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 - 1600 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 dynamic 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 component (IMF B-y much less than 0) and a small or zero north-south component. The upstream dynamic pressure variations were of two types, reflecting two distinct origins: the unperturbed solar wind and the bow shock. The underlying reconnection interpretation, with the necessary magnetic shear across the magnetopause being provided by the strong westward component of the IMF, is substantiated by a study of ionospheric flows. A twin-cell pattern of standard polarity is evident in which strong asymmetries about noon are seen, as expected from the predominantly westward pointing IMF. Flow perturbations are superimposed on this general pattern. The auroral observations consist of a latitudinally narrow zone of persistent auroral emission, at the poleward edge of which 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 activation of discrete forms at the cleft equatorward boundary. A comparison with results obtained from mapping the magnetopause and its boundary layers to the ionosphere by means of particle precipitation characteristics indicates that the persistent aurora consists mainly of low- latitude boundary layer precipitation on closed field lines. The transients poleward of the persistent aurora are coming from the cusp or near cusp. We find that the auroral transients are correlated with a sequence of magnetic impulses of 50 nT or less amplitude seen at ground stations around the optical site. We also find that there is a further, good correlation between the transients and the arrival at Earth of rapid and large (> 20 % increase relative to background levels) upstream dynamic pressure pulses. Smaller dynamic pressure changes do not seem to affect the aurora noticeably. Our interpretation is in line with some recent studies, namely, that an increase of dynamic pressure on the magnetopause enhances the reconnection rate there. Clearly, theoretical work is needed to understand this important, if indirect, effect of dynamic pressure on the magnetosphere.