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

摘要

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