Coordinated ionospheric observations indicating coupling between preonset flow bursts and waves that lead to substorm onset

Y. Nishimura, L. R. Lyons, M. J. Nicolls, D. L. Hampton, R. G. Michell, M. Samara, W. A. Bristow, E. F. Donovan, E. Spanswick, V. Angelopoulos, S. B. Mende

Research output: Contribution to journalArticlepeer-review

Abstract

A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm expansion phase onset. Recent Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) array observations have shown a repeatable preonset sequence, which is initiated by a poleward boundary intensification (PBI) and is followed by auroral streamers moving equatorward (earthward flow in the plasma sheet) and then by substorm onset. On the other hand, substorm onset is also preceded by azimuthally propagating waves, indicating a possible importance of wave instability for triggering substorm onset. However, it has been difficult to identify the link between fast flows and waves. We have found an isolated substorm event that was well instrumented with the Poker Flat incoherent scatter radar (PFISR), THEMIS white-light ASI, and multispectral ASI, where the auroral onset occurred within the PFISR and ASI fields of view. This substorm onset was preceded by a PBI, and ionospheric flows propagated equatorward from the polar cap, crossed the PBI, and reached the growth phase arc. This sequence provides evidence that flows from open magnetic field lines propagate across the open-closed boundary and reach the near-Earth plasma sheet prior to the onset. Quasi-stable oscillations in auroral luminosity and ionospheric density are found along the growth phase arc. These preonset auroral waves amplified abruptly at the onset time, soon after the equatorward flows reached the onset region. This sequence suggests a coupling process where preexisting stable waves in the near-Earth plasma sheet interact with flows from farther downtail and then evolve to onset instability. Key Points An isolated substorm was preceded by a PBI, flow channel, and waves Precursor weak waves are found to connect to onset waves We suggest a coupling process of preonset waves and flows for onset instability

Original languageEnglish (US)
Pages (from-to)3333-3344
Number of pages12
JournalJournal of Geophysical Research: Space Physics
Volume119
Issue number5
DOIs
StatePublished - May 2014

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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