NATURE AND ORIGIN OF WAVE MODES IN THE DAYSIDE EARTH MAGNETOSHEATH

The day side magnetosheath displays turbulence at frequencies lower than the local proton frequency. The nature of the waves modes is studied from ISEE-1-2 electron density and magnetic field data. We compare the polarization and the compressibility of the observed fluctuations to the polarization and the compressibility of the linear wave modes in a Maxwellian kinetic plasma model with an anisotropic proton temperature. Downstream of the undershoot of a supercritical quasi-perpendicular shock we successively observe Alfven Ion Cyclotron (AIC) modes, mirror modes and then AIC modes. From the crossing of an oblique shock to the inner sheath we observe AIC modes propagating toward the magnetopause. Along the Sun-Earth line, from the inner sheath to the magnetopause, AIC modes are observed during one crossing while mirror modes are seen during another crossing. In the sheath depletion layer we observe right-hand polarized AIC modes. At 0600 LT and at equal distance from the magnetopause and from the shock both types of modes are observed, the mirror mode appearing when the magnetic field is draping the magnetopause. From these analysis we infer the source of the propagating AIC modes to be the magnetosheath frontier regions, while the mirror modes grow in situ in the inner magnetosheath when specific conditions are reached.