Fine-Scale Structuring of Pre-Breakup Arcs: Observations and Implications

Low-frequency waves (or wavy structures) often emerge in a localized region of the near-Earth plasma sheet and midnight auroral arcs in the course of substorm onset. One of pre-existing auroral arcs becomes active with some spatial structuring into fine-scale forms (also called auroral beads/rays/waves) in several/a few minutes prior to auroral breakup and the evolving individual structures propagate either westward or eastward along the arc. Whereas such a pre-breakup arc signature is believed to reflect the development of near-Earth plasma sheet instability that may trigger the subsequent abrupt expansion of aurora, it still remains to be understood regarding the physical processes operating behind the connection. Here we report on recent progress of our understanding of the pre-breakup auroral arcs, with a particular emphasis on highlighting their key aspects obtained by the ground-based auroral observations. Some of the examples will include comparison with in situ field and particle measurements from spacecraft (such as THEMIS, Van Allen Probes) in the near-Earth tail that were favorable for magnetic conjunction with pre-breakup auroral arc. We will also discuss the implications of these observations on magnetospheric processes involved in the deformation of pre-breakup auroral arcs into smaller structures.