Surfing On The Shock Wave Front (Physics)

Reproduced in the laboratory, through plasma physics experiments, the so-called “collisionless shocks”. The results show that the relativistic energies to which the particles are accelerated by the shock surfing acceleration process are sufficient to trigger a process capable of bringing them to energy values typical of cosmic rays. Among the authors of the study, published today in Nature Physics, Marco Miceli of the University of Palermo and Salvatore Orlando Inaf of Palermo

Our planet is constantly bombarded with highly energetic particles from outer space called cosmic rays . Years of studies and observations have shown that these particles can be accelerated to the energy values observed by shock waves ( shock ) which propagate in different astrophysical environments, mainly in supernova remnants in expansion in the interstellar medium (a supernova remnant is a rapidly expanding nebula produced by a supernova explosion). Researchers have identified several mechanisms that, depending on the conditions of the shock and the surrounding environment, can accelerate particles to the typical values of cosmic rays.

The figure shows the setup of the experiment. 
In panel a) you can see the ambient gas, the solid target, the direction of the laser and that of the magnetic field.  Panel b) also shows a three-dimensional rendering of the expanding plasma in the ambient gas obtained from Flash simulations.  Panels c) and d) show the gas density (integrated along the line of sight) 4 ns after the incidence of the laser, measured in two different planes (xy and xz).  The direction of the magnetic field is also indicated.  Credits: W. Yao et al., Nature Physics, 2021

To study the triggering of the acceleration process, a team of researchers led by physicists Weipeng Yao and Julien Fuchs (École Polytechnique, Sorbonne Université) reproduced in the laboratory the conditions of an expanding shock in an ambient gas, immersed in a magnetic field. uniform, and used magnetohydrodynamic simulations to study the evolution of the system. Ambient gas is made with low density hydrogen (10 18  cm -3), in which a solid Teflon target is immersed. This is then hit with a 1 ns high-powered laser pulse, which produced hot plasma that began to expand into the ambient gas. The initial rate of expansion of the resulting shock was 1500 km / s. Furthermore, the shock is initially characterized by parameters that describe the collisions between particles – mean free path and collision time of the collisions between ions – typical of conditions in which collisions are not important during the expansion of the shock wave ( collisionless shock). The whole system is immersed in a uniform magnetic field, aligned transversely with respect to the direction of propagation of the laser. This configuration reproduces the conditions of shock wave propagation in various astronomical environments, such as supernova remnants interacting with dense molecular clouds and the solar wind.

In addition to verifying the possibility of reproducing an expanding shock in the laboratory, the researchers measured, thanks to two spectrometers, the speed of protons accelerated during the experiment. From these measurements it was possible to demonstrate the presence of particles that were accelerated to energies of several hundred keV by the shock. Both the experiment and the numerical simulations show how these particles were accelerated in the first 2-3 ns of the system’s evolution, when the shock propagated at a speed greater than 1000 km / s. The dominant acceleration mechanism in this initial phase is shock surfing acceleration , a process in which the charged particles placed in front of the shock wave are accelerated by the electric field associated with the shock. «The process ofshock surfing acceleration allows the protons to be accelerated while “riding” the shock wave front, a bit like surfers, due to the effect of the electric field in the shock region “, one of the co-authors of the study explains to Media Inaf , Marco Miceli of the University of Palermo.

The experiment demonstrated that the relativistic energies at which the particles are accelerated by the shock surfing acceleration are sufficient to trigger a further process – known as diffusive shock acceleration and not reproduced by the experiment – capable of bringing the particles to typical energy values. of cosmic rays. “With laboratory experiments,” adds another of the co-authors, astrophysicist Salvatore Orlando of the INAF of Palermo, “we were able to recreate, on a scale, physical conditions extremely similar to those observed in some supernova remains, showing the surfing processit can trigger the acceleration mechanism of cosmic rays in these sources ». Finally, both the experiment and the simulations have shown the importance of the magnetic field, without which it is not possible to form the expanding shock.

This study therefore identifies for the first time in shock surfing acceleration the process responsible for the first phase of acceleration of cosmic rays by shocks similar to those that characterize supernova remnants. More generally, there are several astrophysical environments where this process plays an important role in the acceleration of charged particles. For example, during highly energetic phenomena on the Sun, such as in the shock front caused by coronal mass ejections, in interplanetary space in the region of interaction of the terrestrial magnetosphere with the solar wind, at the termination shock of the solar wind and, more generally, at the termination shock of the solar wind. in stellar winds.

To know more:

  • Read in Nature Physics the article ” Laboratory evidence for proton energization by collisionless shock surfing “, by W. Yao, A. Fazzini, SN Chen, K. Burdonov, P. Antici, J. Béard, S. Bolaños, A. Ciardi , R. Diab, ED Filippov, S. Kisyov, V. Lelasseux, M. Miceli, Q. Moreno, V. Nastasa, S. Orlando, S. Pikuz, DC Popescu, G. Revet, X. Ribeyre, E. d ‘Humières and J. Fuchs

Provided by INAF

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