DISTINGUISHING ALFVEN 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 δE and δB perpendicular to the Earth's magnetic field B0 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) = μ0|δE(f)/δB(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) = Σp−1 where Σp is the height‐integrated Pedersen conductivity of the ionosphere. Above 0.1 Hz we find Z(f) > Σp−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. Copyright 1990 by the American Geophysical Union.