Evolution of mirror structures in the magnetosheath of Saturn from the bow shock to the magnetopause

Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiter's and Saturn's magnetosheaths. In particular, in one crossing of Saturn's subsolar magnetosheath, mirror waves are present almost continuously from the bow shock to the magnetopause. Therefore in this crossing, taking advantage also of relatively steady interplanetary conditions, we can track the evolution of mirror structures from a quasi-perpendicular bow shock to a low-shear magnetopause. We find that these structures evolve from quasi-sinusoidal waves to nonperiodic structures, consisting of both magnetic field enhancements and wells, and, finally, to dips in the plasma depletion layer (PDL) close to the magnetopause. Both the amplitude and wavelength of the fluctuations tend to increase with increasing distance from the bow shock, except in the PDL, where they decrease toward the magnetopause. The waves are always compressional, and the direction of maximum variance forms an angle of similar to 30 degrees with B in the outer magnetosheath and a smaller angle in the inner magnetosheath. A comparison with the predictions of a nonlinear theory of the mirror instability shows some discrepancies, indicating that further theoretical studies are necessary.