[P-EM07] Dynamics in magnetosphere and ionosphere

[PEM07-05] Near-Earth magnetotail and auroral arc development associated with substorm onset: A new interpretation of substorm triggering

*宮下 幸長¹、平木 康隆²、Angelopoulos Vassilis³、家田 章正¹、町田 忍¹ (1.名古屋大学宇宙地球環境研究所、2.電気通信大学情報理工学部、3.Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles)

キーワード：substorm, auroral arc, auroral breakup, magnetotail, magnetic reconnection, substorm triggering

Using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and ground-based observatories at high temporal and spatial resolutions, we studied the time sequence of near-Earth magnetotail and auroral arc development associated with a substorm onset. We discuss four steps of auroral development, auroral fading, initial brightening of an auroral onset arc, enhancement of the arc's wave-like structure, and poleward expansion, and link them to magnetotail changes. A case study shows that near-Earth magnetic reconnection began at X ~ -17 Re at least ~1 min before auroral fading and ~3 min before initial auroral brightening. Large-scale ionospheric convection was also enhanced just before auroral fading and before initial auroral brightening. Then low-frequency waves were amplified in the plasma sheet at X ~ -10 Re, with the pressure increase likely due to arrival of an earthward flow from the near-Earth reconnection site ~4 min after initial auroral brightening and ~50 s before enhancement of the wave-like auroral structure. Dipolarization began ~7 min after initial auroral brightening and ~30 s before auroral poleward expansion. On the basis of these observations, we suggest that near-Earth magnetic reconnection plays two roles in substorm triggering. First, it generates a fast earthward flow and Alfven waves. When the Alfven waves, which propagate much faster than the fast flow, reach the ionosphere, large-scale ionospheric convection is enhanced, leading to auroral fading, initial brightening, and gradual growth of the wave-like auroral structure. Second, when the reconnection-initiated fast flow reaches the near-Earth magnetotail, it promotes rapid growth of an instability, such as a ballooning instability, and the wave-like auroral structure is further enhanced. When the instability has grown sufficiently, dipolarization and auroral poleward expansion are initiated.