  We report the spontaneous formation of bipolar magnetic structures in direct
numerical simulations of stratified forced turbulence with an outer coronal
envelope. The turbulence is forced with transverse random waves only in the
lower (turbulent) part of the domain. Our initial magnetic field is either
uniform in the entire domain or confined to the turbulent layer. After about
1--2 turbulent diffusion times, a bipolar magnetic region of vertical field
develops with two coherent circular structures that live during one turbulent
diffusion time, and then decay during 0.5 turbulent diffusion times. The
resulting magnetic field strengths inside the bipolar region are comparable to
the equipartition value with respect to the turbulent kinetic energy. The
bipolar magnetic region forms a loop-like structure in the upper coronal layer.
We associate the magnetic structure formation with the negative effective
magnetic pressure instability in the two-layer model.
