On the transmission of waves at discrete frequencies from the solar wind to the magnetosphere and ground: A case study

We analyze a case event in which several fluctuations at discrete frequencies (f ≈ 1.3–1.5, 3.3–3.6, 4.4–4.6, and 5.9–6.2 mHz; i.e., close to the “CMS” frequencies) were observed in the magnetosphere, after the impact of a sharp shock wave that, in the interplanetary medium, was followed by intense fluctuations in the solar wind parameters. The comparison between interplanetary, geosynchronous, and ground-based observations revealed that following the Sudden Impulse, magnetospheric modes at the same discrete frequencies were detected at geostationary orbit by spacecraft located in the morning and the dawn sector, and, ubiquitously, at ground-based stations: all of them revealed a one-to-one correspondence with those ultimately identified in the high-velocity stream following the shock wave. It reveals that the occurrence of such global modes is directly related to the transmission of external fluctuations and the observed geomagnetic fluctuations might be interpreted as the ground magnetic response to magnetospheric compressional modes forced by oscillations of the solar wind pressure at the same frequencies. By contrast, we did not find any evidence for magnetospheric oscillations possibly related to other mechanisms such as the velocity shear, the impact of the shock wave itself, etc.