Magnetized accreting stars: Studies of the inner accretion-disk edge by a magnetohydrodynamic approach. II. Ideal instabilities of the inner disk edge

The interaction between an accretion disk and the magnetosphere of a rotati ng compact star is studied in two- and three-dimensional simulations to add ress the role of ideal (nonresistive) magnetohydrodynamic instabilities of the differentially rotating plasma. Two-dimensional simulations show the do minance of the gravitationally driven Rayleigh-Taylor instability over the shear-flow-driven Kelvin-Helmholtz instability. Three-dimensional simulatio ns starting from the full equilibria obtained by a numerical magnetofrictio nal method rather than a radial cut at z = 0 generally show smaller growth rates and larger mode wavelengths due to magnetic line-tying. Thus growth r ates of instability that cannot be calculated analytically because of the c omplicated magnetic field and plasma model can now be computed using numeri cal magnetohydrodynamic simulations. Variations of the accretion rate of a pulsar can be explained in terms of fast gravitational instability of the i nner accretion disk edge and slow reloading of disk plasma.