Introduction

Geomagnetic activity can be divided into two main categories, storms and substorms. Storms, the main contributors to space weather, are initiated when enhanced energy transfer from the solar wind (and related IMF) into the magnetosphere leads into intensification of ring current. The ring current development can be monitored with the Dst index. The following storm definition has been proposed by Gonzales et al. (1994):

Storm is an interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current strong enough to exceed some key threshold of the quantifying storm time Dst index.

Storm strength

Dst [nT]

Bz [nT]

dT [h]

Intense

-100

-10

3

Moderate

-50

-5

2

Small (typical
substorm!)

-30

-3

1

The electric field mentioned is composed of solar wind velocity V and southward IMF (Bz). Of these, the magnetic field is found to be more important, indicating that the mechanism for the energy transfer includes magnetic field merging. The thresholds and driving parameters could then be as as shown in the table.

According to the classical substorm injection hypothesis, ring current is enhanced via energization and injections of plasma sheet particles from the tail towards the inner magnetosphere during substorms, which are typical for storm times. However, this view has been under attack for some time now, and according to the works by Iyemori and Rao (1996) and Siscoe and Petschek (1997) the substorm expansion phases act as energy dissipation term and the southward IMF as an input term in the energy balance equation (see also McPherron, 1997).

The largest storms are often related to coronal mass ejections from the Sun (e.g., Gosling et al., 1991). In these cases, the related enhancements of solar wind velocity accompanied by southward IMF direction result into Sudden Storm Commencements (SSC). These storms are typically nonrecurrent or transient. The more moderate storms are often recurrent, i.e., they recur with the solar rotation period; see geomagnetic activity for more discussion about the observed periodicities in activity.

During a storm, auroral ovals become greatly disturbed, broadening and expanding equatorwards, particularly on the nightside. This brings the aurora to the skies of middle and low latitudes (see great aurora).

Phases of a storm

Storms are typically divided into three distinct phases according to the signatures in Dst:

References


See also Wikipedia on storms.