Possible Connection of Field and Particle Fluctuations in Near-Earth Tail to Pre-Breakup Evolution of Auroral Arcs

Abstract:

The onset process of substorm is one of the most controversial issues of magnetospheric physics. Attention has been paid to wave signatures in the near-Earth plasma sheet observed during several minutes prior to the onset because they are believed to be a manifestation of a plasma instability that may trigger the substorm. The ballooning instability in the near-Earth tail has been considered as one of onset instability candidates, but there is still a controversy about its excitation and the role in the auroral substorm initiation. On the other hand, recent auroral observations from the ground and spacecraft have shown that in the pre-breakup arc, there exists a characteristic small-scale auroral structure, which consists of azimuthally arrayed and propagating wave-like forms. The typical azimuthal wave number is ~100-300, comparable to the ion gyro radius in the near-Earth plasma sheet. The auroral forms move at azimuthal propagation speeds usually of the order of several km/s, much faster than the ionospheric convection. Based on these results, one may interpret the auroral forms as a visible counterpart of ballooning modes developing in the near-Earth tail. Despite recent efforts to find such a manifestation, however, little one-to-one correspondence between pre-breakup wave processes in the optical aurora (ionosphere) and in the near-Earth tail has been investigated. In this presentation we report simultaneous obaservations of the evolution of pre-breakup auroral arc using ground-based all-sky imager(s) and near-Earth spacecraft (THEMIS, Van Allen Probes) that were favorable for magnetic conjunction with the arc. The near-Earth spacecraft measured field and particle oscillations with almost the same period as that deduced from the fine-scale structures of pre-breakup auroral arc. We will discuss the possible connection based on such simultaneous observations.