Abstract
We study the radio emission from young supernova remnants by means of three-dimensional numerical MHD simulations of the Rayleigh-Taylor instability in the shell of the remnant. The computation is carried out in spherical polar coordinates (r, θ, ϕ) by using a moving grid technique that allows us to resolve the shell finely. The three-dimensional result shows more turbulent (complex) structures in the mixing region than the two-dimensional result, and the instability is found to deform the reverse shock front. Stokes parameters (I, Q, and U) are computed to study the radio properties of the remnant. The total intensity map shows two distinctive regions (inner and outer shells). The inner shell appears to be complex and turbulent, exhibiting loop structures and plumes as a result of the Rayleigh-Taylor instability, while the outer shell is faint and laminar because of the shocked uniform ambient magnetic fields. The inner shell resembles the observed radio structure in the main shell of young SNRs, which is evidence that the Rayleigh-Taylor instability is an ongoing process in young SNRs. When only the peculiar components of the magnetic fields generated by the instability are considered, the polarization B-vector in the inner radio shell is preferentially radial with about 20%-50% of fractional polarization, which is higher than the observed value. The fractional polarization is lowest in the turbulent inner shell and increases outward, which is attributed to the geometric effect. The polarized intensity is found to be correlated with the total intensity. We demonstrate that the polarized intensity from the turbulent region can dominate over the polarized intensity from the shocked uniform fields if the amplified field is sufficiently strong. Therefore, we conclude that the Rayleigh-Taylor instability can explain the dominant radial magnetic field in the main shell of young supernova remnants. However, the outer faint shell shows a dominant tangential field orientation due to the shock-compression because this region is not mixed by the Rayleigh-Taylor instability, which is contrary to observations. Therefore, another mechanism is necessary to produce the radial components of the magnetic field at the outer shock, which we suggest to be a clumpy medium model.