CLUSTER Observation of Magnetic Reconnection in the Magnetotail: Comparison with Hall Physics Effects.

During the August 17, 2001 ~16:20 substorm event, CLUSTER S/C were at apogee (~20 R_E) in the midnight region (~0100 MLT) and very close to the reconnection site. High velocity (~1000 km/s) tailward ion field aligned and E x B flows were observed while the B_Z component of the magnetic field was negative. These tailward flows were followed by two Earthward reversals. Low energy O⁺ was obseved in the inflow region with a dawnward direction while energetic O⁺ was observed tailward of the reconnection site. The plasma sheet was very thin, ~1100 km, very variable in thickness and tilted in the Y-Z plane due to a strong B_y component of the IMF. Due to this very thin plasma sheet CLUSTER S/C probed almost simultaneously all regions, from the neutral sheet to the lobes. Variations in the magnetic field are consistent with the quadrupole structure produced by the Hall current system while the presence of electron beams towards the reconnection site indicate that we are observing the Hall current carriers. Electric field data show very strong low frequency fields. These observations will be compared to the theoretical models of collisionless magnetic reconnection and more specifically to the Hall physics effects.