Large scale magnetic fields and their dissipation in GRB fireballs}

We consider possible geometries of magnetic fields in GRB outflows, and their evolution with distance from the source. For magnetically driven outflows, with an assumed ratio of magnetic to kinetic energy density of order unity, the field strengths are sufficient for efficient production of gamma -rays by synchrotron emission in the standard internal shock scenario, without the need for local generation of small scale fields. In these conditions, the MHD approximation is valid to large distances (ga 10¹⁹ cm). In outflows driven by nonaxisymmetric magnetic fields, changes of direction of the field cause dissipation of magnetic energy by reconnection. Much of this dissipation takes place outside the photosphere of the outflow, and can convert a significant fraction of the magnetic energy flux into radiation.