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While the importance of detecting the Global spectral signatures of thered-shifted 21-cm line of atomic hydrogen from the very early epochs cannot beoverstated, the associated challenges primarily include isolating the weaksignal of interest from the orders of magnitude brighter foregrounds, andextend equally to reliably establishing the origin of the {\it apparent} globalsignal to the very early epochs. This letter proposes a critical dipole testthat the measurements of the monopole component of the spectrum of interestshould necessarily pass. Our criterion is based on a unique correspondencebetween the intrinsic monopole spectrum and the differential spectrum as animprint of dipole anisotropy resulting from motion of observer with respect tothe rest frame of our source (such as that of our Solar system, interpretedfrom the dipole anisotropy in CMBR). More importantly, the spectralmanifestation of the dipole anisotropy gets {\it amplified} by a significantfactor, depending on the monopole spectral slopes, rendering it feasible tomeasure. We describe details of such a test, and illustrate its applicationwith the help of simulations. The letter also alludes to a novelmodel-independent path toward isolating the foreground contribution, using thediurnal pattern readily apparent in drift-scan observations. Such dipolequalifier for the monopole spectrum, when combined with reliable foregroundestimation, is expected to pave way for in situ validation of spectralsignatures from early epochs, important to presently reported and futuredetections of EoR signal.

the astrophysical journal. letters

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