Interferences in Electric Field Spectrograms of WHISPER induced by the Electron Drift Instrument on Cluster

The Electron Drift Instrument (EDI) on CLUSTER determines the ambient electric field through the measurement of the drift of test electrons emitted by two electron guns. The electron beams are intensity-modulated with a pseudo-noise code, in order to facilitate beam detection and to measure the times-of-flight of the electrons. Beam characteristics like current intensity, modulation frequency, code length, and beam firing direction are adjusted onboard according to the magnetic field magnitude and direction. To determine how the weak electron beams affect the measurements of other instruments on CLUSTER was one of the goals of the Interference Campaign (IFC), which was run between Dec 7 and Dec 20,2000. During the EDI-specific experiments of this campaign, beam parameters and the beam pointing with respect to other instruments were varied. With this presentation we want to focus on the effects of EDI on the measurements of the natural emissions between 2 kHz and 80 kHz made by the WHISPER instrument. Data from the IFC and from nominal mission operations show two distinguished types of interferences. Type I or direct signatures are sharp lines at the harmonics of the electron beam pseudo-noise code frequency. These signatures are mainly controlled by the beam current. Type II or indirect signatures indicate wave modes, that coincide with EDI operations. Their frequency is close or equal to the electron gyro frequency and its harmonics. The knowledge of these interference signatures is essential for the interpretation of WHISPER data. Understanding their nature will help to limit their occurrence through appropriate settings of EDI operational parameters that will still allow useful EDI measurements.