Substorm expansion phase
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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
- 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
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
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.
- 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)
- 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.