Multi-spacecraft observations of small-scale fluctuations in density and fields in plasmaspheric plumes

摘要

In this event study, small-scale fluctuations in plasmaspheric plumes with time scales of~10 s to minutes in the spacecraft frame are examined. In one event, plasmaspheric plumes are observed byCluster, while IMAGE measured density enhancement at a similar location.Fluctuations in density exist in plumes as detected by Cluster andare accompanied byfluctuations in magnetic fields and electric fields. Magnetic fluctuationsare transverse and along the direction of the plumes. The E/B ratio issmaller than the Alfvén velocity.Another similar event is briefly presented. We then consider physical properties ofthe fluctuations.Alfvén mode modulated by the feedbackinstability is one possibility, although non-local generation is likely.It is hard to show that the fluctuations represent a fast mode.Interchange motion is possible due to the consistencybetween measurements and expectations. The energy source could be apressure or density gradient in plasmaspheric plumes. When more events are accumulatedso that statistical analysis becomes feasible, this type of study will be useful to understand the time evolution of plumes.