CURRENT SHEET MEASUREMENTS WITHIN A FLAPPING PLASMA SHEET

Slow plasma flows perpendicular to the magnetotail neutral sheet and a ssociated with napping motions of the plasma sheet can occasionally ex ceed the sensitivity threshold of plasma instruments. This is the case in several events selected from 3 months of magnetotail observations by the three-dimensional plasma instrument on the AMPTE/IRM satellite. By comparing the ion now in the Z(GSM) direction and the correlated c hanges of the X-GSM-directed magnetic field, we estimate the current s heet napping velocity to range from a few tens to a few hundreds of ki lometers per second. Our measurements of the current sheet thickness ( 0.1-1 R-E) and current density (10-30 nA/m(2)) were significantly diff erent from estimates of these parameters based on magnetospheric model s, suggesting a stretched field configuration. The observed events too k place during geomagnetically disturbed periods. One of those periods was a 2-hour-long convection bay interval. The largest napping flows were observed during the few events which also exhibited high-speed Ea rthward flows, but the napping velocity was smaller during the more co mmon, slow convection intervals. This observation, as well as the unex pectedly large napping velocities seen as close as 12 R-E (i.e., close to the hinge point), suggests that napping motions originate near the current sheet. Plasma sheet napping tends to occur in conjunction wit h ground Pi2 bursts, but the exact onset times and the dominant period s of the two phenomena are generally different. Thus, although a commo n energy source of both phenomena may be intermittent current disrupti on/reconnection, the two phenomena are likely to couple differently to that free-energy source. We discuss the vertical structure of the pla sma sheet parameters during napping plasma sheet episodes.