University:
Boston University
Course:
CAS PY 105 | Physics 1
Academic year:

2024
Views:

385

Pages:

19
Author:

Reagan Malone

Kinetic Plasma Physics in the Ionosphere Talk Outline  Plasmas in the Ionosphere  Kinetic vs. Fluid Physics  Plasma Instabilities  Kinetic Simulations  Example problems in Space Physics  Limitations of these methods Plasma in the Ionosphere Ionosphere Plasma Composition Ionosphere Plasma Density Variability Arecibo Incoherent scatter radar Coherent Radar reflections from the Ionosphere   Bragg Scatter: wradar >> wp Example:  Scatter off E-region ionosphere    ~90-130 km altitude Electrojet irregularities Meteor plasmas Radar measurements in the F region Radar measurement of plasma density fluctuations Plasma Depletions which bubble up at night (sometimes) Altitude Spread-F Turbulence: Plasma Physics Approaches  Fluid Approaches  Cold  Warm  MHD Approaches  Ideal  Resistive  Hall  Kinetic Approaches  What are the differences? What are simulations?  Views of nature:  Physicists think that the real world approximates equations.  Engineers think that equations approximate the real world.  Mathematicians don’t care...  Simulations are a mathematical description, or model, of a real system typically in the form of a computer program  Simulations explore the behavior of systems too complex for analytical theory  Inhomogeneous systems  Nonlinear systems  Turbulence First Plasma Particle Simulations: Klystrons 1939: Klystron inventors William Hansen and brothers Russell and Sigurd Varian examine early model Proposed ITER Tokomak Fusion Energy Simulations Temperature fluctuations from Numerical Tokamak Turbulence Calculations on the CRAY T3E1 by Lynch, et al., Proceedings of the ACM/IEEE SC97 Conference (SC’97) Where does one need simulations in Ionospheric Physics?  The Auroral Ionosphere: Electrons accelerate from 3000-1500 km altitude by unknown mechanisms FAST Spacecraft measures turbulent auroral plasmas Altitude Altitude Radars Measure Electron Density Irregularities in Ionosphere Plasma Theory in 5 Minutes 1. Charged particles create fields: Maxwell’s Equations 2. Lorentz Force Accelerates Particles: 3. Equation of Motion 4.   e   E  (ni  ne ) 0    B  E   t    B  0     E   B  0   0 0 J t  dvi qi    E ( x , t )  vi  B ( x , t ) dt mi Collisions deflect particles (important in the lower ionosphere and other regimes) Too many particles – Need simplifications! Particle Simulations  Particles move within a box:  Position: xi  Velocity: vi  Particles generate fields which accelerate other particles  Too Slow! Speed proportional to the number of particles squared.  Fij  qi q j   2 40 ( xi  x j ) Electrostatic Kinetic Simulation Method: Particle-In-Cell 1. Gather to determine charge density, r r(x) = åq d(x - x ) i particles 2. Calculate Electric field: 3. Update velocities: 4. Update Positions: 5. Collide particles with neutrals 6. Go to Step 1     E  r 0 dv i qi = [ E(x i ,t) + v i ´ B(x i ,t)] dt mi  dxi   vi dt i Assumptions made by PIC  Short range interactions eliminated   Simulators with a meshes cannot model behavior smaller than the mesh Features must be bigger than the mesh  Each PIC particle models the behavior of more than 10 6 real particles  Fluid Simulators also use a mesh   Only one velocity in one location (unlike kinetic simulators) Misses some physics but is less costly (per cell)  Full kinetic physics represented   Particle trapping – resonant acceleration Landau damping – resonant wave damping