DENSITY AND MAGNETIC-FIELD FLUCTUATIONS OBSERVED BY ISEE-1-2 IN THE QUIET MAGNETOSHEATH

We analyse the fluctuations of the electron density and of the magneti c field in the Earth's magnetosheath to identify the waves observed be low the proton gyrofrequency. We consider two quiet magnetosheath cros sings i.e. 2 days characterized by small-amplitude waves, for which th e solar wind dynamic pressure was low. On 2 August 1978 the spacecraft were in the outer magnetosheath. We compare the properties of the obs erved narrow-band waves with those of the unstable linear wave modes c alculated for an homogeneous plasma with Maxwellian electron and bi-Ma xwellian (anisotropic) proton and alpha particle distributions. The Al fven ion cyclotron (AIC) mode appears to be dominant in the data, but there are also density fluctuations nearly in phase with the magnetic fluctuations parallel to the magnetic field. Such a phase relation can be explained neither by the presence of a proton or helium AIC mode n or by the presence of a fast mode in a bi-Maxwellian plasma. We invoke the presence of the helium cut-off mode which is marginally stable in a bi-Maxwellian plasma with alpha particles; the observed phase relat ion could be due to a hybrid mode (proton AIC + helium cut-off) genera ted by a non-Maxwellian or a non-gyrotropic part of the ion distributi on functions in the upstream magnetosheath. On 2 September 1981 the pr operties of the fluctuations observed in the middle of the magnetoshea th can be explained by pure AIC waves generated by protons which have reached a bi-Maxwellian equilibrium. For a given wave mode, the phase difference between B-parallel to and the density is sensitive to the s hape of the ion and electron distribution functions: it can be a diagn osis tool for natural and simulated plasmas.