Microphysics of cosmic plasmas


    The Universe is filled with energetic particles, electron and fully ionized atoms, that roam space with a velocity very close to the speed of light. Collectively, these particles are known as cosmic rays, and their origin is one of the fundamental unsolved problems in modern astrophysics.

    The processes that determine the energy and spatial distribution of cosmic rays are different from those that shape ordinary gases on Earth, because they rely almost entirely on electric and magnetic fields. In the dilute plasmas that fill interstellar space, nature chooses to endow a small number of particles with an extreme amount of energy. We witness a fundamental self-organization that, through interactions between particles and electromagnetic fields, arranges the atoms and available energy in three components: a cool or warm gas that carries the bulk of the mass, cosmic rays with a wide range of energies, and the turbulent electromagnetic fields that link the two. Interesting questions arise: Why does nature produce cosmic rays? Also, what is the fate of the turbulent magnetic field? Do interactions of cosmic rays generate the large-scale magnetic field that permeates the Universe?

    We conduct intensive so-called Particle-In-Cell simulations in which we follow individual particles as they move in electric and magnetic fields. The movie shows simulation results for drifting cosmic rays: The top panel indicates the turbulent magnetic field and the bottom panel depicts the density of interstellar gas. Initially the interstellar gas is at rest and the cosmic rays drift to the left. After a while the structures in both the magnetic field and the gas density appear to drift as well. In the end there is no relative drift between cosmic rays and interstellar gas, and the growth of magnetic field terminates.