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 mag
netosheath 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.
We give a simple argument for the exclusive existence of mirror and electromaghetic ion cyclotron modes in anisotropic high-$beta$ plasmas. It is shown that, in addition to a large domain of coexistence of both modes, two domains exist in parameter s
pace $(A,beta_perp)$ where solely either mirror modes or electromagnetic ion cyclotron modes can be excited. In the overlap region the modes with the larger growth rate should win. However nonlinear effects may modify such a conclusion.
