ANTISUNWARD DECAY OF POLAR-CAP ARCS

Data from an all-sky imaging photometer (427.8 and 630.0 nm) in the ce ntral polar cap, the IMP 8 and ISEE 2 satellites in the solar wind, an d the DMSP F6, F7, and F8 satellites in the topside ionosphere have be en used to investigate ten cases of polar cap arcs decaying antisunwar d. In all cases, the decay occurs after the interplanetary magnetic fi eld (IMF) turns southward; in eight cases, a period of B-z < 0 lasting greater than or equal to 20 min follows the southward turning. The la gs from the IMF turning to the are decay are corrected for the travel time from IMP 8 or ISEE 2 to the cusp/cleft ionosphere. For eight of t he arcs, the lags range from 15 to 30 min; while in the other two case s, the arcs first appear in the central polar cap 30-40 min after the southward turning, then decay antisunward during the expansion phase o f a small substorm, 60-65 min after B-z turned negative. In all cases, the direction of dawn-dusk drift is in the direction of B-y, and in t he cases in which B-y also reversed, the corrected lags to a change in dawn-dusk drift are < 5 min (in two cases) or 15-20 min (in two cases ). In the five cases in which a DMSP satellite traverses the are or it s conjugate location, the precipitation data indicate a likely origin on open (closed) field lines in two (three) cases. The 15-30 min B-z l ags characterize both open and closed field line arcs, while the two a rcs associated with 60-65 min lags are on closed field lines. Antisunw ard decay of polar cap arcs 15-30 min after B-z turns negative is cons istent with the B-y-dependent convection of new open flux tubes from t he cusp to the center of the polar cap driven by dayside merging. The 60-65 min lags suggest an evolution in polar cap convection driven at first by dayside merging or viscous processes, then by the development of a substorm-related DP 1 current pattern. If polar cap arcs map to the distant tail flanks, they may be associated with instabilities due to velocity shears between the plasma sheet and the low-latitude boun dary layer. Consequently, the dawn-dusk drift and antisunward decay of polar cap arcs 15-30 min after the IMF turns southward suggest the st irring of the tail flanks by dayside-driven merging.