Authors: Hanqing Ma (University of Maryland), J. F. Drake (University of Maryland), M. Swisdak (University of Maryland)

We perform two-dimensional particle-in-cell simulations to investigate the structure of the out-of-plane magnetic field within magnetic islands formed during collisionless magnetic reconnection. The reconnection process generates strong electron temperature anisotropy inside the island, which triggers the Weibel instability. This instability gives rise to multiple magnetic filaments aligned along the reconnection outflow direction. The resulting filamentary magnetic structures act to reduce the temperature anisotropy, scatter energetic electrons, and inhibit further heating. These processes significantly impact the kinetic dynamics within the island and may alter the overall reconnection evolution.