GEM: The Time Sequence of Substorms in GEM Campaigns: A Comparison of Individual Events to Statistical Averages

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A magnetospheric substorms is the collection of processes by which energy is extracted from the solar wind and deposited in the magnetosphere, ionosphere and earth's atmosphere. Substorms have significant effects on the near-earth environment and affect many of man's systems. The represent a natural hazard to human activities that is becoming more important. Unfortunately, the cause of substorms, particularly of the most destructive phase, the expansion phase, is not known. For may years the most popular physical model explaining substorms has been the near-earth neutral line (NENL) model. This model has been remarkably successful in organizing numerous observations in the near-earth plasma sheet and successful predicted the existence and properties of the magnetotail plasmoid, a bubble of plasma and magnetic field that the earth releases to the solar wind during substorms. A number of new observations have not been incorporated in the NENL model, and it appears difficult to do so. It seems that the substorms expansion begins at local midnight very close to synchronous orbit (6.6 Re) and propagates outward into the tail. This expansion begins on closed, quasi-dipolar field lines at the inner edge of the cross-tail current sheet. Subsequently it moves tailward by disrupting the tail current, diverting it along magnetic field lines to the auroral ionosphere. A new model called the current disruption (CD) model is being developed to explain these observations. Some versions of this model predict dynamics of the tail plasma sheet and magnetic field that are diametrically opposite to the predictions of the NENL model, and the observations on which it is based. Such a fortunate circumstance should be easy to resolve. The model either agrees or disagrees with observations and we should be able to decide between them. It is proposed to re-examine the observations in the near-earth plasma sheet and place them on a firmer footing so that it will be pos sible to develop a reasonable physical model of the substorm expansion phase. To this end, data already at UCLA, and additional data from other investigators will be used to study the problems of frequency of occurrence and time delay of phenomena relative to substorms onset. Other ground indicators of substorm onset will be used including brightenings in satellite auroral images, midlatitude positive bays, Pi 2 pulsation bursts, maxima in inclination of synchronous magnetic field, and dispersionless plasma injection at synchronous orbit. Features to be examined include changes in magnetic field, particle fluxes, plasma pressure, plasma flow and temperature. Based on these results, the different models and their success in explaining the observations will be considered.