Principle of a dual-band search coil magnetometer: a new instrument to investigate magnetic fields fluctuation in space

Search-coil magnetometers are still often used in space physics thanks to their ability to measure weak magnetic field (few tens of fT/sqrt(Hz) at kHz) and also their robustness. However, their frequency band is limited by the resonance of the coil. A second coil, used to extend the frequency band, is almost inefficient after the resonance of the first coil. We present a solution, called "mutual reducer", to make it able to extend frequency band. The physical principle is described. The "dual-band" magnetic sensor so-designed will be on board, for the first time, on Bepicolombo mission which aims to investigate Mercury\´s magnetosphere.