GEOMAGNETIC SUBSTORMS

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Geomagnetic Substorms

A geomagnetic substorm is generally divided into 3 phases, termed the growth phase, the expansion phase and the recovery phase. Substorm studies are an extremely active area of research, and many of the details remain controversial. This is due, in part, to the fact that the substorm involves the complete solar-wind/magnetosphere/ionosphere system. Different researchers therefore tend to have a particular slant on the process, depending on which part of the system they consider most important. The following brief description is based on the "Near-Earth Neutral Line" (NENL) model which is one of the more popular interpretations.

The growth phase of the substorm cycle generally begins with a southward turning of the interplanetary magnetic field, which facilitates an increase in the dayside reconnection rate. Reconnected field lines are dragged over the poles of the Earth by the continued flow of the soalr wind, thus creating an increase in the ionospheric dawn-dusk electric field in accordance with Faraday's law. Since the tail does not immediately respond, there is a net increase in the amount of open magnetic flux in the magnetosphere. This manifests itself as an expansion of the ionospheric polar cap equatorwards, and the magnetic field observed by spacecraft in geosynchronous orbit changes from dipole-like to a more stretched tail-like direction. In the magnetotail itself, the extra magnetic flux causes the magnetopause to flare outwards, and the magnetic content of the lobes begins to increase. This in turn causes a thinning of the plasma sheet, and an intensification of the cross tail currents. The magnetic energy content of the magnetotail thus increases during the growth phase. In the late growth phase, reconnection may begin at a new neutral line, at distances of 20-30 Earth radii (RE) down the tail. This is somewhat closer to the Earth than the typical location of the pre-existing quiet time neutral line.

The expansion phase onset represents a switch from relatively slow storage of energy in the magnetotail, to a very rapid dissipation and release. Fast reconnection of open magnetic flux occurs at the NENL, reducing the magnetic flux content of the tail lobes. The energy released creates accelerated plasma flows - energetic particles injections are observed at geosynchronous orbits, and the magnetic field in this region rapidly dipolarizes. This dipolarization is associated with a diversion of the cross tail current as a downward field-aligned current (FAC) on the dawnside of the magnetosphere, and an upward FAC on the duskside. These currents close across the auroral ionosphere, increasing the westward electrojet and causing significant magnetic disturbances on the ground. At the same time auroral arcs brighten and expand, creating magnificent auroral displays visible from the ground. The injection of energetic particles, the joule heating by the increased ionospheric currents and the auroral activity account for the dissipation of about 50% of the substorm energy budget (~10^16 Joules). The remaining 50% is dissipated by processes occurring tailward of the NENL. In this region, the onset of reconnection at the NENL first creates closed loops of magnetic flux as field lines that have already been reconnected at the pre-existing deep-tail neutral line (DNL) are again reconnected. This "bubble" of magnetic flux and plasma, which is called a "plasmoid" grows until the NENL overtakes the DNL and begins to reconnect open field lines. At this stage the plasmoid is magnetically disconnected from the Earth and is rapidly ejected tailwards. Plasmoids may typically be 30-60 RE in length, and 15-20 RE in height and width. They are observed moving down the tail following substorm onset with speeds of order 5-600 km/s. As they are ejected they compress and deflect the magnetic structure of the tail, creating the characteristic "Travelling Compression Region" (TCR) signature which is observed in the tail lobes. The magnetic, thermal and kinetic energy of the ejected plasma accounts for the other ~50% of the substorm energy budget.

The substorm cycle ends with the recovery phase, in which the magnetosphere relaxes back into its quiet time configuration. Auroral activity dies away, the NENL retreats downtail, the plasma sheet thickens and the cross-tail current declines. The typical timescale for an isolated substorms is of order 1-2 hours, although with continued driving (southward IMF Bz) substorms may occur as multiple events in rapid succession. In these events, multiple dipolarizations, injections, auroral intensifications, plasmoids, etc., may be observed.

Schematic showing the formation and release of a plasmoid structure following substorm onset.