Nature Physics: Turbulent magnetic reconnection driven by strong laser

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Laser

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Laser drive

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2023-01-30

It is reported that Chinese scientific researchers, relying on the "Shenguang II" device of Shanghai National Laboratory of High Power Laser Physics, realized the laser-driven turbulent magnetic reconnection physical process for the first time, confirming the importance of turbulent process in the rapid triggering of solar flares, and providing an important basis for understanding the origin and acceleration process of solar flare high-energy particles. On January 17, 2023, relevant papers were published in the journal Nature Physics with the title of "Turbulent magnetic connection generated by intensive lasers".

Schematic diagram of solar flare with coronal mass ejection (picture from: People's Daily)
Solar flare is a violent solar activity phenomenon. A typical flare explosion is equivalent to billions of hydrogen bombs. Flares can produce multi-band radiation, and severe flares will seriously affect the solar-terrestrial space environment, and even affect human life. Therefore, it is of great significance to understand and understand solar flares.

Turbulence magnetic reconnection is considered as a trigger point of solar flares. It often occurs in current sheets that are stretched and fragmented for a long time. In this paper, researchers demonstrated the turbulent magnetic reconnection produced by laser-generated plasma when it irradiates solid targets. Turbulence is generated by strongly driven magnetic reconnection, which fragments the current sheet. Researchers have also observed the formation of multiple magnetic islands and flux tubes. The findings of scientific researchers reproduce the key features of solar flare observation. With the support of dynamic simulation, researchers have revealed the mechanism of electron acceleration in turbulent magnetic reconnection. This mechanism is dominated by parallel electric field, while the electron acceleration mechanism plays a cooling role, and Fermi acceleration can be ignored. Because the conditions of laboratory experiments of scientific researchers can be extended to the conditions of astrophysical plasma, the results of scientific researchers are applicable to the study of solar flares.

Magnetic reconnection is the process of annihilation of magnetic lines of force in the opposite direction driven by the free energy generated by the current distribution or the energy exerted by the external force. It is a fast way to release and transfer a large amount of magnetic energy in the laboratory, space and astrophysical plasma, so it is considered to be the main trigger factor of some explosive astrophysical events. In astrophysical plasma, random magnetic field distribution is often observed.

In the experiment of simulating solar flares in a laser-driven laboratory, it was also found that electrons can be accelerated to the relativistic state. Plasma collimation, as well as the tip and plasma-like configurations are observed, which indicates that this reconnection process is on the electronic scale and is driven by the electron dynamics in the laser-generated plasma. This paper introduces the debris ammeter caused by turbulent magnetic reconnection in the laboratory astrophysics experiment driven by high energy laser. The researchers also studied the characteristics of turbulence and electronic acceleration.

The experiment of this study was carried out in the Shenguang II laser equipment. Figure 1a shows the setup and analysis of the laser. The bright spots on the target (aluminum) irradiated by four laser beams are shown in Fig. 1b and c. Two of them are above and the other are below. The energy of each laser beam is 0.26 kJ, and the wavelength is λ L = 0.351 μ m. Is a nanosecond square pulse. Each beam is focused to the focal spot with a diameter of 50-100 μ M (half maximum full width), making the incident laser intensity 1015 W cm − 2. The distance between the upper and lower points in Case I (Figure 1b) and Case II (Figure 1c) is 200 respectively μ M and 400 μ m。 The slit width between two targets is 600 μ m。 Four driving laser beams are irradiated on the target, and four extended annular magnetic field structures are generated through the Bilman cell effect. Then a current sheet is formed on the back plate of the slit between the targets. Compared with the previous two laser-driven reconnection experiments, the current sheet of this four-beam laser experiment is much longer.

Figure 1: Laser-driven turbulent magnetic reconnection experiment.

In order to further analyze the properties of the turbulent region in these experiments, two-dimensional particle unit (PIC) simulation was carried out, and the key processes were checked by three-dimensional PIC simulation. In these simulations, the initial parameters are basically the same as the experiments of researchers. For example, four magnetic field bubbles are generated by four laser irradiation. The structure of these magnetic field bubbles corresponds to the experimental configuration in Case I and II. In the calculation, researchers assume that the spatial scale di in the simulation is equal to the spatial scale di in the experiment. The simulated current sheet structure and turbulence characteristics are shown in Figure 2. It can be clearly seen that the density fragments are similar to the measured values (Figure 1b, c). At the same time, multiple magnetic islands are generated during the drive magnetic reconnection (Fig. 1). In the 3D PIC simulation, the magnetic flux tube replaces the 2D magnetic island (as shown in Figure 4 of the extended data).

Figure 2: Current sheet structure and turbulence spectrum of two-dimensional PIC simulation.

Figure 3a and b show the average electron energy distribution in the simulation domain of Case I. More energetic electrons are excited in the current sheet area and then trapped in the magnetic island. As the reconnection continues, the current sheet breaks and forms islands (Fig. 1b). Then, the width of the island will be further expanded, and the captured high-energy electrons will lose their energy over time. In contrast, in case II, more energetic electrons are generated and trapped in the current sheet, as shown in Fig. 3d. In addition, as shown in Figure 3e, the current slice fragments are not as significant as in case I, and the islands are still connected. Fig. 3c, f shows that the moving pressure of ions and electrons is much greater than the magnetic pressure when x=0; In case I, the ratio of ionic motion pressure to magnetic pressure is 150, and in case II, it is~60-70. Figure 3c, f shows the three stages of the process.

Figure 3: 2D PIC simulation results.

Generally, the reconnection process is accompanied by plasma heating and electron acceleration. The measurement results show that the electron acceleration in case II is stronger than that in case I. According to the previous research on turbulent collision-free impact, when two laser beams only illuminate the upper target (case III), the electron energy spectrum is analyzed. Figure 4a shows the corresponding power spectrum with non-thermal part in Case I and II, while in Case III, the energy spectrum approximates the Maxwell distribution. In case I, the power law index of the spectrum is − 1.1 (Figure 4a, yellow line) and − 2.8 (Figure 4a, gray line). However, in case II, except that the power-law index is − 1.1, ε = At 0.17-0.21 MeV, the part with higher energy（ ε = 0.21-0.6 MeV) can be fitted with a power law index of − 1.6 (Figure 4a, light blue dotted line). Therefore, the measurement of researchers shows that different acceleration mechanisms play different roles in these reconnection processes. It can be seen from Figure 4c-f that the cooling mechanism of the electron induction accelerator plays a leading role in the outflow zone, especially in Case I. Therefore, for the electron transmission along the outflow direction, in case I, more electrons are decelerated and more energy is deposited into the exhaust.

Figure 4: Electronic acceleration mechanism.

In short, the experiments of scientific researchers provide a typical example for the study of turbulent magnetic reconnection. In addition, non-thermal electron spectra with significant acceleration in the ion inertia region were also observed. Because of its light weight, the electrons are completely magnetized in the current sheet, so most of the electrons move along the magnetic field line.

However, the gyroscopic radius of the ion is larger than the turbulent scale, so the ion is not magnetized. Therefore, the propagation of protons can be described as a random walk between the combined reconnected field lines until they leave the current sheet. This study will encourage researchers to continue to study turbulent reconnection in different plasma environments. Based on this research, researchers can also study other astrophysical phenomena, especially events in the solar active region, such as the generation of the solar needle, the appearance of the magnetic field, the formation of the corona ring, and so on. This is also helpful for the research of reconnection model of magnetotail turbulence and gamma ray burst turbulence.