Abstract. The onset of anomalous resistance in a layer on auroral field lines is shown to be accompanied by the excitation of an Alfvénic impulse (AI). The generated AI marks the transition of the global magnetosphere-ionosphere instability into an explosive phase with positive feedback. The spatial structure of this impulse both in space and on the ground has been described analytically and numerically under the thin layer approximation. The field-aligned currents transported by the Alfvénic impulse are concentrated near the edges of the layer with anomalous resistivity, whereas the reverse currents are spread throughout the layer. For some parameters of the layer the ionospheric attenuation of even small-scale structures is not dramatically large, so the magnetic response to the generated AI may be observed on the ground. The considered event on 3 January 1997 with magnetic-auroral intensification observed by ground magnetometers and Polar UV imager agrees with the model proposed.