© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
Multi-spacecraft observations of small-scale fluctuations in density and fields in plasmaspheric plumes
1Space Science Center, University of New Hampshire, 8 College Road, Durham, NH 03824, USA
2Belgian Institute for Space Aeronomy (IASB-BIRA), 3 Avenue Circulaire, 1180 Brussels, Belgium
3Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228, USA
4AER, Inc., 131 Hartwell Ave., Lexington, MA 02421, USA
5Swedish Institute of Space Physics, Box 537, 751-21 Uppsala, Sweden
6Alfvén Laboratory, Royal Institute of Technology, 100-44 Stockholm, Sweden
7Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
Abstract. 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 by Cluster, while IMAGE measured density enhancement at a similar location. Fluctuations in density exist in plumes as detected by Cluster and are accompanied by fluctuations in magnetic fields and electric fields. Magnetic fluctuations are transverse and along the direction of the plumes. The E/B ratio is smaller than the Alfvén velocity. Another similar event is briefly presented. We then consider physical properties of the fluctuations. Alfvén mode modulated by the feedback instability 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 consistency between measurements and expectations. The energy source could be a pressure or density gradient in plasmaspheric plumes. When more events are accumulated so that statistical analysis becomes feasible, this type of study will be useful to understand the time evolution of plumes.