Low Altitude Image of Particle Acceleration and Magnetospheric Reconfiguration at Substorm Onset

Abstract

We compare meridional distributions of auroral (0.3-20 keV) and energetic (30-300 keV) particle fluxes measured by two low-altitude polar-orbiting spacecraft which crossed the auroral oval before and shortly after the sudden substorm onset. 3-4 min after the substorm onset a very strong particle energization was observed with the particle flux increased 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 energetic electrons detected prior to the substorm onset. Interpreting the 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 configuration of equatorial magnetotail. The major acceleration/precipitation region in the center of auroral zone was inferred to be confined in the region of dipolarized magnetic field in the magnetotail. Further poleward (tailward in the magnetotail) the isotropic electron distributions indicated the presence of current sheet with very stretched magnetic field lines. Based on magnetic flux conservation in the flux tube, we estimate the width of transition region between dipolarized region and current sheet to be ∼0.5RE (about the gyroradius of plasma sheet proton in the equatorial magnetic field ∼5 nT). Such sharp transition may be formed by fast Earthward flow in the current sheet colliding with the dipolarized magnetic shells. An evidence supporting this interpretation is that the explosive westward electrojet was found at latitudes occupied by the current sheet but poleward of the dipolarized region. We also show another event in which the dipolarized region expanded up to the poleward boundary of auroral oval in 7 minutes after the substorm onset.