Solution (source code)

= Solution

The stated one-sided temperature contrast is $\beta=0.00105$, so the <Doppler dipole> gives
$$
\boxed{v=0.00105c\simeq3.15\times10^5\,\mathrm{m\,s^{-1}}=315\,\mathrm{km\,s^{-1}}.}
$$
The motion is \b[toward the hotter region]. The contrast already gives one side relative to the mean; the hot-to-cold difference would be twice as large.