Properties of Plasma Waves Observed Upstream From Mars
J. S. Halekas, S. Ruhunusiri, О. Л. Вайсберг, Yuki Harada, J. R. Espley, D. L. Mitchell, C. Mazelle, Norberto Romanelli, G. A. DiBraccio, D. A. Brain
Abstract
Abstract We describe a new method to analyze the properties of plasma waves and apply it to observations made upstream from Mars by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The slow measurement cadence of most charged particle instrumentation has limited the application of analysis techniques based on correlations between particle and magnetic field measurements. We show that we can extend the frequency range of applicability for these techniques, for a subset of waves that remain nearly coherent over multiple wave periods, by subsampling velocity distribution function measurements and binning them by the wave phase. This technique enables the computation of correlations and transport ratios for plasma waves previously inaccessible to this technique at Mars. By computing the cross helicity, we find that most identified waves propagate upstream in the plasma frame. This supports the conclusions of previous studies but enables a clearer determination of the intrinsic wave mode and characteristics. The intrinsic properties of observed waves with frequencies close to the proton cyclotron frequency have little spatial variability but do have large temporal variations, likely due to seasonal changes in the hydrogen exosphere. In contrast, the predominant characteristics of waves at higher frequencies have less temporal variability but more spatial variability. We find several indications of the presence of multiple wave modes in the lower frequency wave observations, with unusual wave properties observed for propagation parallel to the magnetic field and for background magnetic fields nearly perpendicular to the solar wind flow.