Measurement of the Anisotropy Power Spectrum of the Radio Synchrotron
  Background
  We present the first targeted measurement of the power spectrum of
anisotropies of the radio synchrotron background, at 140 MHz where it is the
overwhelmingly dominant photon background. This measurement is important for
understanding the background level of radio sky brightness, which is dominated
by steep-spectrum synchrotron radiation at frequencies below 0.5 GHz and has
been measured to be significantly higher than that which can be produced by
known classes of extragalactic sources and most models of Galactic halo
emission. We determine the anisotropy power spectrum on scales ranging from 2
degrees to 0.2 arcminutes with LOFAR observations of two 18 square degree
fields -- one centered on the Northern hemisphere coldest patch of radio sky
where the Galactic contribution is smallest and one offset from that location
by 15 degrees. We find that the anisotropy power is higher than that
attributable to the distribution of point sources above 100 micro-Jy in flux.
This level of radio anisotropy power indicates that if it results from point
sources, those sources are likely at low fluxes and incredibly numerous, and
likely clustered in a specific manner.
