Formation of Beading Auroral Arcs at Substorm Onset: Implications of Its Variability into the Generation Process

The formation of auroral beads, wavy onset arcs, is often discussed in terms of substorm initiation. The present study addresses several aspects of their development and propagation by examining ground all-sky images for representative events for each aspect. The results are summarized as follows: (a) the expansion and propagation of auroral beads do not depend on whether they form east or west of a preceding equatorward flow; (b) Auroral beads propagate either eastward or westward when the westward auroral electrojet intensifies apparently not preceded by the approach of an equatorward flow in neighboring sectors; (c) A nearby arc does not seem to be affected even if auroral beads form only a few tenths of degree apart in latitude; (d) two wavy arcs occasionally form next to each other and propagate in different directions; (e) the evolution of beading arcs is not always conjugate in two hemispheres. Results (a) and (b) suggest that meso- or large-scale ionospheric convection as inferred from ground magnetic disturbances does not regulate the development and propagation of auroral beads. Results (c) and (d) imply that the development of auroral beads can be attributed to a small-scale process, possibly a convection flow, at a latitudinal scale of a few tenths of degree or less. Such a small-scale process may not be conjugate in two hemispheres, which potentially explains Result (e). These results do not necessarily mean that the cause of auroral beads is ionospheric, but suggest that the magnetosphere-ionosphere coupling plays an important role in their development.