Long-period oscillations at discrete frequencies: A comparative analysis of ground, magnetospheric, and interplanetary observations

The power spectra of remarkable events of long‐period fluctuations of the geomagnetic field (as identified at a low‐latitude station, L'Aquila, λ ≈ 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.