How Magnetic Reconnection Jolts Electrons
What connects solar flares that induce space weather, geomagnetic storms that cause auroras, and magnetic disruptions that spoil confinement in magnetically confined fusion devices? All these events rapidly convert stored magnetic energy into kinetic energy of surrounding electrons and positively charged ions in the plasma state of matter. The energy conversion occurs via a fundamental process called magnetic reconnection [1]. But some aspects of reconnection remain poorly understood, despite decades of scrutiny through theoretical studies, ground- and satellite-based observations, lab experiments, and numerical simulations [2]. A key unresolved problem is determining how much of the released magnetic energy goes to the electrons and how much goes to the ions, and by what physical mechanisms this energization occurs. Now Louis Richard at the Swedish Institute of Space Physics and his colleagues have used a novel approach to make headway on this problem [3].
In magnetic reconnection, magnetic field lines that originally point in opposite directions break apart and then reconnect [4]. The newly reconnected field lines are highly bent and, when they straighten, they release energy into the surrounding plasma [5, 6]. Effects analogous to reconnection take place in neutral fluids and superfluids and in interactions of solitary waves called solitons in quantum optics. Such effects are even predicted by some versions of string theory [7].
Recent developments in satellite-based diagnostic technology are providing unprecedented opportunities to probe reconnection in Earth’s magnetosphere—the plasma-filled region around Earth where the planet’s magnetic field dominates the dynamics of the plasma’s charged particles. In 2015, NASA launched the Magnetospheric Multiscale mission [8]. This mission’s four spacecraft have set records for being the tightest satellite constellation and for being the most-distant objects from Earth to use GPS signals. The satellites’ diagnostics measure the full velocity distribution of the magnetosphere’s charged particles and were designed to record electron velocity distributions 100 times faster than any previous mission.
Velocities of particles are often expected to follow the Maxwell-Boltzmann distribution, under the assumption that the particles are in local thermodynamic equilibrium. But collisions of charged particles in space plasmas are sufficiently rare that the particle ensemble is typically not in equilibrium. Magnetospheric Multiscale mission measurements of these particles’ nonequilibrium distributions have been a game changer for the study of reconnection. They have provided insights into the microscale physics that produces the electric fields that enable reconnection, into how reconnection generates waves and turbulence in the plasma, and into what determines the rate at which reconnection proceeds (see [9] and the references therein).
A leading candidate for the physical mechanism by which charged particles gain energy through reconnection is via the electric fields associated with reconnection. When an electric field is perpendicular to the magnetic field, the charged particles do not gain much energy because they circle around the magnetic field and, in doing so, repeatedly lose any received energy. But for an electric field parallel to the magnetic field, the particles can steadily gain energy from the electric field.
Much has been learned in the past two decades about how and where such parallel electric fields are generated during reconnection [10]. When a magnetic field line is about to reconnect, it bends in toward the point where reconnection takes place, decreasing the local magnetic field strength. To conserve magnetic flux, the surrounding plasma must then expand. This expansion preferentially affects the electrons because they respond more rapidly than the ions, which makes the net local charge positive. A parallel electric field is thus created. This parallel electric field has been seen in simulations, and evidence for it exists in isolated observational studies.
Richard and his colleagues used the Magnetospheric Multiscale mission satellites to conduct an extensive and astute analysis of parallel electric fields during reconnection in Earth’s magnetosphere. First, they studied a dataset featuring 140 reconnection events measured in Earth’s plasma sheet—the gigantic sheet of current that resides on the side of Earth away from the Sun and that arises because the solar wind stretches Earth’s magnetic field into the shape of a wind sock. Second, they used keen insight to infer the strength of the parallel electric field from the electron velocity distributions measured so exquisitely by the satellites’ diagnostics. The key idea was that the parallel electric field drives a beam of electrons into the reconnection region, and this process excites microscale plasma instabilities that result in the electrons developing a “flattop” velocity distribution (Fig. 1). The researchers then used the fact that the highest electron velocity within the flat region of this distribution is set by the strength of the parallel electric field to infer this strength.
A statistical analysis of the large dataset revealed excellent agreement with previous theoretical work. Moreover, Richard and his colleagues used their dataset to estimate the efficiency of electron heating by the parallel electric field, providing an empirical relation that can be used to infer the increase in electron temperature as a function of ambient plasma properties. The researchers also made an empirical prediction for the ratio of electron heating to ion heating, addressing that key unresolved problem.
Richard and his colleagues’ work is exciting for three reasons. First, it provides the most robust observational study to date of how electrons get heated by reconnection, showing that the electron temperature can increase by as much as an order of magnitude. Second, the derived empirical relation for electron heating could be applied to other settings where reconnection occurs—especially to those that are inaccessible to in situ satellite observations, such as solar and astrophysical settings. And third, the findings could improve our understanding of how the energy released during reconnection affects the surrounding environment. For example, in the wind sock portion of Earth’s magnetosphere called the magnetotail, the heated plasma is injected toward Earth and impacts the planet’s radiation belts, contributing to space weather events that can disrupt satellite communications. By having a clear understanding of the temperature of the plasma, scientists will be better suited to predict how systems involving reconnection will react to the process. This research activity will be crucial for our technological infrastructure and preparedness for space weather.
References
- E. G. Zweibel and M. Yamada, “Magnetic reconnection in astrophysical and laboratory plasmas,” Annu. Rev. Astron. Astrophys. 47, 291 (2009).
- H. Ji et al., “Magnetic reconnection in the era of exascale computing and multiscale experiments,” Nat. Rev. Phys. 4, 263 (2022).
- L. Richard et al., “Electron heating by parallel electric fields in magnetotail reconnection,” Phys. Rev. Lett. 134, 215201 (2025).
- J.W. Dungey, “LXXVI. Conditions for the occurrence of electrical discharges in astrophysical systems,” Lond. Edinb. Dubl. Phil. Mag. 44, 725 (1953).
- E. N. Parker, “Sweet’s mechanism for merging magnetic fields in conducting fluids,” J. Geophys. Res. 62, 509 (1957).
- H. E. Petschek, “Magnetic field annihilation,” AAS-NASA Symposium on the Physics of Solar Flares, edited by W. N. Hess (NASA, 1964).
- M. Hesse and P. A. Cassak, “Magnetic reconnection in the space sciences: past, present, and future,” J. Geophys. Res.: Space Phys. 125, e2018JA025935 (2020).
- J. L. Burch et al., “Magnetospheric multiscale overview and science objectives,” Space Sci. Rev. 199, 5 (2015).
- J. L. Burch and R. Nakamura, “Magnetic reconnection in space: An introduction,” Space Sci. Rev. 221, 19 (2025).
- J. Egedal et al., “A review of pressure anisotropy caused by electron trapping in collisionless plasma, and its implications for magnetic reconnection,” Phys. Plasmas 20, 061201 (2013).