Simultaneous closed magnetic field line polar arcs and substorms

Viking UV images of the northern hemisphere auroral region were examined du ring simultaneous substorms and polar arcs. Of 120 substorm events, 43 reco rded polar arcs. In all events the substorm. and polar are were identified with the images, and a strict definition of the term polar arc has been app lied to rigorously eliminate any ambiguous cases from this study. In additi on, simultaneous events of substorms and polar arcs were separated into fou r cases: dawnside polar area before substorm onset, dawnside polar arcs aft er onset, duskside polar arcs before onset, and duskside polar arcs after o nset. Superposition epoch plots of the average polar are intensity over tim e with respect to substorm onset were created. Overall, these plots suggest a weak coupling between the polar are and the substorm mechanisms. In most of the simultaneous cases, little or no motion of the polar are was measur ed. This observation, combined with the various intensity minimum determine d in the superposition epoch plots, suggests little or no change occurs in the thickness of the plasma sheet as many substorm models predict. Furtherm ore, magnetic field line mapping frequently indicates magnetic field lines of polar arcs stretch far down the magnetotail (i.e., greater than 50 Earth Radii), while substorms appear to be located within 20 R-e. This concept, combined with the theory that the interaction between the polar arcs and su bstorms is weak, suggests that substorms cannot be a distant magnetotail ev ent, otherwise there would be a stronger interaction between the two. (C) 2 001 Elsevier Science Ltd. All rights reserved.