ALFVEN WAVES IN THE AURORAL IONOSPHERE - A NUMERICAL-MODEL COMPARED WITH MEASUREMENTS

We solve a linear numerical model of Alfven waves reflecting from the high-latitude ionosphere, both to better understand the role of the ionosphere in the magnetosphere/ionosphere coupling process and to compare model results with in situ measurements. We use the model to compute the frequency-dependent amplitude and phase relations between the meridional electric and the zonal magnetic fields due to Alfven waves. These relations are compared with measurements taken by an auroral sounding rocket flown in the morningside oval and by the HILAT satellite traversing the oval at local noon. The sounding rocket's trajectory was mostly parallel to the auroral oval, and it measured enhanced fluctuating field energy in regions of electron precipitation. The rocket-measured phase data are in excellent agreement with the Alfven wave model, and the relation between the modeled and the measured amplitudes is fair, leading us to conclude that the rocket-measured fields are dominated by interfering Alfven waves, forming a standing wave pattern. The field amplitudes measured by HILAT are related by the height-integrated Pedersen conductivity SIGMA(p), indicating that the measured field fluctuations were due mainly to structured field-aligned current systems. A reason for the relative lack of Alfven wave energy in the HILAT measurements could be the fact that the satellite traveled mostly perpendicular to the oval and therefore quickly traversed narrow regions of electron precipitation and associated wave activity. Alternatively, the lower velocity and eastward flight path of the rocket lead to Doppler-shifted frequencies of L shell aligned current structures which are much smaller than HILAT would measure and which in fact may be below our range of interest (0.1-1 Hz), leaving Alfven waves as the dominant source of fluctuating field energy in the rocket frame of reference.