X- and O-lines during magnetic reconnection
This illustration shows the configuration of a region, in the tail of Earth's magnetosphere, where magnetic reconnection is taking place.
Magnetic reconnection occurs when the magnetic field lines of two colliding flows of plasma are broken and reconfigure immediately afterwards in a different geometry. In the process, the energy stored in the magnetic field is transferred to the kinetic energy of particles in the plasma, accelerating them in the form of two jets of high-speed particles launched in opposite directions.
In the insert, two plasma flows with anti-parallel magnetic fields are pushed together, flowing in from above and below. At the crossing of the two flows, an X-line arises, whereas smaller X-lines as well as O-lines – whirlpool-shaped vortices where the magnetic field lines swirl together – may appear throughout the broader diffusion region.
As revealed in a study based on data from ESA's Cluster mission, most of the energy dissipated during a reconnection event is not released at the X-lines, but rather at the O-lines. The Cluster data were from 9 October 2003, when the four spacecraft were flying through the diffusion region, only about 200 km apart. During their passage they encountered several potential sites of X- and O-lines.
The new finding, which contradicts the accepted consensus, is an important step in the process of understanding the mechanisms that accelerate particles in space plasma.