Satellites glimpse the microphysics of magnetic reconnection

Because of Earth’s magnetic field, the stream of electrons and ions that constantly blows from the Sun gets diverted around the planet. The particles mostly travel with the solar-wind magnetic field lines on their way past Earth, but sometimes they breach the magnetosphere, the region of space where the dominant magnetic field is that of Earth. Such breaches eventually manifest themselves as auroras and geomagnetic storms. Driving the large-scale bursts of energy released in those space weather events are electron interactions that may be an important mechanism for energy conversion throughout the solar system.

When oppositely directed magnetic field lines approach each other in astrophysical plasmas, they can break and reconnect in a lower-energy configuration (see the article by Forrest Mozer and Philip Pritchett, Physics Today, June 2010, page 34 ). Bent tightly at first, the field lines abruptly straighten, which sends charged particles streaming away from the reconnection locus. So far, only with laboratory experiments and computer simulations have researchers been able to probe the details of the process at such scales that they can determine how efficiently energy is converted from magnetic to kinetic. However, reconnection in laboratory plasmas proceeds much too slowly, so researchers have not been able to explain geomagnetic storms and other explosive phenomena observed in space.

Now a new observational window has opened. The four formation-flying spacecraft of NASA’s Magnetospheric Multiscale (MMS) mission have witnessed the electron interactions that drive reconnection events in Earth’s magnetosphere. ¹

The magnetosphere offers a local, natural laboratory in which to study two regions that host frequent reconnection events. ² On Earth’s dayside, the solar wind abuts Earth’s magnetic field at a region called the magnetopause. On the nightside, Earth’s field lines sweep out into a trailing magnetotail. Both regions are shown in figure 1. The MMS was launched in 2015 to make the first high-resolution in situ measurements of plasma and fields and to map reconnection events on both sides of Earth.

Figure 1.

Reconnection reconnaissance

In a plasma in a strong magnetic field, the field lines act like wires: Charged particles tightly orbit the field lines, and the net electric current is guided on fixed paths through the plasma. Since the 1970s plasma physicists have understood that for reconnection to occur, the magnetic field must first become “unfrozen” from the plasma’s electrons and ions in a process called demagnetization.

As magnetized plasmas flow toward each other, field lines get squeezed together from above and below the mid-plane toward a notional central locus called the X-line, as illustrated in figure 2. When the radius of curvature of the field lines becomes comparable to the radius of gyration of charged particles’ spirals about the field lines, the particles resist that squeezing and break away. Positive ions, with their larger radius of gyration, leave their field lines while electrons stay tied to the magnetic field lines and continue to stream into a confined region, typically just tens of kilometers across, known as the electron diffusion region. There the electrons finally leave their field lines and set off the reconnection process. As the tightly bent magnetic field lines straighten, the electrons are flung outward in two oppositely directed jets.

Figure 2.

Reconnection had never before been observed directly and completely. In 1999 NASA’s Wind spacecraft filled in part of the picture, when it detected magnetic fields and electron currents established by inflowing electrons during reconnection (see Physics Today, October 2001, page 16 ). Now the MMS is peering inside the electron diffusion region to investigate the processes that unfreeze those electrons and drive reconnection. The mission’s four identical satellites each carry eight electron sensors and travel in an adjustable 10-km-scale tetrahedron to measure the three-dimensional electron distribution and the electric and magnetic fields when the spacecraft fly through the epicenter of a reconnection event.

Efficient reconnection

The MMS spent the first part of its mission observing Earth’s dayside where unequal magnetic fields, that of the solar wind and that of Earth, reconnect in an asymmetric fashion. Since 2017, the satellites have been observing the nightside. On 11 July 2017, the MMS detected jets of ions and electrons streaming toward and away from Earth, providing evidence of an electron diffusion region.

The four spacecraft travelled in a pyramid formation; they were 17 km apart and stayed within 50 km of the most probable region for a reconnection event to occur. During a 10-minute period, the spacecraft moved together from south to north across the magnetotail midplane, 22 Earth radii from Earth. For six seconds, the satellites straddled the electron diffusion region of a reconnection event. Those seconds marked the first in situ observation of terrestrial magnetic field lines reconnecting with themselves in a symmetric fashion.

When Roy Torbert (University of New Hampshire) and colleagues looked at the 2017 data, they found an electron velocity that exceeded 15 000 km/s. The high-speed jets carried a strong current away from the X-line, at speeds near the theoretical limits expected for highly efficient conversion of magnetic to kinetic and thermal energy. Figure 3a shows observations from the MMS magnetotail encounter. ¹

Figure 3.

One of the MMS’s goals was to determine the reconnection rate in the electron diffusion region or, as Torbert’s coauthor James Burch (Southwest Research Institute) says, “what fraction of the lines reconnect when the plasmas are squished together.” The aspect ratio of the electron diffusion region represents the ratio of plasma outflow to inflow and is considered an indicator of the reconnection rate. If the sides and the ends of the diffusion region were of equal length, then the outflow rate would equal the inflow rate. Since all the plasma flowing in to the diffusion region must flow out the ends, the outflow region acts like a pair of nozzles that regulate the inflow rate. Measurements taken as the MMS probes transited revealed an aspect ratio of 0.1 to 0.2, which is consistent with simulations of fast reconnection. ³

Curious crescents

In the 1990s Michael Hesse and colleagues at NASA’s Goddard Space Flight Center had proposed a laminar mechanism for dissipating magnetic energy, in which thermally mobile electrons rapidly transit through and carry energy away from the electron diffusion region. From their dayside work three years ago, Burch, Torbert, and colleagues reported MMS observations of electron demagnetization and acceleration. ⁴ The results included identification of a crescent-shaped feature in the velocity distributions of electrons at the reconnection site, such as the ones shown in figure 3b. The crescent feature was predicted to be a result of electrons whose orbits meander across a boundary between oppositely directed magnetic fields. The meandering is part of the demagnetization of electrons. ⁵

Torbert and colleagues now report multiple discrete structures in electron velocity distributions during reconnection in the magnetotail. As shown in figure 3b, the structures appear crescent shaped in a 2D plot of two velocity components. The velocity distributions from the MMS vindicated predictions that laminar flow, rather than turbulence, dominates the electron dynamics during reconnection.

The crescent structures remained unperturbed during rapid fluctuations in the electromagnetic fields during reconnection. That stability implies that turbulent effects, which would scatter electrons and hence eliminate distinct features like crescents, do not dominate the particle dynamics in the electron diffusion region during reconnection. Rather, the reconnecting field can continuously accelerate the electrons and drive them into high-speed jets, possibly as a consequence of confinement in the symmetric magnetic structure.

Surprises and predictions

The observations provide the first evidence of how reconnection works at the electron scale, and they confirm that reconnection releases magnetic energy efficiently. MMS scientists hope that more data 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.

Torbert’s colleague Tai Phan (University of California, Berkeley) has already found one surprise in data transmitted from the MMS while it transited Earth’s turbulent magnetosheath, the region between the magnetosphere and the bow shock produced when the solar wind speed decreases as it approaches the magnetopause (see figure 1). There, diverging electron jets provided the telltale sign of reconnection. But in contrast to standard reconnection, ions were bystanders. Phan concluded that reconnection driven by electron interactions alone can facilitate energy transfer in a turbulent plasma. ⁶

By studying reconnection on both sides of Earth, the MMS also helps astronomers understand reconnection elsewhere, such as in the atmospheres of stars, near black holes and neutron stars, and at the boundary between our solar system’s heliosphere and interstellar space.