Antisunward decay of polar cap arcs

• Published in: Journal of Geophysical Research: Space Physics 1997-12, Vol.102 (A12), p.27227-27247
• Main Authors: Rodriguez, J. V., Valladares, C. E., Fukui, K., Gallagher, H. A.
• Format: Article
• Language: English
• Subjects: Airglow. Aurora; Earth, ocean, space; Exact sciences and technology; External geophysics; Physics of the high neutral atmosphere
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/97JA01727

Data from an all‐sky imaging photometer (427.8 and 630.0 nm) in the central polar cap, the IMP 8 and ISEE 2 satellites in the solar wind, and the DMSP F6, F7, and F8 satellites in the topside ionosphere have been used to investigate ten cases of polar cap arcs decaying antisunward. In all cases, the decay occurs after the interplanetary magnetic field (IMF) turns southward; in eight cases, a period of Bz < 0 lasting ≥ 20 min follows the southward turning. The lags from the IMF turning to the arc decay are corrected for the travel time from IMP 8 or ISEE 2 to the cusp/cleft ionosphere. For eight of the arcs, the lags range from 15 to 30 min; while in the other two cases, the arcs first appear in the central polar cap 30–40 min after the southward turning, then decay antisunward during the expansion phase of a small substorm, 60–65 min after Bz turned negative. In all cases, the direction of dawn‐dusk drift is in the direction of By, and in the cases in which By 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 arc or its conjugate location, the precipitation data indicate a likely origin on open (closed) field lines in two (three) cases. The 15‐30 min Bz lags characterize both open and closed field line arcs, while the two arcs associated with 60–65 min lags are on closed field lines. Antisunward decay of polar cap arcs 15–30 min after Bz turns negative is consistent with the By‐dependent convection of new open flux tubes from the 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 boundary layer. Consequently, the dawn‐dusk drift and antisunward decay of polar cap arcs 15–30 min after the IMF turns southward suggest the stirring of the tail flanks by dayside‐driven merging.