  A type Ia supernova explosion starts in a white dwarf as a laminar
deflagration at the center of the star and soon several hydrodynamic
instabilities (in particular, the Rayleigh-Taylor (R-T) instability) begin to
act. In previous work (Ghezzi, de Gouveia Dal Pino, & Horvath 2001), we
addressed the propagation of an initially laminar thermonuclear flame in
presence of a magnetic field assumed to be dipolar. We were able to show that,
within the framework of a fractal model for the flame velocity, the front is
affected by the field through the quenching of the R-T instability growth in
the direction perpendicular to the field lines. As a consequence, an asymmetry
develops between the magnetic polar and the equatorial axis that gives a
prolate shape to the burning front. We have here computed numerically the total
integrated asymmetry as the flame front propagates outward through the
expanding shells of decreasing density of the magnetized white dwarf
progenitor, for several chemical compositions, and found that a total asymmetry
of about 50 % is produced between the polar and equatorial directions for
progenitors with a surface magnetic field B ~ 5 x 10^{7} G, and a composition
C12 = 0.2 and O16 = 0.8 (in this case, the R-T instability saturates at scales
\~ 20 times the width of the flame front). This asymmetry is in good agreement
with the inferred asymmetries from spectropolarimetric observations of very
young supernova remnants, which have recently revealed intrinsic linear
polarization interpreted as evidence of an asymmetric explosion in several
objects,such as SN1999by, SN1996X, and SN1997dt. Larger magnetic field
strengths will produce even larger asymmetries. We have also found that for
lighter progenitors the total asymmetry is larger.
