By numerical simulations, it is shown that the evolution of a westward-traveling surge can be identified with the aurora deformation due to the appearance of a westward plasma flow at the boundary between the open and closed field lines. It is assumed that the westward flow is generated from nonadiabatic proton acceleration in the region of magnetic field reconnection by enhancement of the westward inductive electric field. The numerical simulation is performed by using the two-dimensional electrostatic particle code, which is capable of studying plasma dynamics in a plane perpendicular to the geomagnetic field. The present model for the westward-traveling surge assumes the following background state (1) and (2) in the plasma sheet boundary layer magnetically connected to the high-latitude auroral ionosphere. (1) A well potential (Δϕ<-10 kV) is distributed uniformly in the azimuthal direction, being responsible for a large-scale inverted-V structure. (2) A number of azimuthally aligned sheets of dense plasmas (which are called arc sheets) exist in the large-scale potential well.

The electrons in such arc sheets can be accelerated by field-aligned electric fields distributed at low altitudes, and they can thus produce a number of discrete arcs longitudinally elongating at the ionospheric altitude. Initially, a westward plasma flow appears (near local midnight) at the last closed magnetic shell, inside which a number of the arc sheets are located. The temporal and spatial evolution of the discrete arcs are numerically followed in a plane perpendicular to the geomagnetic field. The simulation results show that some discrete arcs can convolute in a few minutes and form the surge structure. The surge head is found to typically move westward with a speed of ~1 km/s. Convolution of discrete arcs is due to local accumulation of positive space charges carried by the accelerated protons. Such positive charge accumulation in the magnetosphere is not strong so that the auroral electrons can still be accelerated toward the ionosphere by the field-aligned potential drop resulting from the potential well. Both effects of the diminished field-aligned potential drop and electron acceleration in the reconnection region may lead to production of the flat and hard differential energy spectrum of precipitating electrons observed inside a surge. A remarkable similarity in dynamics between the numerically simulated surges and the observed ones strongly suggests that the westward-traveling surge is driven by a local westward plasma flow at the boundary between the open and closed field lines. [Recently, the high-altitude (~10,000 km) observations from the EXOS-D satellite (T. Yamamoto et al. manuscript in preparation, 1993) have first revealed the existence of a plasma jet just poleward of a surge.> An expanding auroral bulge can also be reproduced by assuming wider (in latitude) spreading of the particles accelerated in the reconnection process. ¿ American Geophysical Union 1993