Substorm expansion phase

spaceweb@oulu.fi - last update: 4 September 1998, 1010 UT (RR)

• 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
• Recently McPherron and Hsu (ICS-4 meeting, 1998) advocated a concept of "main substorm onset" to deal with observations of multiple onsets. See also substorm fine structure.

• 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. Pi2 and PiB 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)

References

• 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.

See also:

• Space Physics Text Book of Oulu