Bursty bulk flows without a near-Earth neutral line: Generation of fast intermittent flow in a highly curved magnetic field

摘要

Despite the heightened interest in bursty bulk flows (BBFs) in the magnetotail, there have been relatively few studies on how these fast intermittent flows occur in a region where slow and steady convection is expected. Conventional wisdom attributes BBFs to the formation of a near-Earth neutral line (NENL) on the appropriate side of the flow. We demonstrate that this view is not inevitable or necessary, We develop an alternate mechanism of BBFs generation based on the magnetohydrodynamics of a plasma situated in a Harris-type current sheet. We find that without a NENL or other extraneous assumptions, a highly curved current sheet (characterized by a field ratio B-z/B-chi much less than 1) can generate high-speed cross-field flows of > 100 km/s when perturbed by modest impulses of similar to 10 km/s at its boundary. The flow has an Earthward sense in a thinning current sheet. The dependence on the field ratio of the maximal speed has the form c(1) + c(2)B-chi/B-z (2), where c(1) and c(2) are constants. For reasonable parameters the BBF threshold of 400 km/s is reached when B-z/B-chi is approximately 0.1. The investigation also reveals attendant effects and signatures of the BBF-generating process, and these effects and signatures are listed as our theoretical predictions, against which better resolved satellite data from upcoming missions may be compared. We conclude with discussions on the potential role fast intermittent flows might play in the excitation of the substorm and other interesting magnetotail phenomena. [References: 22]