The anomalous self-absorption of a high-power radio wave measured during recent ionospheric modification experiments exhibits a hysteresis effect. These results are consistent with the assumption that the growth of field-aligned irregularities due to the interaction of a high-power pump wave and the ionospheric plasma occurs in two stages. During the first stage, striation growth is due to an instability which has a threshold independent of striation amplitude. When the striation level is sufficiently high, the threshold of the second stage is reached. The second-stage threshold is dependent on the striation amplitude. The state of background ionosphere is also found to be an important factor influencing the observed effects.