RECONNECTION-ASSOCIATED AURORAL ACTIVITY STIMULATED BY 2 TYPES OF UPSTREAM DYNAMIC PRESSURE VARIATIONS - INTERPLANETARY MAGNETIC-FIELD B-Z-SIMILAR-TO-0, B-Y-MUCH-LESS-THAN-0 CASE

We present observations of transient auroral activity in the 1300 - 16 00 MLT range at geomagnetic latitudes of 76 degrees and interpret them in terms of modulations in the reconnection rate at the magnetopause which are, in turn, stimulated by the arrival at Earth of upstream dyn amic pressure pulses. The period studied is 1000 - 1200 UT on December 17, 1992. The interplanetary magnetic field (IMF), monitored by IMP 8 , was aligned in a Parker spiral direction with a strong westward comp onent (IMF B-y much less than 0) and a small or zero north-south compo nent. The upstream dynamic pressure variations were of two types, refl ecting two distinct origins: the unperturbed solar wind and the bow sh ock. The underlying reconnection interpretation, with the necessary ma gnetic shear across the magnetopause being provided by the strong west ward component of the IMF, is substantiated by a study of ionospheric flows. A twin-cell pattern of standard polarity is evident in which st rong asymmetries about noon are seen, as expected from the predominant ly westward pointing IMF. Flow perturbations are superimposed on this general pattern. The auroral observations consist of a latitudinally n arrow zone of persistent auroral emission, at the poleward edge of whi ch auroral forms are seen moving eastward (tailward) at speeds of 1-2 km s(-1). This intermittent activity, which is often detached from the persistent aurora, has a red line intensity (Line of sight) > 1.5 kR, a broad intensity-versus-zenith angle profiles, and a weak green line intensity (< 1 kR). The brightenings are also associated with the act ivation of discrete forms at the cleft equatorward boundary. A compari son with results obtained from mapping the magnetopause and its bounda ry layers to the ionosphere by means of particle precipitation charact eristics indicates that the persistent aurora consists mainly of low- latitude boundary layer precipitation on closed field lines. The trans ients poleward of the persistent aurora are coming from the cusp or ne ar cusp. We find that the auroral transients are correlated with a seq uence of magnetic impulses of 50 nT or less amplitude seen at ground s tations around the optical site. We also find that there is a further, good correlation between the transients and the arrival at Earth of r apid and large (> 20 % increase relative to background levels) upstrea m dynamic pressure pulses. Smaller dynamic pressure changes do not see m to affect the aurora noticeably. Our interpretation is in line with some recent studies, namely, that an increase of dynamic pressure on t he magnetopause enhances the reconnection rate there. Clearly, theoret ical work is needed to understand this important, if indirect, effect of dynamic pressure on the magnetosphere.