Distinguishing Alfvén waves from quasi‐static field structures associated with the discrete aurora: Sounding rocket and HILAT satellite measurements

We present and analyze sounding rocket and HILAT satellite measurements of the low frequency (< 1 Hz) electric and magnetic fields δ E and δ B perpendicular to the Earth's magnetic field B 0 in the auroral oval. By examining the time‐domain field data it is often difficult to distinguish temporal fluctuations from static structures which are Doppler shifted to a non‐zero frequency in the spacecraft frame. However, we show that such a distinction can be made by constructing the impedance function Z(f) = μ 0 |δ E ( f )/δ B ( f )|. Using Z(f) we find agreement with the static field interpretation below about 0.1 Hz in the spacecraft frame, i.e. Z(f) = Σ p −1 where Σ p is the height‐integrated Pedersen conductivity of the ionosphere. Above 0.1 Hz we find Z(f) > Σ p −1 , which we argue to be due to the presence of Alfvén waves incident from the magnetosphere and reflecting from the lower ionosphere, forming a standing wave pattern. These waves may represent an electromagnetic coupling mechanism between the auroral acceleration region and the ionosphere.