LOW-ALTITUDE IMAGE OF PARTICLE-ACCELERATION AND MAGNETOSPHERIC RECONFIGURATION AT SUBSTORM ONSET

We compare meridional distributions of auroral (0.3-20 keV) and energe tic (30-300 keV) particle fluxes measured by two low-altitude polar-or biting spacecraft which crossed the auroral oval before and shortly af ter the sudden substorm onset. 3-4 min after the substorm onset a very strong particle energization was observed with the particle flux incr eased by a factor 100-1000 at energies up to >300 keV. It was found in the middle of auroral oval, poleward of the isotropic boundary of ene rgetic electrons detected prior to the substorm onset. Interpreting th e variations of Energetic Particle (EP) flux and anisotropy (ratio of precipitated to trapped particle flux) in terms of particle scattering in the equatorial current sheet, we were able to characterize the con figuration of equatorial magnetotail. The major acceleration/precipita tion region in the center of auroral zone was inferred to be confined in the region of dipolarized magnetic field in the magnetotail. Furthe r poleward (tailward in the magnetotail) the isotropic electron distri butions indicated the presence of current sheet with very stretched ma gnetic field lines. Based on magnetic flux conservation in the flux tu be, we estimate the width of transition region between dipolarized reg ion and current sheet to be similar to 0.5R(E) (about the gyroradius o f plasma sheet proton in the equatorial magnetic field similar to 5 nT ). Such sharp transition may be formed by fast Earthward flow in the c urrent sheet colliding with the dipolarized magnetic shells. An eviden ce supporting this interpretation is that the explosive westward elect rojet was found at latitudes occupied by the current sheet but polewar d of the dipolarized region. We also show another event in which the d ipolarized region expanded up to the poleward boundary of auroral oval in 7 minutes after the substorm onset.