Magnetic reconnection is the fundamental mechanism by which magnetic energy is dissipated in the universe. Observationally, energy is released in bursts rather than in a continuous manner, driving phenomena such as solar flares and magnetospheric substorms. The basic process of reconnection has been understood from the late 1950s. If two parcels of magnetized plasma have oppositely directed magnetic fields and there is a region of weaker or zero field between them, then under the right circumstances the parcels can approach each other. The oppositely directed magnetic flux can cancel out (annihilate), and the plasma can jet outward along the weaker field directions at a characteristic speed called the Alfvén speed.
Magnetic flux is lost from the structures on the “inflow” sides, while a new magnetic structure, formed from “reconnected” magnetic field lines, grows on the “outflow” sides. As the plasma approaches the central region, the magnetic field may change direction very quickly, producing intense channels of electric current density that can heat the plasma. The current typically takes the form of filaments or sheets, and the magnetic field often takes on a characteristic X-point shape. This process is called magnetic reconnection or merging, and it is thought to be the main means by which magnetic fields in space plasmas change the way they are linked with one another.
Recent observations in the magnetosphere and in the solar corona provide mounting evidence of the key role of reconnection in space plasmas. There are important outstanding questions concerning the small-scale structure of the reconnection region, in which plasma kinetic effects are dominant. In these crucial small-scale regions, particles can be accelerated to high energies, magnetic field lines break and reconnect, plasma jets are formed, heat is released, and energy can be transferred from one region to another. In reconnection, large-scale dynamics and small-scale plasma physics come face to face: This is an essential feature of multiscale, nonlinear space plasma physics.