Aug 7 2019
Researchers from MIT and an international team of collaborators have now reproduced in the lab, with high fidelity, massive interstellar events in which charged matter clouds dash into each other and eject high-energy particles.
An example of an interstellar collisionless shock is seen in this photo of a bow shock in the Orion Nebula. (Image credit: NASA and the Hubble Heritage Team (STScI/AURA))
The study could help find solutions to longstanding debates over precisely what happens in these enormous shocks.
A majority of the events at the largest scale, like the expanding bubble of matter ejected outward from a supernova, undergo a phenomenon known as collisionless shock. As part of these interactions, plasma or gas clouds are so rarefied that a major portion of the involved particles, in fact, miss each other; however, they still interact electromagnetically or through other means to produce visible shock waves and filaments.
Thus far, it has been challenging to reproduce these high-energy events under laboratory conditions that emulate those in an astrophysical environment, resulting in disagreements among physicists regarding the mechanisms underlying these astrophysical phenomena.
At present, scientists have successfully reproduced critical conditions of these collisionless shocks in the lab, thereby enabling in-depth study of the processes that occur inside these giant cosmic smashups. The new outcomes have been reported in the Physical Review Letters journal, in a paper by MIT Plasma Science and Fusion Center Senior Research Scientist Chikang Li, five others at MIT, and 14 others from across the globe.
In the universe, almost all visible matter is in the form of plasma, a form of soup of subatomic particles in which negatively charged electrons swim freely together with positively charged ions rather than being linked to each other in the form of atoms. Plasma is found in the stars, the sun, and most clouds of interstellar material.
A majority of these interstellar clouds are highly ethereal and have such a low density that real-time collisions between their constituent particles are rare even if one cloud hurtles into another at very high velocities of more than 1,000 km/second. However, the outcome can be a dramatically bright shock wave, at times revealing substantial structural detail including long trailing filaments.
According to Li, astronomers have discovered that at these shock boundaries, various changes occur, where physical parameters “jump.” However, interpreting the mechanisms that occur in collisionless shocks has been challenging as the combination of extremely low densities and high velocities has been very difficult to achieve on Earth.
Although collisionless shocks had been previously predicted, the first one to be directly detected, in the 1960s, was the bow shock induced by the solar wind, a fine stream of particles radiating from the sun, when it comes into contact with Earth’s magnetic field.
Soon, several such shocks were identified by astronomers in interstellar space. However, since then, “there has been a lot of simulations and theoretical modeling, but a lack of experiments” to decipher how the processes work, stated Li.
Li and his coworkers discovered a method for mimicking the phenomena in the lab by using a set of six powerful laser beams to produce a jet of low-density plasma, at the OMEGA laser facility at the University of Rochester, and targeting it at a thin-walled polyimide plastic bag filled with low-density hydrogen gas.
The outcomes replicated several of the detailed instabilities noticed in deep space, thereby confirming that the conditions are in close agreement to enable a detailed, close-up study of these elusive phenomena. According to Li, the mean free path of the plasma particles was found to be considerably greater than the widths of the shock waves, thereby fulfilling the formal definition of a collisionless shock.
There was a drastic spike in the density of the plasma at the boundary of the lab-produced collisionless shock. The researchers could measure the detailed effects on the upstream as well as downstream sides of the shock front, enabling them to start differentiating the mechanisms that govern the transfer of energy between the two clouds, something that physicists have spent years attempting to find.
The outcomes are in agreement with a set of predictions based on the so-called Fermi mechanism, stated Li; however, more experiments will be required to certainly rule out some other mechanisms that have been suggested.
“For the first time we were able to directly measure the structure” of significant parts of the collisionless shock, stated Li. “People have been pursuing this for several decades.”
The team also revealed the precise amount of energy transferred to particles that travel through the shock boundary, which increases their speeds to a significant fraction of the speed of light, thereby generating the so-called cosmic rays. Better insights into this mechanism “was the goal of this experiment, and that’s what we measured” stated Li, explaining that they captured a complete spectrum of the energies of the electrons accelerated by the shock.
This report is the latest installment in a transformative series of experiments, annually reported since 2015, to emulate an actual astrophysical shock wave for comparison with space observations.
Mark Koepke, Professor of Physics, West Virginia University
Koepke added, “Computer simulations, space observations, and these experiments reinforce the physics interpretations that are advancing our understanding of the particle acceleration mechanisms in play in high-energy-density cosmic events such as gamma-ray-burst-induced outflows of relativistic plasma.”
Koepke is also chair of the Omega Laser Facility User Group and was not involved in the study.
Researchers in the international team were from the University of Bordeaux in France, the Czech Academy of Sciences, the National Research Nuclear University in Russia, the Russian Academy of Sciences, the University of Rome, the University of Rochester, the University of Paris, Osaka University in Japan, and the University of California at San Diego. The study was supported by the U.S. Department of Energy and the French National Research Agency.