SPRG Seminars
November 27, 2012:
"Plasma Pressure Generated Auroral Current System"
S. B. Mende, SSL/UC Berkeley
In a fairly well accepted model of he magnetosphere anti-sunward directed azimuthal pressure gradients are expected to drive meridional currents perpendicular to the magnetic field in the dusk and dawn closed field line regions. The currents are closed through the auroral ionosphere consisting of a pair of upward and downward sheet field aligned currents (FAC) that are 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 are seen to 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 pair of up and down FAC currents. To investigate whether the magnetospheric pressure gradients are sufficient to generate the aurora 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 for several reasons. In one case it was shown that the region between the up and down going FAC maps to 15 RE and with such a high degree of field line stretching quiet time azimuthal pressure gradient of 0.01 nPa/Re is sufficient to generate the current needed to feed the observed auroral current loops. The pressure gradient generator is proportional to 1/B and to the area between “adjacent” field lines. Field line stretching therefore enhances the current generation the generator is most effective at high latitudes. The high impedance of this type of generator has also important implications on auroral conjugacy.