Simultaneous Occurrence of Magnetospheric Fluctuations at Different Discrete Frequencies ($f\approx $ 1 – 5 mHz): A Review

In the last 30 years, many papers reported the almost simultaneous occurrence of magnetospheric fluctuations at different frequencies and latitudes (basically, in the range ${f}$ f ≈ 1–5 mHz; ${T}$ T ≈ 200–1000 s) and the possible existence and stability of sets of favorite frequencies (in particular: ${f}$ f $_{1}\approx $ ≈ 1 1.3, ${f}$ f $_{2}\approx $ ≈ 2 1.9, ${f}$ f $_{3}\approx $ ≈ 3 2.6–2.7, and ${f}$ f $_{4}\approx $ ≈ 4 3.2–3.4 mHz) has been proposed, determining controversial results. In the present paper we review these investigations focusing particular attention on several critical aspects that may have influenced the results and the comparison of these analyses (particularly, the correspondence between magnetospheric and solar wind fluctuations; the role of the short and long term variations of the solar wind and magnetospheric characteristics; the effects of the great variety of analytical methods adopted for the evaluation of power spectra and for the identification of relevant events). The results of this global analysis do not support the existence of a stable and persistent absolute set of favorite frequencies for magnetospheric oscillations; nevertheless, in the range of frequency explored by most investigations ( ${f}$ f ≈ 1.5–4.0 mHz), they reveal a strong predominance of cases between ${f}$ f ≈ 1.5–2.5 mHz, with percentages maximizing in the bin centered at ${f}$ f = 2.0 mHz (a feature mostly due to events occurring at ${f}$ f ≈ 1.9 mHz) and rapidly decreasing with increasing frequency; small evidence for an additional peak emerges at ${f}$ f = 3.5 mHz; these aspects are much more explicit in the geomagnetic events than in the ionospheric and magnetospheric ones. Among other processes, the impact of the “mesoscale” solar wind density structures on the magnetosphere might be related with the onset of magnetospheric fluctuations at the observed frequencies.