The circuit analogy for the response of the coupled magnetosphere-ionosphere system to changes in day side field line reconnection rate is applied to real conditions and is used to calculate the expected variation in magnetopause position during the erosion event described by Aubry et al. (1970). Generally good agreement between observation and theory is found. The role of the dawn-dusk electric field responsible for magnetospheric convection is examined in some detail and is treated in the circuit analogy as the field due to an external generator connected across the circuit. It is found that the erosion process requires two distinct time constants for a proper description: (1) the time needed for magnetosheath plamsa to travel down the freshly reconnected field lines to the ionosphere and (2) roughly, the time required for the foot of a reconnected field line to travel one quarter of the total noon-midnight dimension of the polar cap. The second time constant is the dominant one and is not related to the ionospheric conductivity, as has been suggested previously. Examination of high-latitude magnetograms obtained during the erosion event discussed shows that the electric field oscillations predicted by the theory and observed by the spacecraft in terms of oscillations in the magnetopause position are also reflected in oscillations in ionospheric current flow.