Large scale magnetic fields and their dissipation in GRB fireballs

We consider possible geometries of magnetic fields in GRB outflows, and the ir evolution with distance from the source. Far magnetically driven outflow s, with an assumed ratio of magnetic to kinetic energy density of order uni ty, the field strengths are sufficient for efficient production of gamma -r ays by synchrotron emission in the standard internal shock scenario, withou t the need for local generation of small scale fields. In these conditions, the,MHD approximation is valid to large distances (greater than or similar to 10(19) cm). In outflows driven by nonaxisymmetric magnetic fields, chan ges of direction of the field cause dissipation of magnetic energy by recon nection. Much of this dissipation takes place outside the photosphere of th e outflow, and can convert a significant fraction of the magnetic energy fl ux into radiation.