AURORAL EVENT SEQUENCES AT THE DAYSIDE POLAR-CAP BOUNDARY FOR POSITIVE AND NEGATIVE INTERPLANETARY MAGNETIC-FIELD BY

The quasi-periodic sequence of moving auroral forms at the dayside pol ar cap boundary observed during southward directed interplanetary magn etic field (IMF) is one of the most interesting features of dayside au roral activity. Examples from the prenoon and postnoon sectors, corres ponding to positive and negative IMF B(Y), respectively, are presented . Dual-site ground-based optical observations of one of the auroral ev ents on January 12, 1991, are combined with ''snapshot'' information o n the particle precipitation environment and ionospheric ion flow obta ined from the polar-orbiting satellite DMSP F9. A characteristic seque nce 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 B(Z) and positive B(Y). The optical events fade d out near the 08 MLT meridian, within the regime of mantle precipitat ion (magnetosheath origin). The fading phase of the optical event trav ersed by the satellite along the 09 MLT meridian is characterized by s tructured mantle precipitation and associated ion flow shears within 7 2-degrees - 78-degrees magnetic latitude (MLAT). The integrated potent ial drop across this event in the north-south direction, corresponding to the westward ion drift component within 72-degrees - 78-degrees ML AT, is 25 kV. IMP 8 observations of solar wind dynamic pressure outsid e the how shock do not show good correlation between pressure variatio ns and major optical event occurrence in the present case. Negative IM F B(Y) events observed near the noon meridian and in the postnoon sect or are moving eastward, in the same direction as the ionospheric conve ction. The optical, particle, and ion drift observations in combinatio n with IMF and solar wind plasma data indicate that the events represe nt dynamical structures of merging cell convection over the polar magn etosphere, possibly initiated by pulses of enhanced merging rate (B(n) not-equal 0) at the dayside magnetopause. The fading of the optical s ignature and the structured ionospheric ion flow/field-aligned current at mantle latitudes are consistent with a high degree of spatial stru cture in the high-latitude boundary layer and of the associated solar wind-magnetosphere dynamo action (E.j < 0), provided the field line ma pping implied by the present understanding of the particle precipitati on characteristics is correct. The contribution from these events to t he transpolar voltage (phi(pc)) may be estimated on the basis of lumin osity area and event repetition time (congruent-to 10 min). A wide ran ge of spatial scales of optical events as well as their internal struc ture is observed. This study indicates that a nonnegligible fraction o f the transpolar voltage may at times be generated by pulsed magnetopa use merging. Discrete auroral activity and irregular magnetic pulsatio ns with long periods are observed at cleft latitudes in the vicinity o f the prenoon convection reversal and further north during the event s equence taking place.