Observations of Changes to the Auroral Distribution Prior to Substorm Onset

  1. Joseph R. Kan,
  2. Thomas A. Potemra,
  3. Susumu Kokubun and
  4. Takesi Iijima
  1. R. D. Elphinstone1,
  2. J. S. Murphree1,
  3. L. L. Cogger1,
  4. D. Hearn1,
  5. M. G. Henderson1 and
  6. R. Lundin2

Published Online: 19 MAR 2013

DOI: 10.1029/GM064p0257

Magnetospheric Substorms

Magnetospheric Substorms

How to Cite

Elphinstone, R. D., Murphree, J. S., Cogger, L. L., Hearn, D., Henderson, M. G. and Lundin, R. (1991) Observations of Changes to the Auroral Distribution Prior to Substorm Onset, in Magnetospheric Substorms (eds J. R. Kan, T. A. Potemra, S. Kokubun and T. Iijima), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM064p0257

Author Information

  1. 1

    Department of Physics and Astronomy, University of Calgary, Calgary, Alberta

  2. 2

    Swedish Institute of Space Physics, Kiruna, Sweden

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1991

ISBN Information

Print ISBN: 9780875900308

Online ISBN: 9781118663981



  • Magnetospheric substorms—Congresses


The temporal development of the auroral distribution is described for a time period (≈40 minutes) before substorm expansion phase onset. Initial auroral activity is observed to begin in the dayside ionosphere and then shift to the nightside. The dayside precursor events are associated with the growth of eastward (afternoon) and westward (morning) electrojets shortly after the interplanetary magnetic field turns southward. During this time, at both noon and midnight, equatorward motions of the peak intensity aurora are observed with speeds of 300 to 500 m/s. Longitudinal motions within the auroral “oval” during this “growth” phase tend to be eastward in the afternoon sector and westward in the morning. These motions are very rapid, sometimes exceeding 15 km/s in the ionosphere. It appears that the stable afternoon sector aurora near 16 MLT is likely to be associated with nightside magnetospheric activity. On some occasions, however, the dayside auroral dynamo may be associated with transient auroral forms superimposed on the more stable aurora originating from the nightside magnetotail. A remarkable set of combined observations by the Viking satellite and ground magnetometer data illustrates this just a few minutes before the start of an auroral substorm. A brief outline of a possible magnetospheric interpretation to these observations is presented whereby a solar wind disturbance separately affects the dayside and nightside magnetospheric dynamos. This first creates the observed dayside auroral phenomena, and eventually perturbs the cross-tail current circuit triggering the substorm expansion phase in the near Earth region.