How do particle currents, magnetic fields and energies behave in the interstellar medium – the space between the stars? So far, models have only been able to reconstruct this to a limited extent because the interactions of magnetic fields and currents in this cosmic environment are extremely complex. Now astrophysicists have been able to understand this magnetized turbulence in the interstellar medium in a magnet hydrodynamic simulation for the first time using a supercomputer in Germany – and this in high resolution. Thanks to this simulation, researchers can now examine these galactic, interstellar processes with a high level of precision and on various scales. The first analyzes already show interesting deviations from common models and theories.
The interstellar medium of the Milky Way and other galaxies play a crucial role for many galactic and cosmic processes – from star formation to cosmic radiation to the distribution of plasma, gases and elements. But the behavior of this medium has only been researched in parts so far. One of the reasons for this: Although there is only a small partial density in the interstellar medium, the movements of this largely invited particle create a magnetic field. This in turn influences the turbulence of the interstellar medium and in turn affects the turbulent currents. “This coupling between turbulence and magnetic fields plays an important, diverse role,” explain James Beattie from Princeton University and his colleagues. “The presence of magnetic fields fundamentally changes the nature of the turbulent currents.”
Cascading interactions and a supercomputer
But the physics and dynamics of these magnetized turbulence are so far difficult to grasp. “Turbulence remain one of the greatest unsolved problems in classical mechanics,” says Beattie. “This despite the fact that turbulence is omnipresent: swirling milk in our coffee to chaotic currents in the oceans to sun wind, interstellar medium and the plasma between galaxies.” Most of the previously existing theories and high-resolution models only describe the nature of magnetized turbulence in non-compressible, differently fast plasma and mostly with a uniform background magnetic field. “But this limits their application to the compressible turbulence of the interstellar medium driven by a dynamo,” the physicists explain this assumption, according to a whole cascade of turbulent currents of different sizes and energies. However, this requires enormous computing effort.
But the team around Beattie has now succeeded – also thanks to a German supercomputer. The Supermug Super Muc-NG is located in the Leibniz Supercomputing Center in Garching near Munich and comprises 6,480 compensation notes with 48 cores each. The astrophysicists used the performance of this computer to carry out the most extensive simulation of magnetized turbulence in astrophysical environments. “These are the first magnet hydrodynamic simulations that dissolve both the under and overshafly cascades in a self-preserving dynamo-driven magnetic field,” explains the team. “The resolution of our simulation is almost an order of magnitude higher than previous modeling of this regime.” The modeling required more than 80 million CPU hours, spread over 140,000 computing cores. It shows the particle and energy flows as well as magnetic fields in a cube-shaped neckline of space, which includes a room volume of around 30 light years on the largest scale. On the smallest scale, the neckline can be reduced by a factor of 5000. “This is the first time that we can examine these phenomena with such a precision level and on different scales,” says Beattie.
Deviations from common theories
The first analyzes based on the new model already showed surprising. Because in the simulations, the researchers found significant deviations from the astrophysical models that have been common for decades. “In highly magnetized modes, the magnetic energy spectrum forms a local cascade that deviates from every known AB-Initio theory,” write Beattie and his colleagues. Accordingly, the magnetic fields in the interstellar medium change the energy flows in other ways: they suppress movements on a small scale and increase special wave-like disturbances such as the Alfvén waves. These results thus throw a new light on the transport of high -energy particles between the stars and the turbulent structure of our galaxy. “We get one step closer to the decryption of true nature of astrophysical turbulence – from chaotic plasma to the rooster to the huge movements in our galaxy and beyond them,” says Battie.
Thanks to its high resolution, the model also has the potential to provide a deeper understanding of the star formation. “We know that the magnetic pressure hinders star development by counteracting gravity that tries to have a stubborn cloud of fog collapse,” explains Beattie. “Now we can quantify in detail what can be expected from magnetic turbulence in these sizes.” The physicist also checks the results of the new simulation for data that was collected when observing the Sonne-earth system. “We have already started to check whether the model matches the existing data of the sun wind and earth – and it looks very good,” says Beattie. “This is very exciting, because it means that with our simulation we can learn something about space weather.”
Source: James Battie (Princeton University) et al., Nature Astronomy, DOI: 10.1038/S41550-02551-5