The Distribution of Two Flapping Types of Magnetotail Current Sheet: Implication for the Flapping Mechanism

Flapping motion of the current sheets of planetary magnetotails is a common dynamic phenomenon. Previous studies of the Earth's magnetotail suggest that its flapping motion has two forms, that is, kink-like flapping that can propagate as waves toward both flanks and steady flapping that moves up and down but does not propagate. Although some models have been proposed to explain the kink-like flapping, its mechanism remains unclear. This paper surveys 87 flapping events statistically with respect to their flapping types, using the multipoint measurements of Cluster. The statistical results show that the up-down steady flapping events tend to occur around the midnight region, and the kink-like flapping events tend to occur near both flanks of the magnetotail. Thus, we propose that kink-like flapping motion is causally related to steady flapping motion; that is, the up and down motion of steady flapping around the midnight region induces kink-like flapping waves, which propagate toward both flanks of the magnetotail. Plain Language Summary The flapping motion of Earth's magnetotail current sheet is widely observed as a common dynamic phenomenon. As an important dynamic process for releasing the magnetospheric energy, flapping motion has drawn attention increasingly. The mechanism to drive the flapping motion, however, remains unclear although many models and theories have been proposed. Recent study suggested that the flapping motion has two types, that is, kink-like flapping and steady flapping. Most previous studies are focused on the kink-like flapping, and steady flapping has received less attention. A more reasonable mechanism should have to consider how the two flapping types relate to each other and how to incorporate them into a global picture of magnetotail flapping motion. Thus, we are motivated to survey the spatial distributions of both flapping types based on the Cluster multipoint measurement, which can offer a clue to interpret the mechanism driving flapping motion. The distribution of flapping types suggests a new flapping picture that the up and down motion of steady flapping around the midnight region induces kink-like flapping waves, which propagate toward both flanks of the magnetotail.