Strong plasma turbulence in the Earth's electron foreshock

Abstract

A quantitative model is developed to account for the distribution in magnitude and location of the intense plasma waves observed in the Earth's electron foreshock given the observed rms levels of waves. In this model, nonlinear strong‐turbulence effects cause solitonlike coherent wave packets to form and decouple from incoherent background beam‐excited weak turbulence, after which they convect downstream with the solar wind while collapsing to scales as short as 100 m and fields as high as 2 V m−1. The existence of waves with energy densities above the strong‐turbulence wave‐collapse threshold is inferred from observations from IMP 6 and ISEE 1 and quantitative agreement is found between the predicted distribution of fields in an ensemble of such wave packets and the actual field distribution observed in situ by IMP 6. Predictions for the polarization of plasma waves and the bandwidth of ion‐sound waves are also consistent with the observations. It is shown that strong‐turbulence effects must be incorporated in any comprehensive theory of the propagation and evolution of electron beams in the foreshock. Previous arguments against the existence of strong turbulence in the foreshock are refuted.