A new branch of electrostatic fluctuations: the ion-bulk waves

We present the results of kinetic simulations that demonstrate the existence of a novel branch of nonlinear electrostatic waves (dubbed ion-bulk waves) with acoustic type dispersion, excited and sustained by the generation of a population of trapped ions. These waves have phase speed comparable to the ion thermal velocity and are analogous at low frequencies to the so-called electron acoustic waves. During the excitation process, in which an external electric field is used to create the trapped particle population, a secondary instability of the beam-plasma type occurs and brings energy to high wavenumber electric field components. The ion-bulk waves can survive against Landau damping even at low values of the electron to ion temperature ratio, at variance with the ion-acoustic waves. In the long time limit, the electric field waveforms associated with the ion-bulk waves appear as long lived soliton-like structures locked in the phase space trapping vortices. Our results are relevant for the case of the the solar-wind plasmas where the electron to ion temperature ratio is of order unity and a significant level of electrostatic activity is usually recovered from in situ measurements.