Simultaneous geomagnetic pulsation observations at two latitudes: resonant mode characteristics

Simultaneous observations of daytime Pc 24 geomagnetic pulsations recorded at Niemegk (Germany, L about 2.3) and L'Aquila (Italy, L about 1.6) were compared for 21 days of 1985. Local spectral characteristics are confirmed to be mainly determined by the field line resonance mechanism. The frequency response of the local resonances was estimated by evaluating the spectral ratio between the north-south (H) and east-west (D) components. By using this simple technique, we estimated fundamental eigenfrequencies of 30-35 mHz and 85-95 mHz at Niemegk and L'Aquila, respectively, which agree well with theoretical computations. The fundamental resonance at Niemegk was found to be efficient most of the time and its structure showed a clear diurnal pattern which is in agreement with the expected diurnal variation of the magnetospheric-ionospheric conditions. The fundamental resonance at L'Aquila, as well as the higher frequency modes at Niemegk, were found to occur less frequently, and preferentially in correspondence with high interplanetary magnetic field intensities.