Dynamics of plasma, energetic particles, and fields near synchronous orbit in the nighttime sector during magnetospheric substorms

Abstract

We discuss two phases of the substorm-associated magnetospheric dynamics in terms of the particles and fields at synchronous orbit. The first phase corresponds to the 'decreases' of energetic particle flux first identified by Erickson and Winckler (1973) and discussed by Walker et al. (1976) and Erickson et al. (1979). This phase begins one-half hour to one hour before the substorm onset and is characterized by (1) a distortion of the magnetosphere to a more taillike configuration caused by (2) an intensification and/or motion toward the earth of the cross-taft current and of its earthward part, the partial ring current, (3) a shift of trapped particle trajectories closer to the earth on the nightside following contours of constant B causing the particle 'decreases,' accompanied by a change in the pitch angle distributions from 'pancake' to 'butterfly' as observed at geostationary orbit, (4) an initiation of a response of the auroral electrojet (AL e) index. The decreases of energetic particle flux can correspond to the substorm growth phase as defined initially by McPherron (1970) or the growth or precursor phase of Erickson et al. (1979). Plasma motions and currents during decreases tend to be variable, but the description above nevertheless characterizes the large-scale trend. It is suggested that the electric field induced by the increasing tail current near the earth acts opposite to the cross-taft convection field and can temporarily inhibit convection near the geostationary orbit. The second phase ;s the conventional expansion phase that begins with the 'onset,' characterized in our study by (1) a sudden decrease in the tail current and a return of the inflated magnetosphere to a dipolelike configuration, (2) a sudden shift of trapped high-energy particles toward the tail again following contours of constant B, and at the same time (3) a surge of tail plasma toward th6 earth as the induced electric field now increases the total convection field. Separate effects thus result in the dramatic increases of both high energy and plasma particles seen at sUbstorm expansion phase onset, (4) an AE index response and the appearance of bays at stations near midnight local time accompanied by very active aurora as well as the precipitation of high-energy part, icles. The different appearance of the responses at ATS I (on the magnetic equator) and ATS 6 (off the magnetic equator) can be well explained by the above description. True diamagnetic effects of the particle population are clearly evident at the ATS 6 region and must be carefully distinguished from the effects of distant currents. The use of oppositely directed detectors on ATS 6 that permit the evaluation of the guiding center particle density gradients has been of great use in this analysis. • On leave fromCentre d'Etude Spatiale des Rayonnements, Uni-versit6 Paul Sabatier, two opposite directions carried out on board ATS 6 have shown that during these flux decreases the outer radiation belts are actually observed to retract toward the earth in a region loc...

A statistical study of magnetic dipolarization for sawtooth events and isolated substorms at geosynchronous orbit with GOES data

“…particle injection) in energetic particle fluxes near local midnight at geosynchronous orbit is a typical feature of magnetospheric substorms (e.g. Sauvaud and Winckler, 1980). During substorms, a change in the magnetic field configuration is also observed which consists of a gradual stretching and rapid dipolarization at geosynchronous orbit (e.g.…”

Sporadic plasma sheet ion injections into the high‐altitude auroral bulge: Satellite observations

“…Magnetic field observations in the equatorial region of the inner magnetosphere and farther in the magnetotail lobe have shown that a reconfiguration of the magnetic field topology ("dipolarization") linked to a partial disruption of the near-Earth cross-tail current is associated with substorm onsets detected as breakups of auroral forms in the ionosphere [e.g., Sauvaud and Winckler, 1980;Lui, 1988Lui, , 1996 All the processes discussed so far are generally presented as single entities; i.e., an isolated substorm would be associated with a single injection, a plasma sheet thinning, and a plasmoid formation. There are, however, numerous experimental pieces of evidence showing that even single substorms are made of recurrent and impulsive processes.…”

Large Scale Dynamics of the Magnetospheric Tail Induced by Substorms: A Multisatellite Study

“…During periods of southward directed interplanetary magnetic field, when the solar wind energy can be directly coupled to the magnetosphere, the role of the cross tail current sheet has been recognized as fundamental in energy storage, plasma energetization andmagnetic fieldreconftguration (e.g., McPherron, 1972;Russell and McPherron, 1973;Lui, 1978;Sauvaud and Winckler, 1980;Nishida, 1983;Fairfield, 1984;Kaufmann, 1987;Lui et al, 1992).…”