THE ORIGIN OF STRONG MAGNETIC-FIELDS IN YOUNG SUPERNOVA-REMNANTS

Young supernova remnants such as Tycho generally exhibit a bright circ ular clumpy shell in both radio and X-ray emission. For several young remnants, various arguments suggest that the magnetic held is larger t han can be explained by compression of a few microgauss ambient magnet ic field by the shock wave. Radio polarization studies reveal a net ra dial orientation of magnetic fields in the shell which cannot be expla ined by the simple compression either. We model Rayleigh-Taylor instab ility at the interface of the ejecta and the shocked ambient medium to explain these observations. We have performed multidimensional MHD si mulations of the instability in the shell of a Type I supernova remnan t for the first time utilizing a moving grid technique which allows us to follow the growth of the instability and its effect on the local m agnetic held in detail. We find that the evolution of the instability is very sensitive to the deceleration of the ejecta and the evolutiona ry stage of the remnant. As the reverse shock enters the inner uniform density region, Chevalier's self-similar stage ends and the thickness of radio shell increases and the instability weakens. Our simulation shows that Rayleigh-Taylor and Kelvin-Helmholtz instabilities amplify ambient magnetic fields locally by as much as a factor of 60 around de nse figures due to stretching, winding, and compression. Globally, the amount of magnetic held amplification is low and the magnetic energy density reaches only about 0.3% of the turbulent energy density at the end of simulation. Strong magnetic held lines draped around the finge rs produce the radial B vector polarization, whereas thermal bremsstra hlung from the dense fingers themselves produces the clumpy X-ray emis sion. As a result, the X-ray emission peaks inside of the radio emissi on. The surface brightness profile shows no detailed correspondence be tween radio and X-ray emission. The major part of radio and X-ray lumi nosity comes from the mixing region.