Stellar eruptions in the laboratory: First experimental evidence for their suppression in strong magnetic fields

Coronal mass ejections (CMEs) are giant expulsions of huge amounts of magnetised plasma from a star’s atmosphere into space. They play a major role in shaping stellar evolution, driving mass and angular momentum loss, and influencing the space weather environments of orbiting planets. While they are routinely observed on the Sun, convincing detections around other stars have remained surprisingly scarce.

The current study, recently published in the scientific journal “Physical Review Letters”, combines astrophysical simulations, high-energy laser-plasma experiments, and advanced 3D plasma numerical modelling. The results show that strong magnetic fields of active stars can completely suppress CMEs before they escape into space. “The unique combination of theory, laboratory tests, and numerical simulations provides the first experimental evidence supporting a long-standing prediction that stellar magnetic fields can confine these large-scale eruptions.” explains Julián D. Alvarado-Gomez, senior scientist in the Stellar Physics and Exoplanets research group at the Leibniz Institute for Astrophysics Potsdam (AIP). He contributed the astrophysical modeling that connects the laboratory experiments to real stellar environments.

Using scaling laws that faithfully reproduce astrophysical conditions, the international team generated laser-driven plasma flows in the laboratory to mimic the core of stellar CMEs. When exposed to relatively weak ambient magnetic fields, the plasma propagated freely. However, stronger magnetic fields caused the flow to fragment, become unstable, and ultimately stop altogether. “This actually came as a surprise when we were increasing the magnetic field strength and observed the change in behaviour of the propagating plasma, but this is the essence of experimental discovery”, says Julien Fuchs, then senior scientist at LULI/CNRS in France and now professor at Technion in Israel.

Detailed numerical plasma simulations identified a kink instability as the physical mechanism responsible for disrupting the eruption. In the laboratory experiment the eruption was triggered by the laser hitting the target, increasing its thermal pressure and causing the plasmoid to expand and move. The kink instability occurred on the plasma flow when the external magnetic field was strong enough causing it to bend and even travel backwards. This helps to understand why CMEs are so rare for stars: Although active stars produce enormous flares that should also generate powerful CMEs, the new results indicate that many of these eruptions may never escape the star at all, instead becoming trapped by the surrounding large-scale magnetic field.

The findings also have important implications for exoplanets: CMEs represent an extreme form of stellar space weather and can even strip the planetary atmospheres or change their chemistry, thus affecting the long-term habitability of planets. If many CMEs are magnetically suppressed, some exoplanets may be exposed to a less hostile space environment than previously anticipated, improving their prospects for retaining atmospheres over billions of years.

The research was supported by the European Research Council (ERC) Project GENESIS, ELI-NP, the Romanian Ministry of Research and Innovation, the U.S. National Science Foundation (NSF), and major French and European high-performance computing infrastructures.