JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. A11, PAGES 19,427-19,439, NOVEMBER 1, 1991 Multipoint Reconnection in the Near-Earth Magnetotail: CDAW 6 Observations of EnergeticParticles and Magnetic Field NAIGUO LIN, 1 R. L. MCPHERRON, 2 M. G. KIVELSON, 2 AND R.J. WALKER Institute o) e Geophysics and Planetary Physics, University o) e Caloeornia, Los Angeles The Coordinated Data Analysis Workshop (CDAW 6) substorm eventat 1054 UT on March 22, 1979,is studied using energetic particle(• 20 keV) data from the medium- energy particle experiment of ISEE I as well as high-resolution magnetometerdata of ISEE I and 2. Theseanalyses cast new light on the structureand temporalevolution of the plasma sheet near 15/•E during the substorm. Thesechanges are generally consistent with predictions from the near-Earth reconnection modelof substorms, modifiedto include subsidiary plasmaand field structure on scales of a few Earth radii. In particular, the onset of streaming flow, nearly aligned with magneticfield lines, of energeticions and electrons in all availableenergy channels and its reversal and subsidence are closely correlatedwith substorm phases. The energetic particles (which are isotropic duringthe growth phase) begin streamingtailward right after the expansion onset. They reverse their sense of flow in the midexpansion phase. The magnetic field variations suggest that two small magnetic island structures embedded within the plasma sheet moved tailward past the spacecraft shortly after the substorm expansion onset. Thesestructures may be signatures of small- scale multisite reconnection associated with the formation of the near-Earth neutral line. Somemagnetic and particle signatures not anticipatedin the description of the substorm based on a two-dimensional model of the neutral line are discussed and shown to be reasonablefor plausible extensions of the model. INTRODUCTION Dynamics of energetic particles in the magnetotail plays an important role in the substormprocess. By establishing the relationbetween observed streaming of particles and substorm phases onemay gainunderstand- ing of the onset and development of substorms. A sub- storm selected for intensiveanalysis in the Coordinated Data Analysis Workshop (CDAW 6) occurred at 1054 UT on March22, 1979 (see the overview by McPherron andManha [1985, andreferences therein]). Many char- acteristics of the event proved basicallyconsistent with the near-Earth reconnection model of substorms [e.g., McPherronand Manha: 1985; Paschmann et al., 1985; Ipavich et al., 1985;Baker,1984;Fritz et al., 1984], al- though some features were difficult to interpret in terms of a single,two-dimensional X line model. By utilizing energeticparticle data as well as high-resolution mag- netic data which have not beenusedin previous studies of the event we have interpretedthe event usinga mod- ified near-Earth reconnection model of substorms that gives plausibleinterpretations of somepreviously trou- blesome features observed in the substorm event. The CDAW 6 substorm which occurred at the time of equinoctal symmetry is typical of isolated substorms following intervals of magnetic calm. As described in McPherron and Manka [1985], a large increase in so- lar wind velocity and an enhancement of the northward 1 Now at Department ofPhysics, University of Minnesota, Minneapolis. 2Also at Department of Earth and Space Sciences, Uni- versity of California, Los Angeles. Copyright 1991 by the American Geophysical Union. Paper number 91JA01952. 0148-0227/91/91JA-01952505.00 interplanetary magnetic field (IMF) initiated a storm sudden commencement at ~ 0826 UT. At ~ 1010 UT, following a long period of northward Bz, the IMF turned southward for more than I hour. About 10 min later (at ~ 1020UT), the 57 stationAE indexbegan to increase. Although the IMF Bz turned southward at ~ 1010 UT [Baker et al., 1985], implying an increase of energy input from the solar wind, a sharp increase in AE valueswas not seen for more than 40 min, when at about 1100 UT, AE rose from moderate values to more than 1000 nT. Using ground magnetometer measurements and the in- crease of electron fluxes observed at synchronous orbit, 1054 UT was identified as the onset of the expansion phase of the substorm [McPherron and Manka,1985]. After reaching a peak at ~ 1140 UT the AE index de- creased until 1300 UT when another substorm onset in- terrupted its recovery. Overviews of plasmabehavior in the magnetotaildur- ing this substormhave been provided in earlier papers. For example,ISEE I and 2 plasmaobservations of ions with energy below40 keV and of electrons below20 keV were reported by Paschmann et al. [1985]. Ipavich et al. [1985] reported the measurements of energetic protons (10- 130 keV)and oxygen ions O + (~ 130 keV)using the ultralow energy charge analyzer (ULECA) on ISEE 1. Fritz et al: [1984] reported on the energetic electron and magnetic measurements from a set offour satellites locatedfrom 6.6 to 15 RE along the 0200 LT meridian at the time of the 1054 UT substorm. These observa- tions revealed flow patterns of charged particleswhich, if interpreted as temporalvariations associated with the variousphases of the substorm,are basicallyconsistent with the presence of reconnection betweenthe geosyn- chronous orbit andthe ISEE i location (~ 15 RE) near the 0200 LT meridian. 19,427