  We present the first model of full-sky polarized synchrotron emission that is
derived from all WMAP and Planck LFI frequency maps. The basis of this analysis
is the set of end-to-end reprocessed Cosmoglobe Data Release 1 sky maps
presented in a companion paper, which have significantly lower instrumental
systematics than the legacy products from each experiment. We find that the
resulting polarized synchrotron amplitude map has an average noise rms of
$3.2\,\mathrm{\mu K}$ at 30 GHz and $2^{\circ}$ FWHM, which is 30% lower than
the recently released BeyondPlanck model that included only LFI+WMAP Ka-V data,
and 29% lower than the WMAP K-band map alone. The mean $B$-to-$E$ power
spectrum ratio is $0.40\pm0.02$, with amplitudes consistent with those measured
previously by Planck and QUIJOTE. Assuming a power law model for the
synchrotron spectral energy distribution, and using the $T$--$T$ plot method,
we find a full-sky inverse noise-variance weighted mean of
$\beta_{\mathrm{s}}=-3.07\pm0.07$ between Cosmoglobe DR1 K-band and 30 GHz, in
good agreement with previous estimates. In summary, the novel Cosmoglobe DR1
synchrotron model is both more sensitive and systematically cleaner than
similar previous models, and it has a more complete error description that is
defined by a set of Monte Carlo posterior samples. We believe that these
products are preferable over previous Planck and WMAP products for all
synchrotron-related scientific applications, including simulation, forecasting
and component separation.
