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The magnetic reconnection

Magnetic reconnection is an important and universal plasma phenomenon.

 LPP team :

N. Aunai, G. Belmont, P. Canu, O. Le Contel, A. Retino, R. Smets

 Selection of publications

 What is reconnection ?

It occurs within layers of electric current, formed during the interaction of plasma environments within which the magnetic field has different orientations. When these layers become thin enough to reach a dissipative scale, the magnetic field lines can change their connectivity, which otherwise must be conserved. The process is illustrated in figure 1.

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Figure 1
From left to right, one can see the magnetic field lines (black lines) and the plasma (colored particles) interact and change their connectivity/ Green particles, first magnetically connected, become magnetically disjoint, then reconnected to the violet particles. Like an elastic, the magnetic field lines relax and release the energy previously accumulated in the system by accelerating and heating the plasma.

 The reconnection in the LPP space plasma team :

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Figure 2
The Sun-Earth system representation (and more generally Star-Planet). At bottom are depicted, from left to right, the central pulsar of the crab nebula, an artist’s view of Magnetar, and the region of the central black hole of our Milky Way, which are as many astrophysical environments where reconnection could play a crucial role in the acceleration of high-energy particles.

As can be seen in figure 2, the sun-blown plasma wind flows into the interplanetary medium and interacts with the magnetospheres. Reconnection plays an important role at all stages of this way: trigger solar eruptions, affect the propagation of coronal mass ejections in the interplanetary medium, control how these structures interact with the magnetosphere, and finally accelerate and heat the plasma in the magnetotail.

Within the team, we essentially study this process when it takes place in the solar system, the only environment accessible with space missions. We analyze the data measured in situ by satellites in the interplanetary medium and in the terrestrial magnetosphere. These environments constitute a real laboratory for the study of reconnection, from which we can extrapolate certain behaviors to inaccessible astrophysical environments, or compare them to laboratory phenomena occurring in magnetic confinement devices such as the Tokamaks.

In situ measurements, among which those acquired by Cluster, Themis or Magnetospheric MultiScale (MMS) missions, give information on micro physics and collisionless mechanisms that govern reconnection, acceleration and heating of the plasma.

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Figure 3
Example of signatures associated with a magnetic reconnection event as measured by Cluster à the Earth’s magnetopause.

Together with the satellite observations, we study the mechanisms governing the evolution of the reconnection and its impact on its environment via numerical simulation.

According to the scale considered, we use codes with different approximations. Some codes treat plasma as a fluid, others as a set of particles, and others are hybrid, treating ions as particles and electrons as a fluid. The simulations allow, in a complementary way to the observations, to isolate certain physical mechanisms in a simplified and reproducible framework.

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About reconnection... (figures from numerical simulations:hybrid, full-PIC and test-particule). Source: N. Aunai.

 Examples of issues addressed in LPP :

-  What is the impact of the imposed large-scale plasma environment (asymmetries, geometry, etc.) on the reconnection process? For example, what are the differences between reconnection at the magnetopause and in the magnetospheric tail?

-  What are the critical regions where the plasma cannot be assimilated to a fluid but must be treated as particles ? What are the mechanisms of heating and acceleration of particles? How can we reconcile the particle point of view with the fluid point of view?

-  What is the link between reconnection and turbulence? Is reconnection a source of turbulence, and to what extent does it contribute to the dissipation of energy at small scales?

Other questions are addressed by researchers working on laboratory experiments: triggering of sawtteeth in Tokamaks, LASER reconnection, applications to high energy density physics,...

Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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