NATURE, PROPERTIES, AND ORIGIN OF LOW-FREQUENCY WAVES FROM AN OBLIQUESHOCK TO THE INNER MAGNETOSHEATH

We analyze the high time-resolution profiles of the electron density a nd of the magnetic field and the plasma parameters recorded by ISEE 1 and 2 during a crossing of the Earth's magnetosheath at 1430 LT. Compr essive and Alfven ion cyclotron modes (AIC modes) are identified by co mparing the measured magnetic polarization and electron parallel compr essibility with the results of calculations in an unstable kinetic lin ear model. A criterion to discuss the accuracy of the wave vector dire ction of mirror modes is established; an efficient method to disentang le mirror and AIC modes is presented and applied. From the bow shock t o the inner sheath we identify successively (1) compressive modes and AIC modes in the oblique shock, (2) a pure AIC mode region of circular ly and elliptically polarized waves in a layer 0.3 RE thick adjacent t o the undershoot, (3) a mixed region 2 RE thick where both mirror mode s and AIC modes are observed, (4) a pure mirror mode region. The natur e of the dominant mode appears to be controlled by the depth in the ma gnetosheath, more than by the local values of beta(p) and the proton t emperature anisotropy T-p perpendicular to/T-p//. In the outer sheath the unusual identification of a pure Alfvenic region for a large avera ge proton beta beta(p) = 13 and a moderate proton temperature anisotro py could be explained by a relatively low density of alpha particles. The mirror modes are three-dimensional structures with their major axi s along the magnetic field and with their minor axis nearly perpendicu lar to the magnetopause surface. We estimate the dimensions of ordered structures observed in the middle of the magnetosheath for a beta(p) around 7 +/- 1 and T-p perpendicular to/T-p// around 1.5; the minor ax is of regular mirror modes is typically between 1300 and 1900 km long: the intermediate dimension is larger than either 2200 or 2700 km, whi le the major axis is larger than either 2700 or 3400 km. For the first time the measured parallel compressibility of the pure mirror modes i s shown to be in relatively good agreement with the linear model predi ctions for 4 < beta(p) < 11. The absence of AIC modes in the inner she ath suggests that these modes cannot grow or propagate in regions wher e mirror modes are well developed and that AIC wave energy is not tran sferred across a large-amplitude mirror mode region.