ON THE ORIGIN OF RADIAL MAGNETIC-FIELDS IN YOUNG SUPERNOVA-REMNANTS

We study the radio emission from young supernova remnants by means of three-dimensional numerical MHD simulations of the Rayleigh-Taylor ins tability in the shell of the remnant. The computation is carried out i n spherical polar coordinates (r, theta, phi) by using a moving grid t echnique that allows us to resolve the shell finely. The three-dimensi onal result shows more turbulent (complex) structures in the mixing re gion than the two-dimensional result, and the instability is found to deform the reverse shock front. Stokes parameters (I, Q, and U) are co mputed to study the radio properties of the remnant. The total intensi ty map shows two distinctive regions (inner and outer shells). The inn er shell appears to be complex acid turbulent, exhibiting loop structu res and plumes as a result of the Rayleigh-Taylor instability, while t he outer shell is faint and laminar because of the shocked uniform amb ient magnetic fields. The inner shell resembles the observed radio str ucture in the main shell of young SNRs, which is evidence that the Ray leigh-Taylor instability is an ongoing process in young SNRs. When onl y the peculiar components of the magnetic fields generated by the inst ability are considered, the polarization B-vector in the inner radio s hell is preferentially radial with about 20%-50% of fractional polariz ation, which is higher than the observed value. The fractional polariz ation is lowest in the turbulent inner shell and increases outward, wh ich is attributed to the geometric effect. The polarized intensity is found to be correlated with the total intensity. We demonstrate that t he polarized intensity from the turbulent region can dominate over the polarized intensity from the shocked uniform fields if the amplified held is sufficiently strong. Therefore, we conclude that the Rayleigh- Taylor instability can explain the dominant radial magnetic held in th e main shell of young supernova remnants. However, the outer faint she ll shows a dominant tangential field orientation due to the shock-comp ression because this region is not mixed by the Rayleigh-Taylor instab ility, which is contrary to observations. Therefore, another mechanism is necessary to produce the radial components of the magnetic held at the outer shock, which we suggest to be a clumpy medium model.