It has been found that when radio waves of frequency 16 kc./s. and somewhat higher are transmitted over distances of 540 km., the phase and amplitude of the received signal change suddenly about one hour before ground sunrise at the mid-point of the transmission path. This effect results from a lowering of the nighttime reflection height of 95 km. to the daytime value of 70 km.By considering the geometry of the situation, it is apparent that the process of photo-ionization of atmospheric molecules cannot produce the observed effect since the relevant wave lengths are absorbed before reaching the 95 km. height. On the other hand, electrons may be removed from negative oxygen ions by visible and near infrared radiation which does reach the 95 km. level prior to sunrise. The problem is then to determine the intensities of the relevant wave lengths at the 95 km. level, and to compute the rate at which electrons are released from negative ions. It is shown that the pre-sunrise effect at 95 km. is readily explained as a negative ion phenomenon, and the rate of fall of reflection height is discussed.