Detections of the cross correlation signal between the 21cm signal during reionization and high-redshift Lyman Alpha emitters ( LAEs ) are subject to observational uncertainties which mainly include systematics associated with radio interferometers and LAE selection .
These uncertainties can be reduced by increasing the survey volume and/or the survey luminosity limit , i.e .
the faintest detectable Lyman Alpha ( Ly \alpha ) luminosity .
We use our model of high-redshift LAEs and the underlying reionization state to compute the uncertainties of the 21cm-LAE cross correlation function at z \simeq 6.6 for observations with SKA1-Low and LAE surveys with \Delta z = 0.1 for three different values of the average IGM ionization state ( \langle \chi _ { \mathrm { HI } } \rangle \simeq 0.1 , 0.25 , 0.5 ) .
At z \simeq 6.6 , we find SILVERRUSH type surveys , with a field of view of 21 deg ^ { 2 } and survey luminosity limits of L _ { \alpha } \geq 7.9 \times 10 ^ { 42 } erg s ^ { -1 } , to be optimal to distinguish between an inter-galactic medium ( IGM ) that is 50 % , 25 % and 10 % neutral , while surveys with smaller fields of view and lower survey luminosity limits , such as the 5 and 10 deg ^ { 2 } surveys with WFIRST , can only discriminate between a 50 % and 10 % neutral IGM .