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We investigate here the effects of plasma instabilities driven by rapidelectron/positron pair cascades, which arise in the environment of GRB sourcesas a result of back-scattering of a seed fraction of the original spectrum. Theinjection of electron/positron pairs induces strong streaming motions in theambient medium. One therefore expects the pair-enriched medium ahead of theforward shock to be strongly sheared on length scales comparable to theradiation front thickness. Using three-dimensional particle-in-cellsimulations, we show that plasma instabilities driven by these streamingelectron/positron pairs are responsible for the excitation ofnear-equipartition, turbulent magnetic fields. Our results reveal theimportance of the electromagnetic filamentation instability in ensuring aneffective coupling between electron/positron pairs and ions, and may helpexplain the origin of large upstream fields in GRB shocks.

Electron-Positron Pair Loading and the Origin of the Upstream Magnetic Field in GRB Shocks