Secondary island formation in collisional and collisionless kinetic simulations of magnetic reconnection

The evolution of magnetic reconnection in large‐scale systems often gives rise to extended current layers that are unstable to the formation of secondary magnetic islands. The role of these islands in the reconnection process and the conditions under which they form remains a subject of debate. In this work, we benchmark two different kinetic particle‐in‐cell codes to address the formation of secondary islands for several types of global boundary conditions. The influence on reconnection is examined for a range of conditions and collisionality limits. Although secondary islands are observed in all cases, their influence on reconnection may be different depending on the regime. In the collisional limit, the secondary islands play a key role in breaking away from the slow Sweet‐Parker scaling and pushing the evolution towards small scales where kinetic effects can dominate. In the collisionless limit, fast reconnection can proceed in small systems (30× ion inertial scale) without producing any secondary isl...