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Magnetospheric Substorms

A magnetospheric substorms consists of an ordered sequence of physical processes that take place when the interplanetary magnetic field carried past the earth by the solar wind turns southward relative to the earth's magnetic field. A process called magnetic reconnection begins at the subsolar magnetopause allowing the two magnetic fields to interconnect. The earth's magnetic field is stripped from the day side and transported by the solar wind over the polar caps and drawn out into a long magnetic tail. These stretched field lines eventually reconnect and snap back towards the earth. Particles injected by these collapsing field lines both precipitate into the atmosphere creating aurora and ionospheric electrical currents, and enter closed drift paths around the earth forming the radiation belts.

Professor McPherron and his group have been pioneers in the development of a model of this process called the near-earth neutral line model of substorms [button to IGPP List]. For nearly twenty years this model has been the standard paradigm explaining substorm phenomena. Recently the model has come under attack because several sets of new observations do not appear to be easily incorporated in the model. As a consequence, intense controversy exists at the present time about whether

magnetic reconnection is the fundamental process responsible for the collapse of the tail field, or whether another process called current disruption is more important.

One recent group contribution to the subject of magnetospheric substorms includes the work of Professor Jeff Sanney of Loyola Marymount University, a NASA JOVE (Joint Venture) Scholar mentored by Prof. McPherron, and Dr. Tuija Pulkkinen of the Finnish Meteorological Observatory in Helsinki, Finland, a scientific collaborator. Sanny's paper [button to IGPP List] uses ISEE magnetometer data in the tail to show that current sheet thinning is a precursor to the onset of the collapse phase of a substorm. Pulkkinen's paper establishes this result in a different way using global magnetic models. Another contribution is work by Prof. McPherron with Dr. Ed Hones of the Los Alamos National Laboratory. This paper [button to IGPP List] uses ground magnetometer data to establish that past reports of near- earth reconnection occur at the center of the region of tail collapse, not outside as claimed by proponents of current disruption. Mr. Tung-Shin Hsu, a new student working with Prof. McPherron has begun a statistical study of the relation of substorm phenomena in the tail to a variety of indicators of tail collapse in an attempt to better understand which processes are important in its initiation.