Articles | Volume 30, issue 3
Ann. Geophys., 30, 623–637, 2012

Special issue: Cluster 10th anniversary workshop

Ann. Geophys., 30, 623–637, 2012

Regular paper 29 Mar 2012

Regular paper | 29 Mar 2012

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

H. Matsui1, F. Darrouzet2, J. Goldstein3, P. A. Puhl-Quinn4, Yu. V. Khotyaintsev5, P.-A. Lindqvist6, E. Georgescu7, C. G. Mouikis1, and R. B. Torbert1 H. Matsui et al.
  • 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.