Abstract:
Everybody knows that matter on Earth can be commonly found in three states: solid, liquid, and gas. But if we consider the Universe in which we live, 99% of all matter exists in a fourth state, called "plasma": a gas so hot and dilute that electrons are free to escape the electric pull of atomic nuclei. All the stars in the Universe are made of plasma, which also fills the interstellar and intergalactic space; plasmas are also found around black holes and neutron stars; and gigantic jets of plasma moving nearly at the speed of light protrude from the center of many galaxies and extend for hundreds of thousands of light years. Studying astrophysical plasmas is then of prime importance to understand many fundamental processes occurring everywhere in our Universe.
And yet, the study of plasmas is often very difficult: the equations that govern the plasma dynamics are extremely complicated, and usually can only be solved with the help of supercomputers carrying out calculations on thousands of CPUs. In this talk, I will review the basic theory of plasmas, by linking to phenomena that we commonly observe in many astrophysical environments, from our own Sun to the surroundings of supermassive black holes. I will describe the governing equations of plasma dynamics, and then move onto more detailed topics: the Sun's atmosphere, the solar wind and the Earth's magnetosphere, and the surroundings of black holes and neutron stars.
Bio:
Fabio Bacchini is Assistant Professor at the Centre for mathematical Plasma Astrophysics of KU Leuven (Belgium). In his work, he designs and applies numerical simulations on supercomputers to address the most relevant questions in astrophysics concerning space plasmas. His research covers a wide range of astrophysical scenarios, from the heliosphere and near-Earth environment to the surroundings of black holes and neutron stars.
Summary:
Plasma dynamics:
Atomic ions (nuclei with some/all electrons stripped off) and free electrons
99% of matter in universe is a plasma
Examples: auroras, lightning, plasma balls, nuclear fireballs, neon signs, welding arcs
Nuclear fusion: 2 small atoms (e.g. Hydrogen) fused into a larger one (e.g. Helium)
Magnetic confinement: use magnetic fields to confine Hydrogen nuclei into a small space at high temperature where they’ll collide and fuse
Since they’re charged and energetic they act in complex ways that are hard to model and manage
Plasmas in Space:
Sun: giant ball of plasma
Interplanetary/galactic space is filled with diffuse plasma
Black holes surrounded by plasma
Focus: stellar plasmas
Sun’s surface has regular explosions that blow out plasma
These plasma eruptions often hit the Earth
Earth’s magnetic field protects the Earth because it bends the path of the plasma, which is charged
When the diverted plasma particles end up hitting the Earth, we see auroras and can get disruptions to satellites, ground electronic systems
Multiscale spatial scales of plasma
Black hole:
Jets of gas coming out of a galaxy’s central black hole: 10k light years in length
Plasma around a black hole: 40b km wide
Plasma turbulence dynamics: ~1km long
Stars:
Coronal ejections
Complex internal structure: terminal shock, magnetic trap (XRay emissions from trapped gas), reconnection outflows (particle accelerated radiation)
Plasma simulations to understand dynamics
Plasma behavior is described by a complex set of equations:
magnetic/electric forces
contact forces of fluid, etc.
Aggregated fluid-level
Individual particle-granularity
May incorporate relativistic dynamics (key in high gravity and energy regimes)
Used to predict behavior of plasmas and design devices that interact and control them
Range in scale from microseconds to billions of years
Whole universe simulation
Black holes
Neutron stars
Supercomputing for plasma simulation
Need (hundreds of) thousands CPUs and GPUs
Hard to access and significant environmental impact
Scales: Micro->Meso->Macro
Regimes: Kinetic->Hybrid->Fluid
Energy: Newtonian->Transrelativistic->Relativistic
The location of use-case in above matrix determines the details that a simulation must incorporate
Examples:
Low energies: Heliosphere
Main structures: corona, wind, planetary magnetospheres
Phenomena: coronal mass ejections, wind, turbulence
Macro/Fluid/Newtonian
High energies: Black holes
High gravitational field bends spacetime and the path of light and matter
Show paths of all particles that leave, collapse into or permanently orbt
High energies, Large scales: BH/NS Magnetosphere: Macro/Fluid/Relativistic
High Energies/Small Scales: collisionless accretion and coronal acceleration:
Particle acceleration
Emission of X-Rays and Gamma Rays
Micro-Meso/Kinetic/Trans-Relativistic
High energies at mesoscales:
Fluid simulations where you place particles into the dynamics and trace their paths
Must account for relativity and small-scale kinetic effects
Meso/Hybrid-Kinetic/Trans-Relativistic
Simulations validated by
Comparing predicted energy emission spectra (especially X-rays) to real observations
Neutrino emissions
Timescales of events simulated vs astronomical observations