Light cone anisotropy in 21 cm signal from the epoch of reionization
  Using a suite of detailed numerical simulations we estimate the level of
anisotropy generated by the time evolution along the light cone of the 21cm
signal from the epoch of reionization. Our simulations include the physics
necessary to model the signal during both the late emission regime and the
early absorption regime, namely X-ray and Lyman-band 3D radiative transfer in
addition to the usual dynamics and ionizing UV transfer. The signal is analysed
using correlation functions perpendicular and parallel to the line of sight
(LOS). We reproduce general findings from previous theoretical studies: the
overall amplitude of the correlations and the fact that the light cone
anisotropy is visible only on large scales (100 cMpc). However, the detailed
behaviour is different. At 3 different epochs, the amplitude of the
correlations along and perpendicular to the LOS differ from each other,
indicating anisotropy. These 3 epochs are associated with 3 events of the
global reionization history: the overlap of ionized bubbles, the onset of mild
heating by X-rays in regions around the sources, and the onset of efficient
Lyman-alpha coupling in regions around the sources. A 20x20 deg^2 survey area
may be necessary to mitigate sample variance when we use the directional
correlation functions. On a 100 cMpc scale the light cone anisotropy dominates
over the anisotropy generated by peculiar velocity gradients computed in the
linear regime. By modelling instrumental noise and limited resolution, we find
that the anisotropy should be easily detectable by the SKA, assuming perfect
foreground removal, the limiting factor being a large enough survey size. In
the case of the LOFAR, it is likely that only first anisotropy episode will
fall in the observing frequency range and will be detectable only if sample
variance is much reduced (i.e. a larger than 20x20 deg^2 survey, which is not
presently planned).
