This communication describes the study of mirror modes in a plasma with a small or large fraction of cold electrons within a gyrokinetic framework. It was found that if the number of cold electrons is me/mi times smaller than the number of hot electrons, the dispersion relation of the compressional perturbation represents a modification of the magnetohydrodynamics slow mode dispersion relation; if the plasma anisotropy is small, the perturbation is an oscillation, but if the anisotropy is sufficient, the square of the oscillation frequency becomes negative and one of the solutions represents the 'fluid' mirror instability. In the opposite case, when the number of cold electrons is me/mi times larger than the number of hot electrons, the perturbation is damping in the non-oscillatory regime if the anisotropy is small, and growing in the case of large anisotropy. This is the 'kinetic' mirror instability. For both kinetic instabilities, the threshold is the same, but the growth rates are different.
Toward a general quasi-linear approach for the instabilities of bi-Kappa plasmas. Whistler instability