magnetic reconnection

History

Magnetic reconnection was first described by British physicist James Dungey in 1953. He realized that solar flares could occur due to the breaking and reconnecting of magnetic field lines in the solar corona.

How it works

In plasma (an electrically charged gas), the magnetic field is frozen into the material—field lines are tied to the plasma and move with it. When plasma flows with oppositely directed fields collide, in a narrow region the field almost vanishes, and the lines can tear and reconnect differently. It's as if two rubber bands flying toward each other collided and intertwined—their kinetic energy transforms into heat and a jerk.

History

Magnetic reconnection was first described by British physicist James Dungey in 1953. He realized that solar flares could occur due to the breaking and reconnecting of magnetic field lines in the solar corona.

How it works

In plasma (an electrically charged gas), the magnetic field is frozen into the material—field lines are tied to the plasma and move with it. When plasma flows with oppositely directed fields collide, in a narrow region the field almost vanishes, and the lines can tear and reconnect differently. It's as if two rubber bands flying toward each other collided and intertwined—their kinetic energy transforms into heat and a jerk.

History

The key idea of breaking and reconnecting magnetic field lines in current sheets to explain solar flares was proposed by James Dungey in 1953. Later, in 1957–58, Peter Sweet and Eugene Parker independently developed a quantitative model of steady-state reconnection (the Sweet–Parker model), which, however, predicted energy release that was too slow. The problem of fast reconnection stimulated the development of new models in the 1990s: Biskamp included the Hall effect, and work by Shay and Drake showed that charge separation in the ion-inertial region dramatically accelerates the process. Modern space missions (Cluster, MMS) and laboratory experiments (MRX, SPHEX) have confirmed the importance of kinetic effects.

How it works

In plasma, described by the equations of magnetohydrodynamics, the magnetic field is frozen into the medium, and its topology is preserved if the conductivity is infinite. When plasma volumes with antiparallel fields converge, a thin current sheet forms at the boundary, where the current density sharply increases and the magnetic field weakens. Due to finite resistivity or kinetic effects such as electron inertia and charge separation (the Hall term in the generalized Ohm's law), the frozen-in condition breaks down. Then the field lines reconnect, and magnetic energy dissipates. In fast regimes (e.g., Hall reconnection), the process rate is determined by the Alfvén time and depends only weakly on conductivity. Particle acceleration occurs as a result of the direct action of the electric field associated with the changing magnetic flux, and through stochastic processes in turbulent magnetic structures. Reconnection is often accompanied by the formation of narrow plasma jets heated to millions of degrees.

Open problems & fun facts

What is the microscopic mechanism of fast magnetic reconnection in collisionless plasma of the solar corona and magnetosphere?; How exactly are particles accelerated to relativistic energies in a single reconnection event?; What is the role of turbulence and three-dimensional effects in initiating and sustaining reconnection?

💡 Experiments on the Magnetic Reconnection Experiment (MRX) at Princeton showed that the actual reconnection rate can be tens of times higher than predictions of the Sweet–Parker model, confirming the key role of Hall fields.