Studying the Microphysics of Magnetic Reconnection in the Earth’s Magnetosphere and the Solar Wind Electron Heating Michael Shay Department of Physics and Astronomy University of Delaware Precursor: presentations/2012-09-swarthmore-colloquium/presentation.pptx, but I converted to keynote and threw out a huge number of slides. Collaborators • Colby Haggerty – Univ of Delaware • Tai Phan Marit Oieroset – Berkeley • Masaaki Fujimoto • Paul Cassak – Univ of West Virginia • Jim Drake Space Weather • The nature of changing environmental conditions in space. – Plasma: A gas of charged particles. A Solar Flare • Explosive energy release – Up to 1032 ergs 3 x 1018 kW-hr – Takes ~ 20 minutes – Equivalent to: QuickTime™ and a Photo decompressor are needed to see this picture. 40 billion atomic bombs(!) 2005 human energy consumption: 1.4 x 1014 kW-hr Data from TRACE Spacecraft Auroral Substorms • All Sky Images – Nishimura et al., GRL, 115, A07222, 2010. QuickTime™ and a Motion JPEG OpenDML decompressor are needed to see this picture. Overview • Plasma Physics Primer • What is Magnetic Reconnection? • Electron Heating due to Magnetic Reconnection Overview • Plasma Physics Primer • What is Magnetic Reconnection? • Electron Heating due to Magnetic Reconnection Plasma - Large Scale Behavior To Sun Electrons () Ions (+) MHD Magnetohydrodynamics Charge Separation Scale MHD - Magnetohydrodynamics • Fluid Equations – Slow Timescales – Large length scales • Key Physics – Magnetic field lines act like rubber tubes • Alfven Speed : mi n d V B g B dt t t B c E n gnV E – Plasma “Frozen-in” to the magnetic field • Magnetic Topology is conserved: 4 2 B nT 8 V c B Magnetic Topology is Conserved => Magnetic field lines can’t be cut. Everything Breaks Eventually Formation of Boundary Layers Boundary Layers • Tiny layers that separate distinct regions – Small scales => Different Physics – “Effective Larmor Radius:” Inertial Length • δ = c/ωp • Plasma – Different magnetic fields – Diffusion region Overview • Plasma Physics Primer • What is Magnetic Reconnection? • Electron Heating due to Magnetic Reconnection Magnetic Reconnection Vin CA • Simplistic 2D picture • Change of magnetic topology – Releases magnetic energy Diffusion Region MHD not valid Magnetic Reconnection Jz and Magnetic Field Lines QuickTime™ and a GIF decompressor are needed to see this picture. Reconnection Rate D Vin B Vout Vout Vin •• • – •• B Conservation of Mass Conservation Rate: Reconnection of Rate: Energy Vinin ~ (δ/D) cA Reconnection V mi n Vin D ~ mi n Vout δ Last 10 years: ~ O(0.1) Eδ/D out-of-plane ~ Vin B δ Reconnection in Solar Flares • X-class flare: ~ 100 sec. • τA~ L/cA ~ 10 sec. • Fast! – Every day analogy: Speed of sound F. Shu, 1992 •d Reconnection drives macroscale flows Energizes particles Kivelson et al., 1995 A Multi-Scale Challenge • Reconnection Diffusion region scales: 1 km – Microscale process – Macroscale effects • Complete description 300,000 km – Model Macroscales – Resolve Microscales – Impossible! • Grand Challenge Problem Kivelson et al., 1995 Unsolved Reconnection Questions • What makes it turn on and off? • Where does the energy go? – Flows, electron or ion heating? • What about 3 Dimensions? • Turbulence? • But you’ve been studying it for 50 years! Overview • Plasma Physics Primer • What is Magnetic Reconnection? • Electron Heating due to Magnetic Reconnection Observing Magnetic Reconnection • In-situ satellite measurements MMS Mission • Specifically devoted to studying magnetic explosions – Cost: $1 billion – Launch date: 2014 – 4 satellite mission • MMS Movie Example of magnetopause reconnection with electron heating THEMIS-D jet 70 eV heating THEMIS-D jet Electron bulk heating seen in some regions, not in others jet jet jet Solar Wind: No heating (Gosling, 2007) Magnetopause: 10s of eV gain in Te (Gosling et al., 1990) Magnetotail: keV heating Heating in Plasmas • H-Theorem – Gas/Plasma in thermodynamic equilibrium relaxes to a maxwellian particle distribution. • Adiabatic Heating – Compression. Does work. Leads to heating. • Requires thermodynamic equilibrium. • Maxwellian velocity distribution • Joule Heating – Scatter current. Generate heat. – Requires collisions • Solar Corona/Solar Wind/Magnetosphere – Almost collisionless! – Not in thermodynamic equilibrium! Ion Distribution Function • Multiple populations • Non of which are Maxwellian Electron Distribution Functions: Simulation • Chen et al., 2008 T|| > T⊥ Multiple Species Maxwellian Fluid Description not Adequate • Kinetic representation: Boltzmann Equation • f (x,v) • Two options – Discretize x and v • 5 dimensions - Expensive! – Random particles: Follow trajectories Simulating Kinetic Reconnection • Finite Difference – Fluid quantities exist at grid points. • E,B treated as fluids always – Maxwell’s equations • Kinetic Particle in Cell – E,B fluids – Ions and electrons are particles. – Stepping fluids: particle quantities averaged to grid. – Stepping particles: Fluids interpolated to particle position. Grid cell Macro-particle SmallLose Scale theReconnection Forest for theStudies Trees • Include all kinetic physics – Simplistic simulation geometry – Simplistic boundary conditions • Basic physics simulations – What is the basic physics controlling electron heating during magnetic reconnection? • Massively parallel simulations – 4000 - 16000 cores – 100 billion particles • Strong union of simulations/theory • Comparisons with observations Simulation Parameters • • • • • • • Normalizations: L0 = di = c/ωpi, t0 = (Ωci)-1 Simulation Size: 204.8 di X 102.4 di Grid: Δ = 0.05 di mi/me = 25, 100, c = 15, 30 Boundary conditions: periodic Equilibrium: Double Harris equilibrium Simulate until quasi-steady – Time average over a few (Ωci)-1 • Coordinates: “Simulation Coordinates” – Outflow: x – Inflow: y – Out-of-plane: z Initial Conditions • • Basic Reconnection Simulations – Periodic boundary conditions Reconnected flux Density Double current sheet – Reconnects robustly • Initial x-line perturbation • Excellent Testbed for studying basic properties of reconnection • Current along Z Does not include many boundary condition effects Reconnection Rate Time Y Z Z t=0 X X XX Y Z Z t = 1200 Time X X XX Simulation Parameters 3 • Observational events are often in a parameter regime not typically simulated – β relatively small in simulations – Example: GEM Challenge had β ≈ 0.2 Te Te ∞ 1/e, rec (eV) 0.5 Ti/Te ~ 5 e, rec 5.0 nkTe/(Brec2/2 0) Table of All Most Simulations • Currently about 50 simulations • Simulate a range of: Run # 301 Breconn Bguide ninflow Te B2 Ti β⊥ β⊥e β⊥i βtotal 1 0 0.2 0.25 0.25 1.00 0.20 0.10 0.10 0.20 1 1 0.2 0.25 0.25 2.00 0.20 0.10 0.10 0.10 1 0 0.2 0.25 2.25 1.00 1.00 0.10 0.90 1.00 304 1 1 0.2 0.25 2.25 2.00 1.00 0.10 0.90 0.50 305 1 306 1 run307 1 run311 1 302 303 – – – – – run308001 0.447 run312001 0.447 run309 1 run313 1 run315 1 run316 1 run310001 2.236 run314001 2.236 run317001 2.236 run318001 2.236 Reconnection B-field: Br = .4 to 2.3 Reconnection Guide Field: Bg = .4 to 2.3 Density: n = .04 to 1.0 Ti/Te = 1 to 10 β = 0.1 to 6 0 0.2 2.25 0.25 1.00 1.00 0.90 0.10 1.00 1 0.2 2.25 0.25 2.00 1.00 0.90 0.10 0.50 0 1.0 0.25 0.25 1.00 1.00 0.50 0.50 1.00 1 1.0 0.25 0.25 2.00 1.00 0.50 0.50 0.50 0 0.2 0.25 0.25 0.20 1.00 0.50 0.50 1.00 0.447 0.2 0.25 0.25 0.40 1.00 0.50 0.50 0.50 0 0.04 0.25 2.25 1.00 0.20 0.02 0.18 0.20 1 0.04 0.25 2.25 2.00 0.20 0.02 0.18 0.10 0 0.04 2.25 0.25 1.00 0.20 0.18 0.02 0.20 1 0.04 2.25 0.25 2.00 0.20 0.18 0.02 0.10 0 0.2 0.25 2.25 5.00 0.20 0.02 0.18 0.20 2.236 0.2 0.25 2.25 10.00 0.20 0.02 0.18 0.10 0 0.2 2.25 0.25 5.00 0.20 0.18 0.02 0.20 2.236 0.2 2.25 0.25 10.00 0.20 0.18 0.02 0.10 run319 0.447 0 0.2 0.25 2.25 0.20 5.00 0.50 4.50 5.00 run320 0.447 0.447 0.2 0.25 2.25 0.40 5.00 0.50 4.50 2.50 run321 1 0 1.0 0.25 2.25 1.00 5.00 0.50 4.50 5.00 run322 1 1 1.0 0.25 2.25 2.00 5.00 0.50 4.50 2.50 run323 1 0 0.2 0.25 1.25 1.00 0.60 0.10 0.50 0.60 run324 1 1 0.2 0.25 1.25 2.00 0.60 0.10 0.50 0.30 run325 1 0 0.2 0.0625 0.3125 1.00 0.15 0.03 0.13 0.15 run326 1 1 0.2 0.0625 0.3125 2.00 0.15 0.03 0.13 0.08 run327 1 0 0.2 1 5 1.00 2.40 0.40 2.00 2.40 run328 1 1 0.2 1 5 2.00 2.40 0.40 2.00 1.20 run329 1 0 0.2 2.5 12.5 1.00 6.00 1.00 5.00 6.00 run330 1 1 0.2 2.5 12.5 2.00 6.00 1.00 5.00 3.00 Determination of Heating Vez Y Bx, By, Bz Bz X Y Y Jx, Jy, Jz Ey X Y Y Vix, Viy, Viz X • Slice 20 ion inertial lengths downstream of xline. Y Te||, Te⊥ Y Effect of β? • β = thermal energy/magnetic energy ΔTe βr_tot WARNING: DTetot_max is actually DTepar_max + 2*DTeperp_max Energy Budget D Vin B Vout Vout Vin B • α = percentage of available energy δ Scaling of Electron Heating • Energy Conservation • Important Questions – What is αTe? – Is it a constant for a variation of inflow conditions? • If αTe is constant: Scaling with Alfven Speed: Te_tot • Scaling evident ΔTe_tot – αTe is independent of inflow parameters! (CAr)2 Energy Budget • Plot versus 1/2 (CAr • Slope of line = 0.12 )2 ΔTe_max 12% – 12% of energy into electron heating? 1/2 mi (CAr)2 • Average heating in exhaust – Slope of 5% • 5% of magnetic energy converted into heating. ΔTe_av 5% 1/2 mi (CAr)2 Statistical survey of the degree of electron heating at magnetopause 1. Identify reconnection exhausts 2. Determine Te • Determine boundary conditions: , guide field, etc… magnetosphere Diffusion region VA magnetosheath spacecraft Te (eV) Te (eV) Observations inflow VA,rec (km/s) Te VA,rec 2 Slope= 0.069 mi VA,rec2 /2 (eV) Te = 0.069 m VA2 /2 = 0.069 Brec2/(2 0 N) • Simulations: 5% into electron heating • Observations: 7% into electron heating Te (eV) Degree of heating depends on VA VA,rec (km/s) • Solar wind: VA ~ 50 km/s -> practically no heating • Magnetopause: inflow VA ~ 50-400 km/s • Magnetotail: inflow VA ~ 2000 km/s -> 1.4 keV Component Reconnection • Reconnecting field lines may not be anti-parallel • Can think of as: – anti-parallel reconnection – add a uniform B-field perpendicular to reconnection plane. – Guide field. Kivelson and Russel, 1995 Gosling, 1990 45 One Stark Effect: Guide Field • Bg = Br Y – Almost no perpendicular heating! Bx, By, Bz Te|| Vix, Viy, Viz Y X Y Te⊥ Y Te||, Te⊥ X Y Anisotropy • Striking – In General: ΔTe|| ≳ ΔTe⊥ – Guide field Case: No ΔTe⊥ – Guide field has larger ΔTe||? ΔTe|| ΔTe|| All Bg ΔTe⊥ All Bg (CAr)2 Bg = Br (CAr)2 (CAr)2 ΔTe⊥ ΔTe|| Bg = 0 Bg = 0 (CAr)2 ΔTe⊥ (CAr)2 Bg = Br (CAr)2 Te (eV) Observations: Guide field suppresses perpendicular heating Te < Te|| Te|| (eV) Te~ 0.75Te|| Te|| (eV) magnetic shear < 120o (guide field > 0.6) Te (eV) Te (eV) magnetic shear > 150o (guide field < 0.3) Te << Te|| Te|| (eV) Conflicting findings on anisotropy of electron heating: Guide field effect Magnetosheath: Te|| heating only Guide field ~ 1 Magnetotail: ~Isotropic heating [Chen et al., 2008] jet Magnetotail guide field ~ 0 Unanswered Question • What if Te/Ti > 5? – May effect heating • What is the physical mechanism behind the heating? • Acceleration at x-line (e.g. Pritchett et al., 2006, AshourAbdalla et al.) • Acceleration in high field regions (e.g. Birn et al., 2000, 2004, Hoshino et al. 2001) • Contracting Islands (e.g. Drake et al., 2006) • Turbulent electric fields (e.g. Dmitruck et al., 2004) • Parallel Electric Fields (e.g. Egedal et al., 2012) • What if there are many x-lines? (Solar Flares) • Turbulent Reconnection? Conclusions • Magnetic Reconnection – Magnetic Energy Release in Plasma – Multiscale problemf • Satellite Observations and PIC Simulations – Range of inflow parameters, guide field • Simulation/Observations Find Similar Scaling – ΔTe scales with (CAr)2 for wide range of parameters • Universal process – Guide Field Effect • ΔTe⊥ shut off for guide field. – Physics: Isotropization? – Electron Thermal Heating is Generic Physics? • Now comes the hard part. • Focus is on exhaust region – No strong compression at dipole fields, etc. • Easier to create Te|| – Contracting Island Model – E|| near x-line and separatrices • Important issue: Isotropization Vez – Example: Scattering at strongly Te⊥ curved field lines Y Y X X What Controls Electron Bulk (Thermal) Heating in Reconnection? Answer: VA2 and guide field Diffusion region VA Tai Phan, Mike Shay, Masaki Fujimoto, et al. Reconnection converts magnetic energy into: - Kinetic energy (plasma jetting) - Ion heating - Electron heating -> Thermal and Supra-Thermal assumed to always happen, but not true Electron bulk heating seen in some regions, not in others jet jet jet Solar Wind: No heating (Gosling, 2007) Magnetopause: 10s of eV gain in Te Magnetotail: keV heating (Gosling et al., 1990) The degree of electron bulk heating must depend on plasma regime Turbulent Reconnection • This smooth reconnection may be the exception. Solar Wind is Strongly Turbulent • What is the nature of reconnection in turbulence? Solar Turbulence QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Hinode (G-band 430nm and Ca II H 397nm) • Granules – 1000km across – Convection cells across entire sun The Solar Wind QuickTime™ and a YUV420 codec decompressor are needed to see this picture. • Continuous wind – Supersonic – Magnetic Field STEREO Spacecraft QuickTime™ and a GIF decompressor are needed to see this picture. 59