The impact of the ionosphere on ground-based detection of the global
  Epoch of Reionisation signal
  The redshifted 21cm line of neutral hydrogen (Hi), potentially observable at
low radio frequencies (~50-200 MHz), is a promising probe of the physical
conditions of the inter-galactic medium during Cosmic Dawn and the Epoch of
Reionisation (EoR). The sky-averaged Hi signal is expected to be extremely weak
(~100 mK) in comparison to the Galactic foreground emission (~$10^4$ K).
Moreover, the sky-averaged spectra measured by ground-based instruments are
affected by chromatic propagation effects (of the order of tens of Kelvins)
originating in the ionosphere. We analyze data collected with the upgraded
BIGHORNS system deployed at the Murchison Radio-astronomy Observatory to assess
the significance of ionospheric effects (absorption, emission and refraction)
on the detection of the global EoR signal. We measure some properties of the
ionosphere, such as the electron temperature ($T_e \approx$470 K at nighttime),
magnitude, and variability of optical depth ($\tau_{100 MHz} \approx$0.01 and
$\delta \tau \approx$0.005 at nighttime). According to the results of a
statistical test applied on a large data sample, very long integrations lead to
increased signal to noise even in the presence of ionospheric variability. This
is further supported by the structure of the power spectrum of the sky
temperature fluctuations, which has flicker noise characteristics at
frequencies $\gtrsim 10^{-5}$ Hz, but becomes flat below $\approx 10^{-5}$ Hz.
We conclude that the stochastic error introduced by the chromatic ionospheric
effects tends to zero in an average. Therefore, the ionospheric effects and
fluctuations are not fundamental impediments preventing ground-based
instruments from integrating down to the precision required by global EoR
experiments.
