Categorization of the Time Sequence of Events Leading to Substorm Onset Based on THEMIS All-Sky Imager Observations

  • Y. Nishimura
  • L.R. Lyons
  • S. Zou
  • V. Angelopoulos
  • S.B. Mende
Part of the IAGA Special Sopron Book Series book series (IAGA, volume 3)


The sequence of events leading to substorm auroral onset has been a long-standing issue in substorm research. Based on statistical studies using THEMIS all-sky imager data, we have recently reported evidence that most substorm onset events are preceded by a pre-onset auroral form which is a distinct north-south arc originating from an poleward boundary intensification (PBI) and reaches the auroral onset region just before onset. This onset sequence was found to be a repetitive process; it is detected in 84% of 249 events between November 2007 and April 2008. A high occurrence of PBIs (84%) emphasizes an abrupt flux transport across the open-closed field line as initiation of the onset sequence. Here we present a variation of the onset sequence we have previously reported and two less frequently observed types of onset time sequence: poleward boundary contact and Harang aurora deformation. While poleward boundary contact events also start with PBIs, the auroral oval width becomes much narrower (∼2° MLAT) prior to onset, indicating that the plasma sheet is thin and the nightside magnetic separatrix is located closer to the near-Earth onset region. Harang auroral deformation events are not associated with an observed PBI, but the equatorward portion of a pre-existing Harang aurora bends equatorward, which indicates a rapid convection change leading to onset. All of those three categories of events suggest that new plasma intrusion toward onset location changes the pressure profile in the near-Earth region and leads to onset instability.


Plasma Sheet Magnetic Local Time Auroral Oval Substorm Onset Onset Location 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



This work was supported by National Science Foundation grants ATM-0646233 and ATM-0639312, NASA grant NNX07AF66, NASA contract NAS5-02099, and JSPS Research Fellowships for Young Scientists. Deployment of the THEMIS ASIs was partly supported by CSA contract 9F007-046101. Alaska magnetometer data were obtained from Geophysical Institute of University of Alaska, Fairbanks. NOAA satellite data were provided through the NOAA’s National Geophysical Data Center.


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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  • Y. Nishimura
    • 1
    • 2
  • L.R. Lyons
    • 1
  • S. Zou
    • 1
    • 3
  • V. Angelopoulos
    • 4
  • S.B. Mende
    • 5
  1. 1.Department of Atmospheric and Ocean SciencesUniversity of CaliforniaLos AngelesUSA
  2. 2.Solar-Terrestrial Environment LaboratoryNagoya UniversityFurocho, Chikusa, NagoyaJapan
  3. 3.Department of Atmospheric, Oceanic and Space SciencesUniversity of MichiganAnn ArborUSA
  4. 4.Institute of Geophysics and Planetary Physics, University of CaliforniaLos AngelesUSA
  5. 5.Space Science LaboratoryUniversity of CaliforniaBerkeleyUSA

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