THEMIS observation of a magnetotail current sheet flapping wave

A flapping wave was observed by THEMIS-B (P1) and THEMIS-C (P2) probes on the dawn side of the magnetotail, while the solar wind was generally stable. The magnetic activity was quite weak, suggesting that this flapping wave was generated by an internal instability, which normally occurs during magnetic quiet times. Our analysis shows that the flapping wave was propagating downward with a tail-aligned scale of at least 3.7 R_E and did not show much change in shape during its propagation from P1 to P2. Correlation analysis employed to estimate the time lag between the corresponding half waveforms of P1 and P2 shows that the propagating velocities along the current sheet normal directions were close to each other in the beginning, but increased linearly later on. The average wavelength of the flapping wave is approximately 4 R_E. Theoretical analysis suggests that the ballooning type wave model may not be the mechanism for the observed flapping wave, but that the magnetic double-gradient instability model is a more plausible candidate.