridium are shown on the left. The red lines show the cross track component of the magnetic field just caused by the Birkeland currents, the geomagnetic field has been subtracted out. The black arrows show the reconstructed magnetic field (using a bit of mathematics).

Using the electromagnetic equation of Ampere and the magnetic data above, we can reconstruct the Birkeland currents. These are shown at the right. The Birkeland current pattern is shown over the northern hemisphere for 23 November, 1999. There are typically two current systems, known as region 1 and region 2 (space physicists are very creative). The region 1 currents are poleward of the region 2 currents.

All plots are shown in polar coordinates, viewed from above the north geomagnetic pole. Noon is toward the top, midnight at the bottom with dawn on the right and dusk on the left. Poleward latitudes map further out into space compared with the more equatorward latitudes. The region 1 current on the dawn side (right) is the blue coded ring located near 70 degrees and flows downward toward the Earth while the dawnside region 2 current is the red coded ring equatorward near 65 degrees and flows up out of the ionosphere. On the dusk side the region 1 current is red (radially outward) while the region 2 current is now coded blue as it flows in toward the Earth here.

Typical values for the peak currents are around 1 uA per square metre. Over the whole current system, this adds up to around 7 million Amps.

We can go one step further and estimate the net electromagnetic energy density or Poynting flux into the ionosphere for this large circuit. Taking the radar data to estimate the electric fields, the Poynting flux is shown on the left where the peak values (red) are up near 15 mW per square metre. This adds to be around 50 GW of power into the ionosphere. The large power dump in mid-afternoon is located between the region 1 and 2 current systems where the Birkeland currents flow horizontally across the ionosphere where heating accurs due to collisions with the neutral gas in the ionosphere mix. This power density figure only goes to 60 deg latitude.