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.