Shall we carry sounders onboard future magnetospheric missions? Lessons learned from Cluster

Since 2000, the four ESA/NASA Cluster satellites orbit the Earth from 20 Re down to 1 Re. Each spacecraft is equipped with the same set of 11 plasma instruments, among which a relaxation sounder named WHISPER. Scientific highlights, where WHISPER data played a key role, will be first presented with an emphasis on plasmaspheric wave research [1]. Then, we will illustrate how WHISPER is key to estimate the electron density in various regions (sometimes unforeseen) of the magnetosphere, on its own and in conjunction with other plasma instruments [2]. These lessons learned from Cluster will help to answer the following question: shall we carry sounders onboard future magnetospheric missions?