Electrostatic fluctuations in the solar wind: An evidence of the link between Alfvénic and electrostatic scales

Electrostatic activity in the frequency range of few kHz represents a very common phenomenon observed in the solar wind since first observation from Helios spacecraft. In this paper we present a detailed comparison between electrostatic fluctuations detected by STEREO spacecraft and the results of kinetic numerical simulations. By using a novel approach, made by a combination of two different numerical models, we provide a convincing interpretation of the space observations in terms of the electrostatic branch of the so-called ion-bulk waves able to survive against Landau damping even at small values of Te/Tp. The comparison between data and numerical simulations allows to characterize the chain of physical mechanisms, able to efficiently transfer energy from the Alfvénic scales down to scales of the order of the Debye length, through the excitation of the observed electrostatic fluctuations.