Role of ULF/ELF waves in the dynamics of the solar wind; measurements on solar orbiter

Solar wind ions are known to be heated, in particular in the inner solar wind regions to be covered by the Solar Orbiter. In this collisionless plasma the heating can only be achieved via ULF/ELF waves and/or turbulence. While important the proper characterization of waves/turbulence, is a difficult problem due to the Doppler shift. We briefly describe the kind of waves/turbulence that could be observed aboard SO, and take the Earth's magnetosheath as an example. We show that very intense, electromagnetic fluctuations, with wide-band spectra (extending far above the proton gyrofrequency), are often observed in the magnetosheath, and we discuss their possible interpretation and their potential consequences on particle heating. We stress the importance of measuring the 3 magnetic components of the waves (together electric), in order to be able to identify wave mode in spite of the Doppler shift. Search Coil magnetometers have been developed and implemented on several spacecraft; we describe shortly the recent developments of this technique to measure AC magnetic components.