On the origin of electric turbulence in the polar cap ionosphere

We compare scaling properties of electric fields measured by the low-altitude polar-orbiting Dynamics Explorer 2 satellite in the auroral zone and the polar cap under interplanetary magnetic field (IMF) southward conditions. The techniques of logscale diagrams (LDs) and probability density function (PDF) are applied to demonstrate the scale-free structure of electric fluctuations on scales from 0.5 km to 256 km in both regions. It is shown that while the amplitudes of electric field fluctuations are much smaller in the polar cap than in the auroral zone, the scaling characteristics of the fluctuations in the two domains are basically the same. To examine the possibility that electric turbulence in the polar cap may be driven directly by turbulent solar wind variations, we searched for the relationship between the RMS values of the electric fluctuations in the polar cap and solar wind variability and did not find a clear relationship. We also demonstrate that the Poynting flux associated with electric and magnetic fluctuations in the polar cap tends to subside from the flanks toward the center of the polar cap. These findings are more consistent with plasma shear flow on open field lines being the driver of turbulence in the polar cap ionosphere.