In the Earth auroral zone, the electron acceleration by Alfven waves is sometimes a precursor of the non-propagating acceleration structures. In order to investigate how Alfven waves could generate non-propagating electric fields, a series of simulations of counter-propagating waves in a uniform plasma is presented. The waves (initially not configured to accelerate particles) propagate along the ambient magnetic field direction. It is shown that non propagating electric fields are generated at the locus of the Alfven waves crossing. These electric fields have a component orientated along the direction of the ambient magnetic field, and they generate acceleration and a significant perturbation of the plasma density. The non-linear interaction of down and up-going Alfven waves might be a cause of plasma density fluctuations (with gradients along the magnetic field) on a scale comparable to those of the Alfven wavelengths.
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