Low-frequency magnetic field fluctuations in Earth's plasma environment observed by THEMIS

Abstract. Low-frequency magnetic wave activity in Earth's plasma environment was determined based on a statistical analysis of THEMIS magnetic field data. We observe that the spatial distribution of low-frequency magnetic field fluctuations reveals highest values in the magnetosheath, but the observations differ qualitatively from observations at Venus presented in a previous study since significant wave activity at Earth is also observed in the nightside magnetosheath. Outside the magnetosheath the low-frequency wave activity level is generally very low. By means of an analytical streamline model for the magnetosheath plasma flow, we are able to investigate the spatial and temporal evolution of wave intensity along particular streamlines in order to characterise possible wave generation mechanisms. We observe a decay of wave intensity along the streamlines, but contrary to the situation at Venus, we obtain good qualitative agreement with the theoretical concept of freely evolving/decaying turbulence. Differences between the dawn region and the dusk region can be observed only further away from the magnetopause. We conclude that wave generation mechanisms may be primarily attributed to processes at or in the vicinity of the bow shock. The difference with the observations of the Venusian magnetosheath we interpret to be the result of the different types of solar wind interaction processes since the Earth possesses a global magnetic field while Venus does not, and therefore the observed magnetic wave activities may be caused by diverse magnetic field controlled characteristics of wave generation processes.