Comparison of a 2-D Particle-in-Cell Simulation to the Structure of the Tail  Reconnection  Region  Observed on  2001/10/01

This paper compares Cluster multi-spacecraft observations of the structure of electric fields, magnetic fields and ion distribution functions in a thin (sub-ion inertial length) current sheet near a reconnection x-line to the results of a Particle in Cell (PIC) simulation.  The primary purpose is to investigate the kinetic process of ion acceleration by shock -like electric fields. The spacecraft data and the PIC simulation provide evidence for the existence of a strong electric potential well centered on the current sheet separator region. The walls of the potential wells coincide with the current sheets/standing waves.   Both simulations and data show that this potential structure grows wide with distance down the x-line as expected from the diverging geometry of reconnection standing waves/ current layers. The data indicates the small-scale potential well has a depth of ~5-8 kilovolts. Both experimental data and simulations provide evidence that the  H⁺ fluid is incident  on the thin current sheets  from both the northern and southern  tail lobes  and is ballistically accelerated  across the magnetic field  by  the potential well  where they  form nearly symmetric H⁺ beams counter-streaming  in the center of the well. The beams have small thermal spreads and beam velocities of V_H+~B/(4pr_H+)^1/2 ~1000 km/s. In both cases, the estimates of the integrated potential drops are consistent with the energies of the ion beams.   The small structure of the electric  fields in the electron  de-coupling  region in the simulation  and in the data  are compared  in an  attempt  to understand  the breaking of the electron frozen -in condition. 

We are also analyzing  sev eral  encounters with the PSBL later  during this day  in an attempt  to determine  the relative contributions  of steady state and Alfvenic  Poynting flux, ion kinetic energy,  and electron kinetic energy to the flow of energy along the magnetic field. These intervals  are also associated with small scale  structures with high E/B ratios similar to those expected for  kinetic Alfven waves as well  as electron distributions which have been energized  in the parallel  direction.  Another  feature  of these crossings is the existence of strong fluxes of energetic  electrons and ions peaking at  the PSBL.