The quasi-periodic sequence of moving auroral forms at the dayside polar cap boundary observed during southward directed interplanetary magnetic field (IMF) is one of the most interesting features of dayside auroral activity. Examples from the prenoon and postnoon sectors, corresponding to positive and negative IMF BY, respectively, are presented. Dual-site ground-based optical observations of one of the auroral events on January 12, 1991, are combined with ''snapshot'' information on the particle precipitation environment and ionospheric ion flow obtained from the polar-orbiting satellite DMSP F9. A characteristic sequence of auroral forms observed within 08--10 MLT expanded northwestward in a region of strong westward ion flow component, during intervals of strongly negative IMF BZ and positive BY. The optical events faded out near the 08 MLT meridian, within the regime of mantle precipitation (magnetosheath origin). The fading phase of the optical event traversed by the satellite along the 09 MLT meridian is characterized by structured mantle precipitation and associated ion flow shears within 72¿--78¿ magnetic latitude (MLAT).

The integrated potential drop across this event in the north-south direction, corresponding to the westward ion drift component within 72¿--78¿ MLAT, is 25 kV. IMP 8 observations of solar wind dynamic pressure outside the bow shock do not show good correlation between pressure variations and major optical event occurrence in the present case. Negative IMF BY events observed near the noon meridian and in the postnoon sector are moving eastward, in the same direction as the ionospheric convection. The optical, particle, and ion drift observations in combination with IMF and solar wind plasma data indicate that the events represent dynamical structures of merging cell convection over the polar magnetosphere, possibly initiated by pulses of enhanced merging rate (Bn≠0) at the dayside magnetopause. The fading of the optical signature and the structured ionospheric ion flow/field-aligned current at mantle latitudes are consistent with a high degree of spatial structure in the high-latitude boundary layer and of the associated solar wind--magnetosphere dynamo action (E⋅J<0), provided the field line mapping implied by the present understanding of the particle precipitation characteristics is correct.

The contribution from these events to the transpolar voltage (ϕpc) may be estimated on the basis of luminosity area and event repetition time (≂10 min.). A wide range of spatial scales of optical events as well as their internal structure is observed. This study indicates that a nonnegligible fraction of the transpolar voltage may at times be generated by pulsed magnetopause merging. Discrete auroral activity and irregular magnetic pulsations with long periods are observed at cleft latitudes in the vicinity of the prenoon convection reversal and further north during the event sequence taking place. ¿ American Geophysical Union 1993