The evolution of mirror mode fluctuations in the terrestrial magnetosheath

Mirror type fluctuations were identified in magnetic field data measured aboard the ISEE-1/2 spacecraft in different regions of the magnetosheath. Minimum variance analysis was applied to find the mirror type fluctuations, and the amplitude of the fluctuations was determined individually. Assuming that the source of the mirror mode instability. is at the bow shock, the growth rate of the field strength perturbations was determined for four selected magnetosheath passes when mirror type fluctuations were observed in the outer, middle, and inner region of the sheath. The obtained growth rate values are in the range 0.002 s(-1) < γ < 0.0035 s(-1) being almost an order of magnitude smaller than the maximum growth rate values calculated by Gary et al. (J. Geophys. Res. 98. (1993) 1481) based on a numerical evaluation of the full kinetic dispersion relation. The significant difference between the observed and calculated growth rate values can be explained if the mirror type fluctuations observed in different regions of the magnetosheath do not always originate from the bow shock, but the source may be somewhere else (e.g. at the magnetopause, inside the magnetosheath, or in localized regions of the bow shock). Also, the linear approximation applied in the above-mentioned calculations may be inappropriate for describing the growth of the observed fluctuations. (C) 2002 Elsevier Science Ltd. All rights reserved.