Global properties of magnetotail current sheet flapping: THEMIS perspectives

Abstract. A sequence of magnetic field oscillations with an amplitude of up to 30 nT and a time scale of 30 min was detected by four of the five THEMIS spacecraft in the magnetotail plasma sheet. The probes P1 and P2 were at X=−15.2 and −12.7 R_E and P3 and P4 were at X=−7.9 R_E. All four probes were at −6.5>Y>−7.5 R_E (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward (dawnward and Earthward) nearly perpendicular to the direction of the magnetic maximum variation (B₁) at velocities of 20–30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between ∂B₁/∂t and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 R_E. The cross-tail scale wave-length was found to be about 5 R_E. Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of >10 R_E. The intermittent plasma motion with the cross-tail velocity component changing its sign, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. Simultaneous observations of the magnetic field variations by THEMIS ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.