Plasmaspheric plumes: CLUSTER, IMAGE and simulations

Plasmaspheric plumes have been routinely observed by the CLUSTER and IMAGE missions. CLUSTER provides high time resolution four-point measurements of the plasmasphere. Electron density is derived from the WHISPER sounder supplemented by data from the electric field instrument EFW. The EUV imager onboard IMAGE provides global images of the plasmasphere. We present coordinated observations of one plume event and numerical simulations for its formation based on the interchange instability mechanism. We compare several aspects of the plume motion as determined by different methods: (i) boundary velocity calculated from time delays of plume boundaries observed by WHISPER on all four spacecraft, (ii) ion velocity derived from the ion spectrometer CIS onboard CLUSTER, (iii) drift velocity measured by the electron drift instrument EDI onboard CLUSTER and (iv) global velocity determined from successive EUV images. These different methods consistently indicate that plasmaspheric plumes rotate around the Earth, with their foot fully co-rotating, but with their tip rotating slower and moving farther out.