AC magnetic field measurements onboard Cross-Scale: Scientific objectives and instrument design

The ACB search-coil magnetometer for Cross-Scale will measure three components of the AC magnetic field up to 4 kHz, and one component up to 100 kHz. Turbulent and coherent magnetic field fluctuations in that frequency range play an important role in the acceleration, scattering, and thermalisation of particles. ACB will, together with the other instruments of the Cross-Scale wave consortium, allow to address the key science objectives associated with plasma waves. Here, we list some of the important issues, based on the experience drawn from Cluster, and describe the instrument.