The Occurrence of Waves at Discrete Frequencies at Geostationary Orbit: the Role of the Data Analysis Techniques and the Comparison with Solar Wind Observations.

Controversial results questioned the existence and stability of sets of favored frequencies in the ULF waves (f 1-5 mHz) observed in the magnetosphere. In a recent analysis [Di Matteo and Villante, 2017], we remarked the critical aspects of the identification of waves and related frequencies in the solar wind (SW) streams following interplanetary shocks, due to the signal characteristics and to the different performances of the spectral analysis methods on complex signals (Welch Method and the Multi-Taper Method). Here, we focus attention on the manifestation of waves in the magnetic field components in structures following Sudden Impulses (SI) at geostationary orbit. We show that the rotation of measurements in the mean field aligned (MFA) coordinates may determine the identification of spurious favored frequencies, with dramatic implications in the statistical analysis. These features do not occur in a fixed MFA system based on the average of the magnetic vector over the entire analyzed interval ( 1 hr, in the present case). In this system, we discuss the possible occurrence of favored frequencies in the magnetic field components, paying attention to their manifestation in different LT sectors and to their relations with the fluctuations in the SW dynamic pressure. Some case events are discussed in details to investigate the possible direct transmission of waves from the SW to the magnetosphere.