On the transmission of compressional fluctuations from the solar wind to the magnetosphere: an analysis of critical aspects

An important aspect of the interaction between the solar wind (SW) and the magnetosphere concerns the relationship between the SW structures/fluctuations and the onset/transmission of the magnetospheric wave modes. Several critical aspects may influence the results of similar analysis: for example, the frequency of fluctuations that are expected to impinge the magnetosphere may be not the same when they are observed by spacecraft at different places in front of the magnetosphere and the choice of the analytical methods adopted for the spectral analysis might influence the frequency estimate (as well as the wave identification) both in the SW and in the magnetosphere (Di Matteo and Villante, 2017, 2018). Focusing attention on these aspects, we present an analysis of SW compressional fluctuations (f ≈ 1-5 mHz), following two interplanetary shocks, as observed by two spacecraft at different places and compared them with the magnetospheric fluctuations following the corresponding sudden impulses, observed by two satellites at geostationary orbit and at several ground-based stations. Our results confirm that the comparison of different methods of spectral analysis is crucial to obtain a definite estimate of the characteristics of fluctuations in each region. For a case study, in which SW fluctuations at the same frequencies were observed by both interplanetary spacecraft, we found that all fluctuations observed in the magnetosphere were related to SW compressional fluctuations approximately at the same frequencies, with no evidence for wave activity of internal origin, or directly driven by the shock impact.