By using ESRO 4 data, this study investigates low-latitude density perturbations during magnetically active periods. It is found that (1) the delay time between substorm activity (AE index) and density perturbations may be of the order of 4 hours only, (2) and equatorial density hump may develop which at times is well separated from the higher latitude disturbance, and (3) thermal and northermal effects contribute to the density perturbation. As to the origin of the disturbance, direct energy injection by the ring current mechanism is only of secondary importance. Instead, traveling atmospheric disturbances and large-scale winds both may contribute to the observed density increase.