Effects Of The Ionosphere On Ground-Based Detection Of The Global 21 CM
  Signal From The Cosmic Dawn And The Dark Ages
  Detection of the global HI 21 cm signal from Cosmic Dawn and Epoch of
Reionization is the key science driver for several ongoing ground-based and
future ground/space-based experiments. The crucial spectral features in the
global 21 cm signal (turning points) occur at low radio frequencies <100 MHz.
In addition to the human-generated RFI, Earth's ionosphere drastically corrupts
low-frequency radio observations from the ground. In this paper, we examine the
effects of time-varying ionospheric refraction, absorption and thermal emission
at these low radio frequencies and their combined effect on any ground-based
global 21 cm experiment. It should be noted that this is the first study of the
effect of a dynamic ionosphere on global 21 cm experiments. The fluctuations in
the ionosphere are influenced by solar activity with flicker noise
characteristics. The same characteristics are reflected in the ionospheric
corruption to any radio signal passing through the ionosphere. As a result, any
ground based observations of the faint global 21 cm signal are corrupted by
flicker noise (or "$1/f$" noise, where "$f$" is the dynamical frequency) which
scales as $\nu^{-2}$ (where $\nu$ is the frequency of observation) in the
presence of a bright galactic foreground ($\propto \nu^{-s}$, where $s$ is
radio spectral index). Hence, the calibration of the ionosphere for any such
experiment is critical. Any attempt to calibrate the ionospheric effects will
be subject to the inaccuracies in the current ionospheric measurements using
GPS ionospheric measurements, riometer measurements, ionospheric soundings,
etc. Even considering an optimistic improvement in the accuracy of GPS-TEC
(Total Electron Content) measurements, we conclude that the detection of the
global 21 cm signal below 100 MHz is best done from above the Earth's
atmosphere in orbit of the Moon.
