Research

Intermittent energy dissipation by turbulent reconnection
 

Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. It is believed that the magnetic reconnection, in the solar corona and the Earths magnetosphere, occurs in a small diffusion region with scale of ion skin depth. Such a region generally follows the two-fluid picture, but at its boundary the antiparallel-merging magnetic field lines may strongly fluctuate, and inside it, fine structuring such as X-lines and O-lines typically appears. During reconnection, the magnetic field lines break and reconnect, and a large amount of energy is released in terms of hard X-rays on the Sun and energetic electrons in the magnetotail. It is still a puzzle how so much magnetic energy is dissipated to particle energy in such a small region. The X-line is traditionally suggested as the main point where energy dissipation happens, as two opposite plasma jets are always produced there. However, recent 3-D simulations indicate that O-lines may also be important. So far, exactly where the energy dissipation happens has been unclear.

 
 
 

Fig 1. Topologies of X-line and O-line, reconstructed from the FOTE method.

To reveal the energy dissipation during reconnection, identifying X-lines and O-lines should be the first step. This requires that (1) multiple spacecraft are located simultaneously inside the diffusion region and have sub-ion-scale separations and (2) a tool is available for reconstructing the fine structures around the spacecraft trajectories. The European Space Agency (ESA) four-spacecraft mission Cluster can readily satisfy the first criterion: in the autumn of 2003, Cluster detected a few magnetic reconnection events in the Earth’s magnetotail, where the ion diffusion region has a scale of 1000 km. In these events, Cluster had a separation of ~200 km, i.e. 1/5 ion scale; hence, all satellites were located simultaneously inside the diffusion region. To meet the second criterion, the research team recently developed and tested a new method, i.e. the first-order Taylor expansion (FOTE), which has been published in 2015 in Journal of Geophysical Research (JGR)a renowned journal in space physicsas the cover paper, and attracted considerable attention from the international colleagues.

 
 
 

Fig 2. Cluster encounter of a reconnection diffusion region in the Earth’s magnetotail

The research team used FOTE method and the Cluster in situ measurements in 2003 to reveal the energy dissipation during reconnection. They discovered numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E?J, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines, not X-lines.

 
 
 

Fig 3. Intermittent energy dissipation by turbulent reconnection

These results have been published in Geophysical Research Letters (GRL)—a renowned journal in space physics—and highlighted as a “Research Spotlight” by the American Geophysical Union (AGU). Respectively, the European Space Agency (ESA) and American Geophysical Union (AGU) had a press release regarding these results. In the press release of AGU, the results were believed to “upend conventional wisdom of magnetic reconnection”; while in the press release of ESA, the results were thought to “challenge the current view of magnetic reconnection”. Also, these results will “spark a great deal of discussion”—as stated by AGU—and “enable scientists to rethink of the standard view of magnetic reconnection”—as stated by ESA.

 
 
 
 

Fig 4. The results were highlighted by AGU as a “Research Spotlight”. Respectively, ESA and AGU had a press release regarding these results.

 

Huishan Fu, professor, school of space and environment, Beihang University, E-mail: hsfu@buaa.edu.cn

 

 

References

[1]Fu, H. S., et al. (2017), Intermittent energy dissipation by turbulent reconnection, Geophys. Res. Lett., 44, 37–43, doi:10.1002/2016GL071787.

[2]Fu, H. S., et al. (2015), How to find magnetic nulls and reconstruct field topology with MMS data. J. Geophys. Res. Space Physics, 120, 3758–3782, doi:10.1002/2015JA021082.

AGU Press Release:

https://eos.org/research-spotlights/for-magnetic-reconnection-energy-o-not-x-might-mark-the-spot

ESA Press Release:

http://sci.esa.int/cluster/58994-o-marks-the-spot-for-magnetic-reconnection/