The impact of Lyman series photons on the intergalactic medium during
  the Epoch of Reionisation
  The role of higher order Lyman series photons on the 21cm absorption or
emission signature of the IGM against the background CMB during the Epoch of
Reionisation is examined. It is shown that, taking into account the diminishing
resonance line scattering cross section with increasing Lyman order, a
non-negligible net scattering rate of higher order Lyman photons is expected.
The resulting radiative cascades will substantially enhance the number density
of Lyman-alpha photons near a radiation source. It is also shown that the
higher order Lyman series photons are able to collisionally heat the IGM by
amounts of tens to hundreds of degrees kelvin. The possibility that the
Wouthuysen-Field effect may be suppressed by the presence of dust near a galaxy
is discussed, and it is shown that the higher order Lyman series photons would
still induce the effect, but with a somewhat reduced 21cm radiation efficiency.
It is also demonstrated that extended low surface brightness emission line
halos will be produced from radiative cascades following the scattering of
higher order Lyman series photons. These halos would provide a unique means of
confirming that reionisation source candidates were surrounded by an IGM that
was still largely neutral on large scales.
