The Identification of Waves at Discrete Frequencies in the Solar Wind and the Role of the Spectral Analysis Techniques.

Waves at discrete frequencies in the solar wind parameters (mostly, density and pressure) have been identified in several investigations with different techniques. We perform a statistical analysis of the pressure fluctuations imbedded in solar wind streams following interplanetary shocks. We preliminary processed a series of synthetic signals reproducing typical solar wind conditions, using two techniques adopted in previous investigations, the Welch method and the Fisher test (F-test) based on multitapering. Our results show that, depending on the signal characteristics, the two methods may provide different results; this aspect is also confirmed by the analysis of the solar wind data. However, with both methods, we identified fluctuations at discrete frequencies in the 1-5 mHz range, with some evidence for a preferential occurrence in bands centered at 1.9, 3.4 and 4.2 mHz.