Vorticity Within Bursty Bulk Flows: Convective Versus Kinetic

摘要

Based on four-point Magnetospheric Multiscale observation, we carefully analyze the vorticity field in the course of a reconnection jet on 27 June 2017. In this event, the convective electric field (Ec) is overwhelmed by the kinetic electric field (E_k). Accordingly, the ω-field in the course of the BBF is dominated by kinetic vorticity (ω_k). The ω-field in the Ek-dominated bursty bulk flow (BBF) is characterized by perpendicular anisotropy. Comparison of velocity-curl vorticity (ω= ? ×V ) with E/B induced vorticity confirms a greater contribution to the BBF vorticity by E (ω_E = (B ? ?)E∕B~2) than by B (ω_B=(E? ?)B∕B~2 ). Power spectrum density reveals that in the E_k-dominated BBF, the Ec and Ek spectra have different power laws. E_c dominates the B-spectrum while Ek dominates the E-spectrum. The Ec(B) spectrum has a ?5/3-like slope below 0.25 Hz but a ?3-like above 0.25 Hz. The Ek(E) spectrum is ?5/3-like at the low-frequency end (below 0.1 Hz) but ?2-like at the high-frequency end (above 2 Hz). Within its medium frequency range, the spectrum is flat. Particularly, the E_k(E) spectrum exhibits the bump at 1–2 Hz in the frequency domain. The solitary/bipolar E-spikes, with typical temporal scale of 0.3–1 s, are likely responsible for the E-bump. Finally, we statistically analyze and compare the vorticity field in Ek-dominated and Ec -dominated BBFs. The result demonstrates that the E_k-dominated BBF tends to have stronger vorticity than the Ec -dominated BBF.