Spectral Index of the Diffuse Radio Background Between 50 and 100 MHz
  We report the spectral index of diffuse radio emission between 50 and 100 MHz
from data collected with two implementations of the Experiment to Detect the
Global EoR Signature (EDGES) low-band system. EDGES employs a wide beam
zenith-pointing dipole antenna centred on a declination of $-26.7^\circ$. We
measure the sky brightness temperature as a function of frequency averaged over
the EDGES beam from 244 nights of data acquired between 14 September 2016 to 27
August 2017. We derive the spectral index, $\beta$, as a function of local
sidereal time (LST) using night-time data and a two-parameter fitting equation.
We find $-2.59<\beta<-2.54 \pm 0.011$ between 0 and 12 h LST, ignoring
ionospheric effects. When the Galactic Centre is in the sky, the spectral index
flattens, reaching $\beta = -2.46 \pm 0.011$ at 18.2 h. The measurements are
stable throughout the observations with night-to-night reproducibility of
$\sigma_{\beta}<0.004$ except for the LST range of 7 to 12 h. We compare our
measurements with predictions from various global sky models and find that the
closest match is with the spectral index derived from the Guzm{\'a}n and Haslam
sky maps, similar to the results found with the EDGES high-band instrument for
90-190 MHz. Three-parameter fitting was also evaluated with the result that the
spectral index becomes more negative by $\sim$0.02 and has a maximum total
uncertainty of 0.016. We also find that the third parameter, the spectral index
curvature, $\gamma$, is constrained to $-0.11<\gamma<-0.04$. Correcting for
expected levels of night-time ionospheric absorption causes $\beta$ to become
more negative by $0.008$ - $0.016$ depending on LST.
