Low-frequency waves in the Earth’s magnetosheath: Present status

Abstract. The terrestrial magnetosheath contains a rich variety of low-frequency ( [ proton gyrofrequency) fluctu-ations. Kinetic and fluid-like processes at the bow shock, within the magnetosheath plasma, and at the mag-netopause all provide sources of wave energy. The domi-nance of kinetic features such as temperature anisotropies, coupled with the high-b conditions, complicates the wave dispersion and variety of instabilities to the point where mode identification is difficult. We review here the ob-served fluctuations and attempts to identify the dominant modes, along with the identification tools. Alfvén/ion-cyclotron and mirror modes are generated by „ M /„E’1 temperature anisotropies and dominate when the plasma b is low or high, respectively. Slow modes may also be present within a transition layer close to the subsolar magnetopause, although they are expected to suffer strong damping. All mode identifications are based on linearized theory in a homogeneous plasma and there are clear indications, in both the data and in numerical simulations, that nonlinearity and/or inhomogeneity modify even the most basic aspects of some modes. Additionally, the deter-mination of the wave vector remains an outstanding ob-servational issue which, perhaps, the Cluster mission will overcome.