PPT

Transcript PPT

Studying the Microphysics of Magnetic Reconnection in the Earth’s Magnetosphere and the Solar Wind Electron Heating

Michael Shay Department of Physics and Astronomy Precursor: presentations/2012-09-swarthmore-colloquium/presentation.pptx, but I converted to keynote and threw out a huge number of slides. University of Delaware

Collaborators

• Colby Haggerty – Univ of Delaware • Tai Phan Marit Oieroset – Berkeley • Masaaki Fujimoto • Paul Cassak – Univ of West Virginia • Jim Drake – Univ of Maryland

Space Weather

• The nature of changing environmental conditions in space.

– Plasma: A gas of charged particles.

A Solar Flare

• Explosive energy release – Up to 10 32 ergs 3 x 10 18 kW-hr – Takes ~ 20 minutes – Equivalent to: 40 billion atomic bombs(!) 2005 human energy consumption: 1.4 x 10 14 kW-hr QuickTime™ and a Photo decompressor are needed to see this picture.

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 (-

MHD Magnetohydrodynamics

Ions (+) )

MHD - Magnetohydrodynamics

• • Fluid Equations – Slow Timescales – Large length scales Key Physics – Magnetic field lines act like rubber tubes • Alfven Speed : – Plasma “Frozen-in” to the magnetic field • Magnetic Topology is conserved:

d m i n dt



g 

4     



2 8     



t n

 

    g nV

E E

 



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/ω p • Plasma – Different magnetic fields – Diffusion region

Overview

• Plasma Physics Primer • What is Magnetic Reconnection?

• Electron Heating due to Magnetic Reconnection



Magnetic Reconnection

V in C A • Simplistic 2D picture • Change of magnetic topology – Releases magnetic energy Diffusion Region MHD not valid

Magnetic Reconnection

J z and Magnetic Field Lines QuickTime™ and a GIF decompressor are needed to see this picture.

Reconnection Rate

D V in B V out V in B • – • Reconnection Rate: n V in D ~ m i n V out δ V E in ~ (δ/D) c out-of-plane A ~ V in B V out δ

Reconnection in Solar Flares

• X-class flare:  ~ 100 sec.

• τ A ~ L/c A ~ 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 – Microscale process – Macroscale effects Diffusion region scales: 1 km • Complete description – Model Macroscales – Resolve Microscales – Impossible!

• Grand Challenge Problem 300,000 km 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

Solar Wind: No heating (Gosling, 2007)

jet

Magnetopause: 10s of eV gain in T e (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

• 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: L 0 = d i = c/ω pi , t 0 = (Ω ci ) -1 • Simulation Size: 204.8 d i X 102.4 d i • Grid: Δ = 0.05 d i • m i /m e = 25, 100, c = 15, 30 • Boundary conditions: periodic • Equilibrium: Double Harris equilibrium • Simulate until quasi-steady – Time average over a few (Ω ci ) -1 • Coordinates: “Simulation Coordinates” – Outflow: x – Inflow: y – Out-of-plane: z

Initial Conditions

• • • • • Basic Reconnection Simulations Double current sheet – Reconnects robustly – Periodic boundary conditions Initial x-line perturbation Excellent Testbed for studying basic properties of reconnection Does not include many boundary condition effects Reconnection Rate Z t = 0 Z t = 1200 Current along Z Time Time Density X X

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

T i /T e  Te (eV) ~ 5  Te ∞ 1/  e, rec 0.5

 e, rec 5.0

nkT e /(B rec 2 /2  0 )

Table of All Most Simulations

Run #

• Currently about 50 simulations

B reconn B guide n inflow T e T i B 2 301

1.00

0.20

302 303 304 305 306 run307 run311 run308001 run312001 run309 run313 run315 run316 run310001 run314001 run317001 run318001 run319 run320 run321 run322 run323 run324 run325

1 1 0 1 0.2

0.2

0.25

1 1 1 1 1 1 1 1 1 1 0.447

0 1 0 0.2

0.2

0.25

2.25

0.25

1 0.2

0.2

0.25

2.25

0.25

1.00

2.00

0 1 1.0

1.0

0.25

1.00

2.00

0 0.2

2.236

0 2.236

0.2

0.25

1 1 1 0.447

1 0.447

0 0.2

0.04

0.25

2.25

0.04

0.25

2.25

– T i 1 0 /T e 1 0.04

= 1 to 10 0.04

2.25

0.25

2.25

0.20

0.40

1.00

2.00

1.00

2.00

5.00

10.00

5.00

2.236

0 2.236

0.2

2.25

0.25

0.447

1 0 0.447

0 0.2

0.2

1.0

0.25

2.25

10.00

0.20

0.40

1.00

1 0 1 0 1.0

0.2

0.25

0.0625

2.25

1.25

0.3125

2.00

1.00

2.00

1.00

0.20

1.00

0.90

1.00

0.50

1.00

0.50

0.20

5.00

0.60

0.15

⊥ 0.10

0.10

0.90

0.02

0.18

0.02

0.18

0.50

0.10

0.03

run326 run327 run328 run329 run330

1 1 1 1 1 1 0 1 0 1 0.2

0.2

0.0625

1 1 2.5

2.5

0.3125

5 5 12.5

12.5

2.00

1.00

2.00

1.00

2.00

0.15

2.40

6.00

0.03

0.40

1.00

⊥

0.10

0.90

0.10

0.50

0.18

0.02

0.18

0.02

4.50

0.50

0.13

2.00

5.00

⊥

0.20

0.10

1.00

0.50

1.00

0.50

1.00

0.50

1.00

0.50

0.20

0.10

0.20

0.10

0.20

0.10

0.20

0.10

5.00

2.50

5.00

2.50

0.60

0.30

0.15

0.08

2.40

1.20

6.00

3.00

β total

Y Y

Determination of Heating

V ez B x , B y , B z X B z J x , J y , J z X E y Y V ix , V iy , V iz X • Slice 20 ion inertial lengths downstream of x line. T e|| , T e ⊥ Y Y Y Y

Effect of β?

• β = thermal energy/magnetic energy ΔT e WARNING: DTetot_max is actually DTepar_max + 2*DTeperp_max β r_tot

Energy Budget

D V in B V out V in B • α = percentage of available energy V out δ

Scaling of Electron Heating

• Energy Conservation • Important Questions – What is α Te ?

– Is it a constant for a variation of inflow conditions?

• If α Te is constant:

Scaling with Alfven Speed: T

e_tot • Scaling evident – α Te ΔT e_tot is independent of inflow parameters!

(C Ar ) 2

Energy Budget

• Plot versus 1/2 (C Ar ) 2 • Slope of line = 0.12

– 12% of energy into electron heating?

• Average heating in exhaust – Slope of 5% • 5% of magnetic energy converted into heating.

ΔT e_max ΔT e_av 12% 5% 1/2 m i (C Ar ) 2 1/2 m i (C Ar ) 2

Statistical survey of the degree of electron heating at magnetopause 1. Identify reconnection exhausts 2. Determine  Te • Determine boundary conditions:  , guide field, etc… V A magnetosphere magnetosheath spacecraft

Observations Slope= 0.069

inflow V A,rec (km/s) m i V A,rec 2 /2 (eV)  Te  V A,rec 2  Te = 0.069 m V A 2 /2 = 0.069 B rec 2 /(2  0 N) • Simulations: 5% into electron heating • Observations: 7% into electron heating

Degree of heating depends on V A V A,rec (km/s) • Solar wind: V A ~ 50 km/s -> practically no heating • Magnetopause: inflow V A ~ 50-400 km/s • Magnetotail: inflow V A ~ 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

Y • B g = B r – Almost no perpendicular heating!

T e|| B x , B y , B z V ix , V iy , V iz Y X Y T e ⊥ T e|| , T e ⊥ X Y

Anisotropy

ΔT e|| All B g • Striking – In General: ΔT e|| ≳ ΔT e ⊥ – Guide field Case: No ΔT e ⊥ – Guide field has larger ΔT e|| ?

ΔT e|| B g = 0 ΔT e|| B g = B r ΔT e ⊥ All B g (C Ar ) 2 ΔT e ⊥ B g = 0 (C Ar ) 2 ΔT e ⊥ B g = B r (C Ar ) 2 (C Ar ) 2 (C Ar ) 2 (C Ar ) 2

Observations: Guide field suppresses perpendicular heating  T e  <  T e||  T e|| (eV) magnetic shear > 150 o (guide field < 0.3) magnetic shear < 120 o (guide field > 0.6)  T e  ~ 0.75

 T e||  T e  <<  T e||  T e|| (eV)  T e|| (eV)

Conflicting findings on anisotropy of electron heating: Guide field effect Magnetosheath: T e|| heating only Guide field ~ 1 Magnetotail: ~Isotropic heating [Chen et al., 2008]

jet

Magnetotail guide field ~ 0

Unanswered Question

• What if T e /T i > 5?

– May effect heating • What is the physical mechanism behind the heating?

• Acceleration at x-line (e.g. Pritchett et al., 2006, Ashour Abdalla 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 – ΔT e scales with (C Ar ) 2 for wide range of parameters • Universal process – Guide Field Effect • ΔT e ⊥ shut off for guide field.

– Physics: Isotropization?

– Electron Thermal Heating is Generic

Physics?

Y • Now comes the hard part.

• Focus is on exhaust region – No strong compression at dipole fields, etc.

• Easier to create T e|| – Contracting Island Model – E || near x-line and separatrices • Important issue: Isotropization V ez – Example: Scattering at strongly curved field lines e Y X X

What Controls Electron Bulk (Thermal) Heating in Reconnection? Answer: V A 2 and guide field V A 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 T e (Gosling et al., 1990) Magnetotail: keV heating 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

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Hinode (G-band 430nm and Ca II H 397nm) • Granules – 1000km across – Convection cells across entire sun

The Solar Wind

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STEREO Spacecraft • Continuous wind – Supersonic – Magnetic Field

QuickTime™ and a GIF decompressor are needed to see this picture.