Plasma waves above the ion cyclotron frequency in the solar wind: a review on observations

Since the very beginning of the space venture, discrepancies between hydrodynamic models and observations have raised the attention to the kinetic effects taking place in the interplanetary medium. After discussing some of these constraints, a review on the state of the art knowledge on the waves in the range fci<f<fpe (fci – about 10−1 Hz – is the ion cyclotron frequency, and fpe – a few 10 kHz – the electron plasma frequency) observed in the free solar wind, i.e., not magnetically connected to the Earth's bow shock, is addressed.