[1] The power spectra of remarkable events of long-period fluctuations of the geomagnetic field (as identified at a low-latitude station, L'Aquila, l % 36°) reveal a tendency for a repeated occurrence of power enhancements in the same frequency bands (f % 0.9-1.0, 1.2-1.4, 2.1-2.3, 2.5-2.7 mHz), often coincident with CMS frequencies. In four cases the data availability and the results of the spectral analysis allowed unambiguous conclusions on the correspondence between ground, magnetospheric, and solar wind fluctuations. For most wave packets, the comparative analysis reveals an unprecedented one-to-one correspondence in terms of frequency, onset, and duration; it suggests that geomagnetic fluctuations can be driven directly by fluctuations of the solar wind density at the same frequencies. On the other hand, the repeated occurrence in the same frequency bands and some evidence for amplification processes suggest additional contributions possibly related with cavity/waveguide resonances.Citation: Villante, U., P. Francia, M. Vellante, P. Di Giuseppe, A. Nubile, and M. Piersanti (2007), Long-period oscillations at discrete frequencies: A comparative analysis of ground, magnetospheric, and interplanetary observations,
Abstract. A statistical analysis of geomagnetic sudden impulses has been performed for data acquired at a low-latitude station during northward interplanetary magnetic field conditions. The local time dependence of the ground response, characterized by very low values in the morning with respect to the afternoon and night sectors and by a clear maximum just after local noon, is very similar to that observed at auroral latitudes, suggesting that signatures of auroral ionospheric currents might be observed also at low latitude. The sense of polarization of the sudden impulses shows a clear reversal from counterclockwise to clockwise a few hours after local noon, consistent with the results obtained for low-frequency geomagnetic fluctuations at the same station. In some cases a transient response (overshoot) is clearly observed; its amplitude is greater when the corrected Dst index is more negative, that is, when the ring current intensity is stronger; moreover, it tends to occur within 1-2 hours after the closest interplanet ary magnetic field northward turning.These results seem to indicate that at our station overshoots characterize sudden impulses occurring a short time after periods with high magnetospheric activity. Since the local field line is embedded in the plasmasphere, the overshoot might tentatively be interpreted in terms of an overcompression of the plasmasphere which, after periods with high magnetospheric activity, is less dense and more elastic.
Abstract. We examined geomagnetic field observations at low and middle latitudes in the Northern Hemisphere during a 50-min interval (12 May 1999), characterized by a complex behaviour of the solar wind dynamic pressure. For the entire interval, the aspects of the geomagnetic response can be organized into four groups of events which show common characteristics for the H and D components, respectively. The correspondence between the magnetospheric field and the ground components reveals different aspects of the geomagnetic response in different magnetic local time (MLT) sectors. For the H component, the correspondence is highly significant in the dusk and night sectors; in the dawn and prenoon sectors it shows a dramatic change across a separation line that extends approximately between (6 MLT, 35 • ) and (13 MLT, 60 • ). For the D component, the correspondence has significant values in the dawn and prenoon regions. We propose a new approach to the experimental data analysis which reveals that, at each station, the magnetospheric field has a close correspondence with the geomagnetic field projection along an axis (M1) that progressively rotates from north/south (night events) to east/west orientation (dawn events). When projected along M1, the geomagnetic signals can be interpreted in terms of a one-dimensional pattern that mostly reflects the field behaviour observed at geostationary orbit. Several features appear more evident in this perspective, and the global geomagnetic response to the SW pressure variations appears much clearer than in other representations. In particular, the MLT dependence of the geomagnetic response is much smaller than that one estimated by previous investigations. A clear latitudinal dependence emerges in the dusk sector. The occurrence of low frequency waves at ∼2.8 mHz can be interpreted in terms of global magnetospheric modes driven by the SW pulse. This event occurred in the recovery phase after the day the SW almost disappeared (11 May 1999): in this sense our results suggest a rapid recovery of almost typical magnetospheric conditionsCorrespondence to: U. Villante (umberto.villante@aquila.infn.it) soon after a huge expansion. Overshoot amplitudes, greater than in other cases, are consistent with a significant reduction of the ring current.
[1] An analysis of a sudden commencement (SC) manifestation reveals a significant contribution of the polar current system which influences the long-term behavior of the H component at low-latitude ground stations located in the subsolar sector. A transition between ''morning'' and ''afternoon'' characteristics of the geomagnetic response occurs at %1300 MLT. ''Morning'' events are characterized by a positive then negative variation (P/N) preceding the longer-term variation and show a gentle latitudinal decrease of the asymptotic response; conversely, ''afternoon'' events, which are characterized by a N/P structure, show a steep latitudinal gradient. PPI and PRI structures are observed on opposite sides of the %1300 MLT boundary. The PPI consists of three dominant fluctuations at well-defined frequencies and the small differences in the signal manifestation at nearby stations can be imputed to the occurrence of local resonance phenomena. Large-amplitude pulsations appear soon after the SC occurrence at subauroral latitudes.
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