The lunar dependence of the occurrence frequency of bright aurora reported by Stoffregen [1968, 1970> and thought by him to be caused by magnetospheric disturbances at the moon's orbit may be due to a hindrance of poleward bound plasma flow by the combined effect of the moon and its magnetohydrodynamic wake when the moon is passing through the plasma sheet in the geomagnetic tail at the geocentric distance of 60RE.