A MECHANISM TO PRODUCE A DAWN-DUSK COMPONENT OF PLASMA-FLOW DURING MAGNETIC RECONNECTION IN THE MAGNETOTAIL

Magnetic reconnection between antiparallel field lines in the magnetot ail is generally thought to produce plasma acceleration in the earthwa rd-tailward direction. However, measurements of the plasma velocity in the magnetotail during substorm activity sometimes reveal a dawn-dusk component of plasma flow. In this paper, we show that a dawn-dusk com ponent of plasma acceleration may be produced during reconnection if t he neutral line is not perpendicular to the magnetic field. In this ca se, MHD simulations show that reconnection between antiparallel field lines will initially produce plasma acceleration that is nearly parall el to the neutral line because the magnetic tension force is not oppos ed by a pressure gradient force in this direction. As the magnetic fie ld topology evolves to a steady state, the plasma flow direction rotat es until it is nearly parallel to the plane that initially contained t he antiparallel magnetic field lines before reconnection (hereafter re ferred to as the initial field plane). However, the time required to r each a steady state (typically several hundred seconds in the magnetot ail region) may be greater than the time during which the reconnection process is active. Consequently, bursts of plasma flow with a dawn-du sk component may occur in the magnetotail. The initial acceleration al ong the neutral line depends on the angle (theta(B)) between the neutr al line and the initial field plane, with the largest burst of plasma flow along the neutral line occurring when theta(B) = 45-degrees.