Statistical analysis of plasmaspheric plumes with Cluster/WHISPER observations

Plasmaspheric plumes have been routinely observed by the four Cluster spacecraft. This paper presents a statistical analysis of plumes observed during five years (from 1 February 2001 to 1 February 2006) based on four-point measurements of the plasmasphere (outside 4 Earth radii) as it is sampled by the spacecraft in a narrow local time sector before and after perigee. Plasmaspheric plumes can be identified from electron density profiles derived from the electron plasma frequency determined by the WHISPER wave sounder onboard Cluster. As the WHISPER instrument has a limited frequency range (2 - 80 kHz) only plumes with densities below 80 cm(-3) can be identified in this way. Their occurrence is studied as a function of several geomagnetic indices (K-p, am and D-st). Their transverse equatorial size, magnetic local time distribution, L position and density variation are discussed. Plasmaspheric plumes are observed mostly for moderate K-p and never for small D-st. They are found mainly in the afternoon and pre-midnight MLT sectors. Comparisons are also made between the density profiles of the plumes as they are crossed on the in- and outbound legs of the orbit, before and after perigee crossing, respectively.