Dipole Anisotropy as an Essential Qualifier for the Monopole Component
  of the Cosmic-Dawn Spectral Signature, and the Potential of Diurnal Pattern
  for Foreground Estimation
  While the importance of detecting the Global spectral signatures of the
red-shifted 21-cm line of atomic hydrogen from the very early epochs cannot be
overstated, the associated challenges primarily include isolating the weak
signal of interest from the orders of magnitude brighter foregrounds, and
extend equally to reliably establishing the origin of the {\it apparent} global
signal to the very early epochs. This letter proposes a critical dipole test
that the measurements of the monopole component of the spectrum of interest
should necessarily pass. Our criterion is based on a unique correspondence
between the intrinsic monopole spectrum and the differential spectrum as an
imprint of dipole anisotropy resulting from motion of observer with respect to
the rest frame of our source (such as that of our Solar system, interpreted
from the dipole anisotropy in CMBR). More importantly, the spectral
manifestation of the dipole anisotropy gets {\it amplified} by a significant
factor, depending on the monopole spectral slopes, rendering it feasible to
measure. We describe details of such a test, and illustrate its application
with the help of simulations. The letter also alludes to a novel
model-independent path toward isolating the foreground contribution, using the
diurnal pattern readily apparent in drift-scan observations. Such dipole
qualifier for the monopole spectrum, when combined with reliable foreground
estimation, is expected to pave way for in situ validation of spectral
signatures from early epochs, important to presently reported and future
detections of EoR signal.
