Hydromagnetic stresses in accretion discs have been the subject of intense theoretical research over the past one and a half decades .
Most of the disc simulations have assumed a small initial magnetic field and studied the turbulence that arises from the magnetorotational instability .
However , gaseous discs in galactic nuclei and in some binary systems are likely to have significant initial magnetisation .
Motivated by this , we performed ideal magnetohydrodynamic simulations of strongly magnetised , vertically stratified discs in a Keplerian potential .
Our initial equilibrium configuration , which has an azimuthal magnetic field in equipartion with thermal pressure , is unstable to the Parker instability .
This leads to the expelling of magnetic field arcs , anchored in the midplane of the disc , to around five scale heights from the midplane .
Transition to turbulence happens primarily through magnetorotational instability in the resulting vertical fields , although magnetorotational shear instability in the unperturbed azimuthal field plays a significant role as well , especially in the midplane where buoyancy is weak .
High magnetic and hydrodynamical stresses arise , yielding an effective \alpha -value of around 0.1 in our highest resolution run .
Azimuthal magnetic field expelled by magnetic buoyancy from the disc is continuously replenished by the stretching of a radial field created as gas parcels slide in the linear gravity field along inclined magnetic field lines .
This dynamo process , where the bending of field lines by the Parker instability leads to re-creation of the azimuthal field , implies that highly magnetised discs are astrophysically viable and that they have high accretion rates .