  Recent full-particle simulations of electron-positron reconnection have
revealed that the Weibel instability plays an active role in controlling the
dynamics of the current layer and maintaining fast reconnection. A four-beam
model is developed to explore the development of the instability within a
narrow current layer characteristic of reconnection. The problem is reduced to
two coupled second-order differential equations, whose growing eigenmodes are
obtained via both asymptotic approximations and finite difference methods. Full
particle simulations confirm the linear theory and help probe the nonlinear
development of the instability. The current layer broadening in the
reconnection outflow jet is linked to the scattering of high-velocity streaming
particles in the Weibel-generated, out-of-plane magnetic field.
