The results of kinetic simulations of magnetic reconnection in Harris current sheets are analyzed. A range of guide fields is considered to study reconnection in plasmas characterized by different β values, ${\mathrm{\beta >m}}_e{\mathrm{/m}}_i.$ Both an implicit particle-in-cell (PIC) simulation method and a parallel explicit PIC code are used. Simulations with mass ratios up to the physical value are performed. The simulations show that the reconnection rate decreases with the guide field and depends weakly on the mass ratio. The off-diagonal components of the electron pressure tensor break the frozen-in condition, even in low β plasmas. In high β plasmas, evidence is presented that whistler waves play a key role in the fast reconnection physics, while in low β plasmas the kinetic Alfvén waves are important. The in-plane and the out-of-plane ion and electron motion are also considered, showing that they are influenced by the mass ratio and the plasma β.

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: (i) it is much faster, developing on scales of the order of the Alfvén time, and (ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed-circulation patterns where the jets going out of the reconnection regions turn around and force their way back in, carrying along copious amounts of magnetic flux.