Theoretical and experimental studies suggest four main classes of magnetopause/boundary layer processes to explain how particles, momentum and energy transport from the solar wind into the magnetosphere-ionosphere system. Assuming that the IMF By-related prenoon-postnoon asymmetry of plasma convection is specific to the magnetic merging coupling mode, we determine statistically the local time distribution of transient auroral events as a function of the IMF By polarity. This is based on concurrent observations of the interplanetary magnetic field components Bz and By and optical auroral observations of cusp/cleft activities within approximately 1000--1400 magnetic local time (MLT) and 71¿--81¿ magnetic latitudes. Intervals of negative IMF Bz and nonzero By are selected for this study. Earlier case studies have shown that transient auroral events moving eastward or westward in the midday sector, depending on the IMF By polarity, are candidate ionospheric signature of magnetopause flux transfer events (FTEs). An example of this auroral event sequence, illustrating some main characteristics of the expected ionospheric footprints of FTEs, is presented. The statistical study reported here shows an asymmetric prenoon-postnoon auroral occurrence distribution which depends on IMF By, consistent with the predictions of the reconnection model within a 95% level of confidence. This result confirms the previously existing evidence in favor of an FTE-related interpretation of the actual auroral phenomenon. ¿ American Geophysical Union 1996