We present a statistical study of the dynamics of the equatorward boundary of the diffuse aurora. This Soft Electron Boundary appears to be best fitted by using the AE index values averaged over the 5 hours preceding the measurements. This inertia of the boundary is interpreted as indicative (1) of the action of a polarization electric field braking the inward motion of the corresponding equatorial plasma during increasing magnetic activity so that several successive perturbations are needed to obtain a consequent inward displacement and (2) of a weak wave-particle interaction during periods of decreasing magnetic activity.