Dipole of the Epoch of Reionization 21-cm signal
  The motion of the solar system with respect to the cosmic rest frame
modulates the monopole of the Epoch of Reionization 21-cm signal into a dipole.
This dipole has a characteristic frequency dependence that is dominated by the
frequency derivative of the monopole signal. We argue that although the signal
is weaker by a factor of $\sim100$, there are significant benefits in measuring
the dipole. Most importantly, the direction of the cosmic velocity vector is
known exquisitely well from the cosmic microwave background and is not aligned
with the galaxy velocity vector that modulates the foreground monopole.
Moreover, an experiment designed to measure a dipole can rely on differencing
patches of the sky rather than making an absolute signal measurement, which
helps with some systematic effects.
