interest ($50 \lesssim \nu
\lesssim 200$ MHz), although it is known to flatten with decreasing frequency due to self-absorption. The intensity of the synchrotron emission and the exact value of the spectral index depend on Galactic coordinate. The amplitude varies over an order of magnitude, between about $200 < T_{gal} < 10,000$ K at 150 MHz (peaking toward the Galactic center and along the Galactic plane), while the spectral index has small variations of order $\sigma_\beta\approx0.1$ dependent largely on Galactic latitude, with the steepest regions occurring at high Galactic latitudes. Free-free emission in the Galaxy and discrete Galactic and extragalactic continuum sources also have spectra that can be reasonably described by power-laws. The integrated flux from extragalactic continuum sources is generally isotropic on large scales and accounts for the majority of the remaining power in the low-frequency radio spectrum, with free-free emission making up only about 1% of the total power. The combined spectrum due to the astrophysical foregrounds is smooth and remains similar to a power-law profile.

On the other hand, as the apparent differential brightness temperature of the redshifted 21 cm background transitions from $T_{21}=0$ mK at very high redshift to $T_{21}\approx-100$ mK before heating of the IGM 