When Shock Meets Turbulence (Astronomy)

A team from the Geophysical and Plasma Physics Astrophysics group of the University of Calabria has developed a numerical model that helps to understand in detail the interaction between shock waves and turbulence. The result, published in the latest issue of PNAS (which dedicated the cover to it), has several applications in the field of astrophysics

It all began in the spring of last year, when a young Calabrian physicist, Domenico Trotta , after a period spent in London – at Queen Mary University – for his doctorate, returned to Italy. And it goes straight to the University of Calabria, in Cosenza. The university where he graduated, and where there is one of the largest research groups for the study of plasma in Europe. A worse time could not have chosen. «I started service on March 1st 2020. Just long enough to say good morning, sign the contract, and the lockdown has started . Suddenly I found myself alone, isolated at home, like everyone else », Trotta now recalls to Media Inaf. A shocking return. But he who has made shocks his object of study does not lose heart, on the contrary. His specialty is in fact the numerical simulation of shock waves – shock waves – which occur everywhere in nature. “Here at the University of Calabria there are many experts in turbulence, in solar wind. I had just finished my doctorate, in fact, on numerical shock simulations . So it was natural to try to reunite the two worlds a little. “

And this is how a research – funded through the H2020 Aida project – started, aimed at understanding what happens when a shock wave crashes against a turbulent environment. An encounter similar to that simulated in wind tunnels, for example, between cars and moving air. Except that air is a viscous material, and the effect it has on a car launched at high speed is to slow it down. The shocks and turbulences studied by Trotta and his colleagues – University of Calabria professors Francesco Valentini and Sergio Servidio , and David Burgess of Queen Mary University of London – are instead plasma- based.: a mixture of mostly free protons and electrons, not bound together. And the effects can be far more surprising.

What effects, then? To reconstruct them, the four physicists simulated a plasma made up of billions of particles on the supercomputer – plasma, in fact. And like in a video game they shot it at supersonic speeds like a shock wave – what astronomers call collisionless shoc k – until it collides with billions of other particles – always a plasma, but this time in a state of turbulence. In practice, they have reproduced in detail what happens all the time almost everywhere in the universe: every time the turbulent solar wind collides with the earth’s magnetosphere, for example, or the shock wave of a supernova when s ‘breaks with the surrounding interstellar medium.

The cover of the latest issue of Pnas, dedicated to the study of D. Trotta et al., Shows the simulation of a supersonic plasma interacting with the surrounding turbulence (left), in which the shock front (which marks the bluer area, right) spreading in the turbulent medium

The result of these simulations can be found on the cover of the latest issue of PNAS , which you see on the side. And beyond the aesthetic appeal – with the magnetic field lines reminiscent of a Van Gogh painting – it is a result that shows what happens to the particles of the shock front when it collides with turbulence.

«The swirling structures that are observed, for example, in the Starry Nightby Van Gogh are frighteningly similar to everyday phenomena: turbulence in the sea, in rivers, in our coffee, and why not: in astrophysical shocks ”, explains Trotta. “In interplanetary space, however, history is more complicated than what we see here on Earth, in classical fluids, and perhaps even more convoluted than what happens in Van Gogh’s paintings. The astrophysical objects are immersed in a very complex fluid, the plasma precisely – a highly ionized state of matter and particularly sensitive to electric and magnetic fields. From the solar corona to the largest systems in the universe, such as galaxy clusters, the vast majority of observable matter is in the plasma state. When the plasma is very diluted (and this is the case with many astrophysical systems), collisions between particles are negligible and interactions between particles are mediated by electromagnetic forces. One of the fundamental characteristics of non-collisional plasmas is the possibility of accelerating particles at high energies ».

“Think of a surfer riding a wave,” adds Sergio Servidio. «For particles it is the same:“ riding ”the shock is accelerated. Now, however, these particles that surf on the shock receive an additional kick from the turbulence, from the swirling plasma areas with which they collide. And then they accelerate (or decelerate) again, gaining additional momentum. Similarly, astrophysical observations suggest that there are phenomena of extreme acceleration to shocks, where the particles also suffer the effect of turbulence and reach unthinkable energies. From this point of view, compared to a fluid, the so-called plasma turbulence is more complex, unpredictable and, despite the years of research, less understood ».

Examples of these interactions between shocks, plasmas and turbulence are numerous. “Think for example of our planet, which represents an obstacle for the plasma emitted by the sun”, observes Francesco Valentini. «The turbulent solar wind is forced by the presence of the terrestrial magnetosphere to stop its course in correspondence with the Earth. Since this flow is supersonic, a shock is created at the interface with the magnetosphere that marks the braking of the solar wind before the planet ( terrestrial bow shock). Other famous non-collisional shocks are those that result from the interaction between supernova explosions and the turbulent interstellar medium. Supernovae are believed to be the primary hotbed of very high energy cosmic rays. In all these cases, turbulence plays a fundamental role in the production of energetic particles, as already intuited in the first brilliant works of Enrico Fermi, in 1949 ».

An example of the new diagnostics used for the identification of particle transport phenomena. Above, the mosaic that identifies areas of acceleration (red) and deceleration (blue) of the plasma. Below, the turbulent electric field responsible for this acceleration. © INAF

Several results are presented obtained with the help of simulations, explain the researchers. Once the particles are accelerated by the shock and begin to propagate in the surrounding medium, the presence of turbulence plays a fundamental role in the evolution of the system. First of all, for higher turbulence levels, the particles are accelerated to higher and higher energies, extracting energy from the turbulent fluctuations. The turbulence is therefore able to “scramble” the particles produced by the shock. It has also been observed that turbulence changes some of the fundamental properties of the shock: for example, it induces strong distortions of the front.

«But turbulence is not a totally random state of matter, in turbulence there are structures, waves, fluctuations of different nature and magnetic reconnection. Therefore turbulence can not only accelerate but also decelerate, “trapping” the particles, which give energy to the surrounding environment. In this regard, we have developed a new diagnostics to quickly and accurately identify plasma areas where acceleration / deceleration or compression / expansion phenomena occur. Using this diagnostic, the plasma becomes a “mosaic” of interesting areas from the point of view of particle energetics. We have established that the turbulent electric field is an important ingredient in the acceleration process. For now, the new diagnostics has been applied to numerical simulations “, concludes Trotta,in-situ “.

Featured image: The three physicists of the University of Calabria, together with David Burgess of the Queen Mary University of London, of the article published in PNAS. From left: Sergio Servidio, Domenico Trotta and Francesco Valentini

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Provided by INAF

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