Satellites reveal the microphysics of magnetic reconnection

When antiparallel magnetic field lines cross or reconnect in astrophysical plasmas, the newly reconfigured lines become highly bent. Unable to sustain the curvature, the lines abruptly straighten, sending charged particles streaming out. Although the effects of magnetic reconnection have been observed in space—in the form of solar flares and brilliant auroras, for example—the actual reconnection process at scales that determine its efficiency has been observed only in the laboratory and simulations. Now NASA’s Magnetospheric Multiscale (MMS) mission has witnessed the smallest-scale electron interactions that drive reconnection events in Earth’s magnetosphere.

As magnetized plasmas flow toward each other, field lines get squeezed together. The charged particles in the plasma resist that squeezing and eventually break away from the field lines in a process called demagnetization. Positive ions break free before the much lighter, negative electrons. The electrons continue to drift inward, 30–40 times farther than the ions. In that tiny diffusion region, the reconnection process begins. The lines join together, bend drastically, and then relax, flinging the electrons outward in two oppositely directed jets.

The MMS mission’s four identical satellites measure electron and ion distributions at the reconnection locus. In 2016, MMS researchers reported observations of electron demagnetization and acceleration where the Sun’s magnetic field meets that of Earth (rectangle at left in the diagram). In the new work, Roy Torbert (University of New Hampshire) and colleagues present observations on the side of Earth away from the Sun (right rectangle), at the magnetotail, where terrestrial magnetic field lines reconnect with themselves. The researchers found that laminar flow, rather than turbulent effects, dominates the electron dynamics during reconnection. In one extremely rapid reconnection event, the laminar structure accelerated electrons to speeds of greater than 15 000 km/s, speeds near the limit allowed by highly efficient reconnection.

MMS scientists hope that more data returned from Earth’s magnetosphere will help explain just how much energy is dissipated by magnetic reconnection throughout the universe and what conditions determine when reconnection begins and ceases. (R. B. Torbert et al., Science 362, 1391, 2018 .)