Magnetic reconnection

Plasma is an electrically conducting state of matter in which charged particles interact with electromagnetic fields. In many plasmas, the field and the moving material are coupled. When regions with differently oriented magnetic fields are brought together, the field can reorganize in a localized region known as a diffusion region. The magnetic connectivity changes, and energy stored in the magnetic field can be converted into particle motion and heat. Depending on conditions, particles may be accelerated to high energies.

The familiar picture of field lines “breaking” and joining other lines is a useful way to visualize the change, but magnetic field lines are conceptual representations of the field, not physical strings. The process is governed by plasma physics, including the motion of charged particles and electric and magnetic fields. Its speed and detailed mechanism vary with plasma conditions; important questions remain under study.

Magnetic reconnection is observed in space and studied in laboratories. At Earth, reconnection can occur where the solar wind interacts with the magnetosphere, changing magnetic connections and transferring energy into near-Earth space. Related activity is associated with auroral and geomagnetic phenomena. In the solar atmosphere, reconnection is an important process in solar eruptions and flares, where it can rapidly release magnetic energy. Reconnection also occurs in magnetically confined fusion experiments, where it can alter plasma confinement.

These settings differ in scale and plasma conditions, so observations in one do not automatically provide a complete explanation for all others. Research combines theory, computer simulations, laboratory experiments and measurements from spacecraft.

NASA’s Magnetospheric Multiscale (MMS) mission was designed to investigate reconnection in Earth’s magnetosphere. Four spacecraft fly in a tetrahedral formation so that measurements can resolve the three-dimensional structure of small, fast-moving regions, including the electron diffusion region. The mission measures plasma and electric and magnetic fields to investigate when reconnection begins, how quickly it proceeds, and how particles are energized.

The localized scale of the diffusion region makes direct measurements challenging. MMS observations, considered alongside laboratory data and modeling, have helped researchers test predictions about the process while leaving further questions open.