Cuts through the 25   Mpc   h^-1 simulation box at redshift z = 4 for a QSO sitting in the void at the lower left corner. In the left panel, the overdensity field 1 + δ is shown. The right panel provides the inverse of the overionisation fraction n_{Hi,Ly − α}/n_Hi,prox due to a L_νLL = 10³¹   erg   Hz^-1   s^-1 QSO. Here n_{Hi,Ly − α} and n_Hi,prox are the neutral hydrogen fractions without and with the influence of the QSO respectively. Clearly visible are the extended overionisation zone and shadowing effects. The scattered white dots arise from numerical noise. The axes are in comoving h^-1   Mpc.