Geomagnetic field line resonances at low latitudes: Pulsation event study of 16 August 1993

The analysis of a pulsation event recorded at a gradient installation in central Italy (L = 1.55) and at a higher latitude station (Nagycenk, Hungary, L = 1.88) is presented. The event occurred on 16 August 1993 during the main phase of a geomagnetic storm. Field line resonance signatures have been identified at the gradient installation by using different spectral techniques, comprising standard methods, such as interstation amplitude ratio, phase difference, and single station H/D amplitude ratio, and more advanced amplitude-phase methods. The capacity of determining the local resonant frequency fr by the different techniques is compared; in particular, we find that the H/D amplitude ratio fails to provide in some cases an estimate of fr. A clear early morning decrease of fr both at L = 1.55 and at L = 1.88 has been found, confirming previous experimental and theoretical results. The estimated fr values at L = 1.55 are significantly higher than those usually reported at these latitudes, suggesting peculiar conditions of the ionosphere-plasmasphere system during this event. The local time variation of the toroidal mode ULF pulsation eigenfrequencies has been numerically calculated at both latitudes using the standard empirical neutral gas model MSIS–86 and the empirical neutral wind model HWM90. Further numerical simulation runs involved a particular disturbed neutral gas composition which is appropriate for stormy periods and an improved meridional wind estimated as best approximation to the observed hmF2 values. The calculated eigenfrequencies agree with the experimentally observed values at L = 1.88 but are smaller than the values observed in the early morning hours at L = 1.55. The effect of the E × B vertical drift is suggested to be the cause of the discrepancy. Possible signatures of the 2nd harmonic at L = 1.55, never reported so far at such low latitudes, have been found. We estimated a harmonic ratio f2/f1 ∼ 1.7–1.8 in agreement with theoretical models. A high-frequency resolution analysis at the gradient installation confirms previous suggestions of the existence of a discrete spectrum of field line resonances driven by some cavity/waveguide modes.