Observation of the Large‐Amplitude andFast‐Damped Plasma Sheet FlappingTriggered by Reconnection‐InducedBallooning Instability

 In this study, we reported the large‐amplitude and fast‐damped flapping of the plasma sheet,
which co‐occurred with magnetic reconnection. Data from the Double Star TC‐1 and Cluster satellites
were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic
reconnection event. The flapping was rapidly damped, and its amplitude decreased from the
magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to
20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet
flapping propagated duskward through a kink‐like wave with a velocity of 100 km/s, which was in
agreement with the group velocity of the ballooning perturbation. A correlation analysis between the
magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated
the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In
this regard, the reconnection‐induced ballooning instability could be a potential mechanism to generate
the flapping motion of the plasma sheet.