Substorm models

spaceweb@oulu.fi - last update: 11 May 1998, 1830 UT (RR)

Introduction

A magnetospheric substorm is a very complicated phenomenon that is not yet fully understood. Perhaps the most difficult questions are as follows. While the substorm related cross-tail current disruption and the formation of the SCW take place in the near-Earth tail, <15 Re, the observed high speed plasma flows indicate that important substorm processes occur at about 20-30 Re. How do these processes relate to each other? What is the exact timing between these processes? And: what are the microphysical processes that create these processes? What is the connection between daytime processes and substorm onset region?

Near-Earth Neutral line, NENL

The near-Earth Neutral line (NENL; e.g., Baker et al., 1996) model assumes that reconnection takes place between the oppositely directed field lines above and below the current sheet at a distance of about 20 - 30 Re (this is considered near-Earth as opposed to the distant tail reconnection).

When reconnection starts, it may take a while before it reaches the open field lines of the lobe (where most of the free energy is). The closed field line reconnection may provide many substorm-like signatures before the main substorm onset.

Note that the observed high speed flows (including plasmoids) are naturally explained by this model. The cross-tail current disruption and the formation of the SCW are considered to be secondary effects.

Cross-field current instability model, CCI

In the cross-field current instability (CCI ; e.g., Lui, 1996) model(s) the substorm is initiated as a current driven plasma instability creates the current wedge, and the reconnection may play no role at all, or only a secondary role in creating the plasmoid. (Note, however, that very few oppose the idea that reconnection does occur at the dayside magnetopause and the distant tail.) This model places the onset region much closer to the Earth than the NENL model.

Others

Many other theories have been proposed:

Fast flow braking model. Development of the NENL model by Shiokawa et al. (1998). Links the SCW and NENL by braking and pileup of the high speed earthward plasma flows.

Phase 1X type reconnection starts (20-30 Re)
Phase 2High speed ion flows start in the neutral sheet (10-20 Re)
Phase 3Flow braking occurs at the boundary between dipolar and tail-like field (10-20 Re)
Pileup of magnetic flux occurs
Dawnward inertia current is formed
Phase 4SCW forms
Compressional pulse is launched towards Earth
Phase 5Initial brightening of aurora
Pi 2 pulsations
Particle injections

It is important to note that this model is applicable only to the initial state of the substorm expansion phase since the duration of high-speed flow is only a few minutes. Some additional process (azimuthal gradient of plasma pressure?) are needed to keep the substorm current system running throughout the whole substorm.

Ballooning models. The models invoking the ballooning mode instability as the substorm initiator place the onset very close to Earth, just like the CCI model (Roux et al., 1991). Samson et al (ICS-4 meeting, 1998) have presented another version of it, called the shear flow ballooning instability.

Note that the ballooning theory has also been objected (Ohtani and Tamao, 1993).

Magnetosphere-ionosphere coupling (MIC) or globally integrated substorm (GIS) model. Magnetosphere-ionosphere coupling (MIC) stresses the role of ionosphere, and especially its conductivity, in the substorm development. The model was recently modified so that it can produce the explosive onset phase previously missing (Kan and Sun, 1996). As a result, the model was effectively reduced to a version of NENL model, the main disagreement being the relative importance of the ionosphere in the substorm process. Later Kan (ICS-4 meeting, 1998) has argued for a globally integrated substorm (GIS) model that directly integrates NENL and MIC models.

Note that it is quite possible that ionosphere plays some role in the substorm process. At least it provides oxygen source for the magnetosphere, and its role in the substorm physics is one of the main open questions. It has also been argued that since the ionospheric conductivity can control the auroral activity (Newell et al., ?), the ionospheric manifestation of substorm development can be affected by it. Ionosphere may also be important because of the decoupling of the magnetospheric convection from the ionosphere through the development of parallel electric fields (Hearendel, ?).

Also the empirical substorm scenario by Maynard et al. (1996) calls for bouncing Alfven waves that provide electromagnetic communication between the ionosphere and plasma sheet.

Convection reduction model. Another work often mentioned is the one by Lyons (1995) which stresses the influence of the IMF in triggering the substorms (note that the possibility of an external trigger is accepted also by other substorm theories). The idea is that a sudden reduction of convection electric field in the near-Earth plasma sheet reduces energization and earthward drift of the plasma sheet particles. The resulting dawn-to-dusk gradient in proton drift speed causes an azimuthally localized pressure minimum in the near-Earth plasma sheet and development of FACs (and SCW).

It has been claimed (Lyons, ICS-4 meeting, 1998) that many ground-based observations support the theory. For example, the disappearance of some dayside convection vortices at substorm expansion phase (Greenwald et al., 1996) is not easily explained by current substorm models, where the onset is considerd to be very localized.

Boundary layer dynamics (BLD) model. Because the substorm related plasma sheet high speed flows were originally observed only within PSBL, the NENL model of substorms was questioned, and a boundary layer model with Kelvin-Helmholtz instability was suggested as an alternative (Rostoker and Eastman, 1987). Recently updated by Rostoker (1996).

Thermal catastrophe model. In this model by Goertz and Smith (1989) waves powered by the solar wind propagate through the magnetotail lobes toward the center of the tail, and transfer energy to the high-latitude plasma sheet through resonant absorbtion of Alfven waves. However, apert from the heating of magnetotail plasma, this model does not explain other substorm features.

References

See also: