How do you study a planetary shockwave without leaving Earth? By firing lasers at plasma and recreating Venus’s unusual planetary conditions. Tim Johnson’s experiment, detailed in an Editor's Suggestion PRL paper, brought Venusian physics to the lab and caught it on camera.
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In a paper recently published in Physical Review Letters (PRL), Timonthy Johnson, a PhD student at MIT’s Plasma Science and Fusion Center (PSFC), and his co-authors presented a scaled laboratory experiment that used laser-generated plasmas to reproduce the formation of Venus’s bow shock — the shock wave generated when a plasma such as solar wind interacts with a planet’s magnetosphere. The paper, Biermann-battery-driven magnetized collisionless shock precursors in laser-produced plasmas, was selected as an Editor’s Suggestion, an honor only given to around 15% of PRL letters.
Magnetized collisionless shocks are very common in astrophysical systems. When plasmas move towards each other and interact, as seen when solar wind encounters the earth’s magnetosphere, they can create magnetized collisionless shocks. These shocks dissipate energy while also accelerating particles into high-energy cosmic rays. Like most others, Earth’s planetary bow shock forms from the interaction between weakly magnetized solar wind and Earth’s strongly magnetized upper atmosphere called the ionosphere; however, Venus has no global magnetic field. Instead, the magnetic field embedded in the solar wind’s flowing plasma interacts with Venus’s ionosphere, resulting in magnetized collisionless shock formation.
Johnson and his team designed an experiment that makes it possible to observe how collisionless shock events can form on Venus—in a lab, not space.
Using the OMEGA facility at the University of Rochester, the team used lasers to create a bubble of plasma traveling faster than the speed of sound, mimicking solar wind. Importantly, no external forces are needed to kickstart the plasma’s magnetic field; when the laser generates the plasma, a phenomenon called the Biermann battery mechanism “freezes” a magnetic field into the plasma. Then, a cloud of charged hydrogen gas that doesn’t have its own magnetic field (like Venus’s ionosphere), is injected to push back against the supersonic plasma. When the laser-driven plasma interacts with the non-magnetized gas, the interaction produces a magnetized precursor to the shock. All of this happens in a few nanoseconds, so the team used advanced plasma diagnostics, including Thomson scattering, proton radiography, and electron spectroscopy, to capture detailed measurements of the plasma as the shock was formed.
Additionally, magnetohydrodynamic simulations were used to model the presence and behavior of Biermann batteries, while particle-in-cell simulations were utilized to study how individual particles behaved during the shock formation to corroborate the experimental results.
Johan Frenje, the PSFC’s head of the High Energy Density Physics division, said of the experiment, “This is a beautiful piece of work by Tim and his team, who used the unique capabilities of the OMEGA laser facility to study the interaction between the solar wind and unmagnetized planetary ionosphere of Venus, resulting in the formation of a collisionless bow shock.”
The paper was generated as part of Johnson’s PhD thesis work, which focuses on the study of astrophysically relevant collisionless plasmas in the laboratory. “It’s really nice to see this project come together after years of hard work,” said Tim. “I’m very grateful for all the help I received from my collaborators.”
Johnson’s team consists of scientists and graduate students from MIT, University of California Los Angeles, University of Rochester, Lawrence Livermore National Laboratory, and University of Bordeaux. Currently, Johnson is a postdoctoral researcher at Lawrence Livermore National Laboratory conducting his research on inertial confinement fusion and laboratory astrophysics.