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The expansion phase starts the active part of the substorm process. It is related to the unloading of energy stored during the growth phase.

  • Lasts typically around 30 min (varying from 10 min to 2 h).
  • Start of the phase is called the onset of the substorm. In optical auroral data, term breakup is typically used.
  • Onset may be triggered externally by changes in the IMF/solar wind characteristics
  • McPherron and Hsu (ICS-4 meeting, 1998) advocated a concept of "main substorm onset" to deal with observations of multiple onsets. See also substorm finestructure.
  • In the magnetosphere, Earthward high speed plasma flows are observed within the near-Earth plasma sheet, and plasmoids are detected in the far tail.
  • Part of the near-Earth cross-tail current disappears in what is called the current disruption. The interrupted current closes itself via the ionospheric current system, creating the substorm current wedge (SCW).
  • As a consequence, the magnetic field returns to a more dipolar configuration ("dipolarization").
  • Injections of accelerated (tens of keV) particles are seen close to the geosynchronous orbit. Drifting electron holes (DEH) are launched.
  • Often low-frequency magnetic field oscillations are during and after the onset (e.g., Bauer et al., 1995; Holter et al., 1995; Ohtani et al., 1998)
  • Sometimes in connection with the injections, high-energy (16-80 keV) field-aligned electron beams have been seen at geosynchronous orbit (Kremser et al., 1988). They start about 4 minutes after the onset (from Pi2) and last for about 1.5 min.
  • X-ray imaging has shown that energetic electron injections at the nightside are very impulsive, with several successive impulses that last from less than a minute to a few minutes (Oestgaard et al, ICS-4 meeting, 1998). It seems that the electrons also drift towards dayside, where they precipitate at 04-12 MLT; the delay time indicate energies between 20 and 40 keV.
  • A new substorm feature may be found by using ENA imaging of the magnetosphere (Henderson et al., 1997). The bursts are spatially confined, with durations from a few tens of minutes to a few hours.
  • Long lasting low-latitude proton precipitation events are seen for several hours after strong substoms (Gvodzdevsky et al., 1997); see also isotropic boundary.
  • On ground, aurora suddenly brightens and expand in the nightside, starting from the southernmost arc near the poleward edge of the diffuse aurora. An auroral bulge with a westward travelling surge (WTS) forms. Note that some observations indicate that dayside auroral activity precedes the nightside onset.
  • Ground based magnetometers show magnetic bays (McPherron et al., 1973; Clauer and Kamide, 1985) due to the SCW related substorm electrojet. Pi 2 and Pi B pulsations can be also used as higher time resolution ground based indicators of the onset.
  • Substorm electrojet shows also signatures of transient burst of enhanced equatorward flow (about 600 m/s for 5 min, repetition period about 8 min; Yeoman and Luhr, 1997)
  • Expansion phase continues until the aurora has reached its most poleward location. This may relate to the SCW propagation tailward.
  • Also IPDP and compressional Pc 5 pulsations are shown to be substorm time phenomena. Supression of cusp related Pc 5 pulsation occurs at the substorm onset (e.g., Pipilenko et al. and Kleimenova et al., ICS-4 meeting, 1998).
  • VLF whistler mode noise, labeled as substorm-related chorus events (SCEs) occur due to gyroresonance instability as clouds of injected electrons drift eastward (Smith et al., 1996)

Note that Mishin et al. (1997) have argued that the active phase could be divided into two separate parts. In this scenario the real expansion phase is preceded by a "phase of multiple onsets". During this period the processes are thought to be, primarily, driven, and only during the second part the unloading part of the substorm commences.


  • Aggson, T. L., J. P. Heppner, and N. C. Maynard, Observations of large magnetospheric electric fields during the onset phase of a substorm, J. Geophys. Res., 88, 3981-3990, 1983.
  • Bauer, T. M., W. Baumjohann, R. A. Treumann, and N. Sckopke, Low-frequency waves in the near-Earth plasma sheet, J. Geophys. Res., 100, 9605-9617, 1995.
  • Clauer, C. R., and Y. Kamide, DP 1 and DP 2 current systems for the March 22, 1979 substorms, J. Geophys. Res., 90, 1343-1354, 1985.
  • Gvodzdevsky, B. B., V. A. Sergeev, and K. Mursula, Long lasting energetic proton precipitation in the inner magnetosphere after substorms, J. Geophys. Res., 102, 24333-24338, 1997.
  • Henderson, M. G., G. D. Reeves, H. E. Spence, R. B. Sheldon, A. M. Jorgensen, J. B. Blake, and J. F. Fennell, First energetic neutral atom images from Polar, Geophys. Res. Lett., 24, 1167-1170, 1997.
  • Holter, O., C. Altman, A. Roux, S. Perraut, A. Pedersen, H. Pecseli, B. Lybekk, J. Trulsen, A. Korth, and G. Kremser, Characterization of low frequency oscillations at substorm breakup, J. Geophys. Res., 100, 19109-19119, 1995.
  • Kremser, G., A. Korth, S. L. Ullaland, S. Perraut, A. Roux, A. Pedersen, R. Schmidt, and P. Tanskanen, Field-aligned beams of energetic electrons (16 keV < E < 80 keV) observed at geosynchronous orbit at substorm onset, J. Geophys. Res., 93, 14453-14464, 1988.
  • McPherron, R. L., C. T. Russell, and M. P. Aubrey, Satellite studies of magnetospheric substorms on August 15, 1968, 9, Phenomenological model for substorms, J. Geophys. Res., 78, 3131-3149, 1973.
  • Mishin, V. M., L. P. Block, A. D. Bazarzhapov, T. I. Saifudinova, S. B. Lunyushkin, D. Sh. Shirapov, J. Woch, L. Eliasson, G. T. Marklund, L. G. Blomberg, and H. Opgenoorth, A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases, J. Geophys. Res., 102, 19845-19859, 1997.
  • Ohtani, S., K. Takahashi, T. Higuchi, A. T. Y. Lui, H. E. Spence, and J. F. Fennell, AMPTE/CCE-SCATHA simultaneous observations of substorm-associated magnetic fluctuations, J. Geophys. Res., 103, 4671-4682, 1998.
  • Smith, A. J., M. P. Freeman, and G. D. Reeves, Postmidnight VLF chorus events, a substorm signature observed at the ground near L = 4, J. Geophys. Res., 101, 24641-24653, 1996.
  • Yeoman, T. K., and H. Luhr, CUTLASS/IMAGE observations of high-latitude convection features during substorms, Ann. Geophysicae, 15, 692-702, 1997.
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