DRAFT: A Method for Wave Analyses in Space Plasmas

Abstract

In this study, we apply a method for wave analyses in space plasmas, to better understand the nature of plasma waves and turbulence. Such a method is based on four-spacecraft measurements of magnetic fields or electric fields; it calculates the phase velocity of each single-frequency wave by using the timing analysis, then divides the wave frequency by the phase velocity to obtain the wavevector, and finally solves the dispersion relation (ω–k relation) by considering all frequency channels, and thus is termed Dispersion RelAtion From Timing (DRAFT). Quantitatively, we define three parameters, match of amplitude (MOA), ratio of half-wavelength to spacecraft separation (λ/2R sc), and correlation coefficient (CC), to judge the reliability of this method. We illustrate the usage of this method and validate the efficiency of the three parameters by analyzing the Magnetospheric Multiscale data. We find that the DRAFT results are generally accurate under conditions MOA > 0.5, λ/2R sc > 1, CC > 0.8. As a support, we compare our method with the previous methods such as Means and singular value decomposition (SVD). We find that if the three criteria (MOA, λ/2R sc, CC) are satisfied, the DRAFT, Means, and SVD are consistent in resolving the wave normal angle. Such a comparison robustly validates the reliability and rationality of the DRAFT method. Using DRAFT, we successfully solve the dispersion relation of an Alfvén wave in Earth’s magnetosheath and obtain its three-dimensional K -spectrum. Our method should be useful to study the low-frequency waves and weak turbulence in space plasmas.