Particle acceleration in the dynamic magnetotail: Orbits in self-consistent three-dimensional MHD fields

The acceleration of protons in a dynamically evolving magnetotail is investigated by tracing particles in the fields obtained from a three-dimensional resistive magnetohydrodynamic (MHD) simulation. The MHD simulation, representing plasmoid formation and ejection through a near-Earth reconnection process, leads to cross-tail electric fields of up to approximately 4 mV/m with integrated voltages across the tail of up to approximately 200 kV. Energization of particles takes place over a wide range along the tail, due to the large spatial extent of the increased electric field together with the finite cross-tail extent of the electric field region. Such accelerated particles appear earthward of the neutral line over a significant portion of the closed field line region inside of the separatrix, not just in the vicinity of the separatrix. Two different acceleration processes are identified: a 'quasi-potential' acceleration, due to particle motion in the direction of the cross-tail electric field, and a 'quasi-betatron' effect, which consists of multiple energy gains from repeated crossings of the acceleration region, mostly on Speiser-type orbits, in the spatially varying induced electric field. The major source region for accelerated particles in the hundreds of keV range is the central plasma sheet at the dawn flank outside the reconnection site. Since this source plasma is already hot and dense, its moderate energization by a factor of approximately 2 may be sufficient to explain the observed increases in the energetic particle fluxes. Particles from the tail are the source of beams at the plasma sheet/lobe boundary. The temporal increase in the energetic particle fluxes, estimated from the increase in energy gain, occurs on a fast timescale of a few minutes, coincident with a strong increase in B(sub z), despite the fact that the inner boundary ('injection boundary') of the distribution of energized particles is fairly smooth.