Generation of magnetic holes in fully kinetic simulations of collisionless turbulence.

The results of three-dimensional fully kinetic simulations of decaying turbulence with the amplitude of the fluctuating magnetic field comparable to that of the mean field are presented. Coherent structures in the form of localized depressions in the magnitude of the magnetic field are observed to form self-consistently in the simulations. These depressions bear considerable resemblance to the so-called magnetic holes frequently reported in spacecraft observations. The structures are pressure-balanced and tend to be aligned with the local magnetic field. In the smallest structures observed, the decrease in the magnetic field strength is compensated by an increase in the electron perpendicular pressure, such that the transverse size of these structures is comparable to the electron gyroradius inside the depression. It is suggested that the structures evolve self-consistently out of the depressions in the fluctuating magnetic field, rather than being the consequence of instability growth and saturation. This is confirmed by additional, small-scale simulations, including those with realistic mass ratio between protons and electrons.