As observed in many previous case studies a typical substorm is characterised by the loss of HF backscatter at substorm onset. Loss of scatter often occurs 20-40 minutes before onset and is most likely effected by soft precipitation in growth phase. Atypical cases, e.g. generation of scatter, exhibit an asymmetry in occurrence times. Examples exist to support different possible mechanisms; blanketing, absorption, suppression of irregularities. Presence of STARE scatter during the same period gives valuable information about level of electric field. Larger scale studies reveal enhancement in the electric field on the dawn and dusk flanks at substorm onset.