  The linear threshold condition of the mirror mode including weak drift and
finite Larmor radius effects is analysed and shown that it can be given to
arbitrary precision in closed analytical form. Combined with observations of
mirror modes in the magnetosheath it is shown that only magnetic `dips' are
produced by the mirror instability. A semi-experimental (non-linear) threshold
condition is derived from the data. The occasionally observed so-called `peaks'
cannot be attributed to mirror modes. They result probably from nonlinear
evolution of the compressive fast mode.
