  Tangled, primordial cosmic magnetic fields create small rotational velocity
perturbations on the last scattering surface (LSS) of the cosmic microwave
background radiation (CMBR). Such perturbations can contribute significantly to
the CMBR temperature and polarization anisotropies at large l > 1000 or so,
like the excess power detected by the CBI experiment. The magnetic contribution
can be distinguished from most conventional signals, as they lead to CMBR
polarization dominated by the odd parity, B-type signal. Experiments like DASI
and WMAP have detected evidence for CMBR polarization at small l. Many
experiments will also probe the large $l$ regime. We therefore calculate the
polarization signals due to primordial magnetic fields, for different spectra
and different cosmological parameters. A scale-invariant spectrum of tangled
fields which redshifts to a present value B_{0}=3\times 10^{-9} Gauss, produces
B-type polarization anisotropies of order 0.3 - 0.4 micro Kelvin between l of
1000-5000. Larger signals result if the spectral index of magnetic tangles is
steeper, n>-3. The peak of the signal shifts to larger l for a lambda-dominated
universe, or if the baryon density is larger. The signal will also have
non-Gaussian statistics. We also predict the much smaller E-type polarization,
and T-E cross correlations for these models.
