Magnetic Reconnection in the Space Sciences: Past, Present, and Future

From the source

1. Magnetic reconnection converts, often explosively, stored magnetic energy to particle energy in space and in the laboratory. Through processes operating on length scales that are tiny, it facilitates energy conversion over dimensions of, in some cases, hundreds of Earth radii. Solar flares release up to 10²⁵ joules — enough, the authors note, to supply the whole world for about 20,000 years if a single large flare could be harnessed.Abstract; Box 1, Plain Language Summary; Section 2.2

2. Reconnection in space is nearly collisionless, so classical electron-ion collisions cannot supply the dissipation, and the process is kinetic — governed by the intricate interaction of charged particles with the electromagnetic fields they create. A signature follows: in antiparallel collisionless reconnection an out-of-plane magnetic field with quadrupolar structure is self-generated near the reconnection site within one ion gyroradius of the field reversal, produced by the Hall effect and setting up a two-scale diffusion region, an inner electron region inside an outer ion region.Section 5.1

3. The normalised reconnection rate — the ratio of the inflowing plasma's bulk speed to the Alfvén speed — comes out close to 0.1 in solar and magnetospheric observations, in two-dimensional simulations largely independent of system parameters, and in the MMS measurements. Hesse and Cassak are explicit about what is still missing: ’What we do not know is why this rate should be universal — or whether it actually is — and what its precise value is.’Section 5.2; Section 6.1

4. MMS settled a long-standing contest about what breaks the field lines. Observations show ’a remarkably quiet structure of electron diffusion regions’, and the evidence increasingly favours the quasi-viscous model based on nongyrotropic electron pressure over anomalous drag from turbulent fluctuations at the reconnection site. The authors' verdict: ’it appears that MMS has accomplished its first objective: to determine the machinery of the electron diffusion region.’Section 5.3

5. How a diffusion region kilometres across drives an eruption millions of kilometres across is the open question the field is now aimed at. The leading idea is that plasmoids — secondary islands — and turbulence spread the effect of a single diffusion region into many, which combined can generate the total effect of a much larger region, accelerating energetic particles and ejecting coronal mass. Parker Solar Probe, transiting to within nine solar radii, is named as the instrument that can address it.Section 6.2, paragraphs on the solar corona

6. The same process runs in fusion machines. Reconnection was quickly recognised in tokamaks after solar-flare theory was developed, and so-called minor disruptions, the sawtooth crashes, are reconnection events that limit plasma temperature and spoil confinement; modern machines are designed to let reconnection happen in controllable small bursts rather than suppress it entirely and store a catastrophic release. Separately, the authors report that in the last decade there has been increased interest in harnessing the conversion of magnetic energy into directed jets that occurs during reconnection for use in plasma thrusters.Section 2.4, Impacts of Magnetic Reconnection to Society; Section 6.2

The way in

How to cite it

M. Hesse, P. A. Cassak (2020) Magnetic Reconnection in the Space Sciences: Past, Present, and Future doi:10.1029/2018ja025935

Where it sits in the curriculum

• Plasmoids, charge clusters and the orbs