A STATISTICAL STUDY OF FLUX-TRANSFER EVENT SIGNATURES IN THE DAYSIDE AURORA - THE IMF B-Y-RELATED PRENOON-POSTNOON ASYMMETRY

Theoretical and experimental studies suggest four main classes of magn etopause/boundary layer processes to explain how particles, momentum a nd energy transport from the solar wind into the magnetosphere-ionosph ere system. Assuming that the IMF B-y-related prenoon-postnoon asymmet ry of plasma convection is specific to the magnetic merging coupling m ode, we determine statistically the local time distribution of transie nt amoral events as a function of the IMF B-y polarity. This is based on concurrent observations of the interplanetary magnetic field compon ents B-z and B-y and optical auroral observations of cusp/cleft activi ties within approximately 1000-1400 magnetic local time (MLT) and 71 d egrees-81 degrees magnetic latitudes. Intervals of negative IMF B-z an d nonzero B-y are selected for this study. Earlier case studies have s hown that transient auroral events moving eastward or westward in the midday sector, depending on the IMF B-y polarity, are candidate ionosp heric signature of magnetopause flux transfer events (FTEs). An exampl e of this auroral event sequence, illustrating some main characteristi cs of the expected ionospheric footprints of FTEs, is presented. The s tatistical study reported here shows an asymmetric prenoon-postnoon au roral occurrence distribution which depends on IMF B-y, consistent wit h the predictions of the reconnection model within a 95% level of conf idence. This result confirms the previously existing evidence in favor of an FTE-related interpretation of the actual auroral phenomenon.