The Identification of Waves at Discrete Frequencies at the Geostationary Orbit: The Role of the Data Analysis Techniques and the Comparison With Solar Wind Observations

Following a recent investigation in which we examined the performance of two methods (the Welch method and the multitaper windowing and F test) on the identification of fluctuations of the solar wind dynamic pressure in streams following interplanetary shocks, we extend our analysis to the magnetospheric field fluctuations detected at the geostationary orbit after the occurrence of the related sudden impulses. First, we show that a contamination of the experimental results, dramatic in statistical investigations, might come from the rotation of measurements in the field-aligned coordinates when the field direction is determined, point by point, by the running averages: this procedure creates spurious events at frequencies related to the length of the running average window. These aspects do not appear in a fixed coordinate system, as determined by evaluating the average field vector for the entire interval. In this system, we examined 124 magnetospheric structures following sudden impulses: the Welch method/multitaper windowing and F test agreement in the wave identification and frequency estimate was achieved for approximate to 50% of events, and some evidence for higher percentages of events occurs at f approximate to 1.5-1.7, f approximate to 2.2-2.4, approximate to 3.9-4.2, and, more explicitly, at f approximate to 4.2-4.7mHz. Oscillation modes with the same characteristics at different local times were rarely observed. An analysis of case events reveals that fluctuations of the solar wind pressure may drive magnetospheric fluctuations at the same frequencies; in addition, the manifestation of modes, not appearing in the solar wind, might be related to the magnetospheric compression or to other processes triggered by the shock arrival.