Plasma Pressure Generated Auroral Current System

Anti-sunward directed azimuthal pressure gradients drive meridional currents perpendicular to the magnetic field in the dusk and dawn closed field line regions. These currents are closed through the auroral ionosphere consisting of a pair of upward and downward sheet field aligned currents (FAC) cross connected in the ionosphere. The upward currents are poleward (equatorward) on the dusk (dawn) side representing the region of visible aurora. Some of these auroras, especially on the dusk side, are in the form of arcs and can be present for several hours and stretch across several hours of local time, implying a large scale steady state current generator on closed field lines. Magnetometers on low earth orbit polar orbiting satellites e.g. FAST are ideal to measure the strength of the up and down FAC currents. To investigate whether the magnetospheric pressure gradients are sufficient to generate these auroral currents we used the T96 field model to define the geometry and map the field lines to their equatorial regions where the pressure gradient current generation is most effective. It was shown that in some cases the region between the up and down going FAC maps to 15 RE and with such a degree of field line stretching quiet time azimuthal pressure gradients of 0.01 nPa/Re are sufficient to generate the current needed to feed the observed auroral current loops. The pressure gradient generator requires either a high degree of field line stretching or larger gradients. With the THEMIS satellites direct measurements of the in situ plasma at the equator and the pressure gradients can be made. The high impedance of such a current generator has also important implications on auroral conjugacy.; Pressure gradient current system driving the auroral current system. FAST and THEMIS satellites make key measurements of the currents and the magnetospheric plasma.