Solar Wind Directional Change Triggering Flapping Motions of the Current Sheet: MMS Observations

We report on a flapping motion event near the substorm onset on 17 June 2017 using Magnetospheric Multiscale (MMS) mission data. A strong current density with a maximum value of 190nA/m(2) is observed during the flapping. The north-to-south (south-to-north) crossing of the neutral sheet corresponds to an increase (a decrease) of the Z(GSM) component of the solar wind V-Z,V- SW. The periods (8min) of the flapping and variations of V-Z,V- SW are almost equal. In addition, dV(Z, SW)/dt and dB(X)/dt observed by MMS exhibit a strong negative correlation. These observations suggest that the flapping motions are triggered by the solar wind directional change via creating a motion of the current sheet in the north-south direction. The pressure difference between the northern and southern lobes caused by the solar wind is expected to be a possible contribution to the formation of the flapping. Plain language summary Flapping motions are large-amplitude waves of the magnetotail current sheet in the north-south direction. These waves are believed to be excited in the magnetotail center and then propagate to its flanks. However, their generation mechanisms are not fully understood so far. In this study, we investigate the relation between a flapping motion event and the solar wind parameters. We find that there is a one-to-one correspondence between temporal variations of the flapping and the Z(GSM) component of the solar wind velocity V-Z,V- SW. Moreover, the vertical movement of the ion flow is also in accordance with V-Z,V- SW. These observations suggest that the flapping can be generated by the solar wind directional change. During the flapping, a strong current density with a maximum value of 190nA/m(2) is generated, much larger than the current reported by previous studies.