Electron-Scale Dissipations During Magnetic Reconnection

The 17th Cluster WorkshopMay 12-15, 2009 at Uppsala, Sweden

Hantao Ji

Contributors: W. Daughton*, S. Dorfman, E. Oz, Y. Ren, V. Roytershteyn*, M. Yamada, and J. Yoo

Princeton Plasma Physics Laboratory

* Los Alamos National Laboratory

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Why Do We Need Experiments ?

• Verify/confront theory– often motivated by theory

• Benchmark/challenge simulation

• Compare with observations

• Discover new physics

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Two Types of Experiments

• All-in-one: many competing processes coexist– e.g. tokamaks

• Problem-specific: one process dominates– e.g. MRX

Controllability is the key: specify conditions, when, and whereto observe how

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Outline• Introduction

– The reconnection problem(s)

– Fundamental physics questions

• Recent results from Magnetic Reconnection Experiment (MRX)– Electron layer dissipation and comparisons with PIC

– Electromagnetic waves and global 3D structure

• Summary

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Many Reconnection Problems

Lab Magneto-sphere

Solar/disk corona

Solar/star interior

Accretion disks

…

Drive Electric field

Solar wind Footpoint motion

T,

Geometry Periodic Dipole (curved) line-tying

Spherical shell

disk

Size/ion skin depth

10-100 10-100 >>1 >> >>1 vary

Collisionality Weakly collisional

Collisionless Weakly collisional

Collisional vary

…

• Vastly different environments; multiple reconnection processes• Part of magnetic, multiple-scale, self-organization processes

• Seek common fundamental physics

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Common Fundamental Physics Questions

• How does reconnection start? (The trigger problem)

• Why reconnection is fast compared to classical theory? (The rate problem)

• How ions and electrons are heated or accelerated? (The heating problem)

• How does reconnection take place in three dimensions? (The 3D problem)

• How do boundary conditions affect reconnection process? (The boundary condition problem)

• How to apply local reconnection physics to a large system? (The scaling problem)

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Samples of Reconnection Experiments

Device Where When Who Geometry Q’s

3D-CS Russia 1970 Syrovatskii, Frank Linear 3D, heating

LPD, LAPD UCLA 1980 Stenzel, Gekelman Linear Heating, waves

TS-3/4 Tokyo 1990 Katsurai, Ono Merging Rate, heating

MRX Princeton 1995 Yamada, Ji Toroidal, merging

Rate, heating, scaling

SSX Swarthmore 1996 Brown Merging Heating

VTF MIT 1998 Fasoli, Egedal Toroidal with guide B

Trigger

RSX Los Alamos 2002 Intrator Linear Boundary

RWX Wisconsin 2002 Forest Linear Boundary

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Magnetic Reconnection Experiment (MRX)

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Experimental Setup in MRX

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Realization of Stable Current Sheet and Quasi-steady Reconnection

Detailed diagnostics: quantitative studies possible

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The Rate Problem:

Why Reconnection Is Fast?(compared to the predictions by classical

theories)

Focusing on electron layers and waves

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Electron Diffusion Layers Crucial in Collisionless Reconnection

(e.g. Drake et al. ‘98)

ion

electron

• Hall effects separate electron layer from ion layer– Ion demagnetized in ion layers with

thickness on order of c/pi

– Electron demagnetized in electron layers with thickness on order of c/pe

– Manifest as quadrupole out-of-the-plane magnetic field [Ren et al. (‘05), Yamada et al. (‘06)]

• Ion layers allow fast mass flows

• Magnetic field reconnects in electron layer to change its topology while electrons are energized

In 2D collisionless reconnection, laminar electron non-gyrotropic pressure dominates the dissipation.

Vasyliuna (‘75), Sonnerup (‘88), Dungey (‘88), Lyons & Pridmore-Brown (‘90)Cai & Lee (‘97), Hesse et al. (‘99), Pritchett (‘01), Kuznetsova et al. (‘01)

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Limited Observations of Electron Layer in Space

• Scudder et al. (‘02) by Polar spacecraft– Reported electron-layer like events including first signatures of

electron nongyrotropic pressure

• Mozer (‘05) by Polar spacecraft– Documented many electron-scale layers but most of them are

magnetized

• Wygant et al. (05) by Cluster spacecraft– A demagnetized electron-scale layer at δ = 3-5 c/pe

• Phan et al. (07) by Cluster spacecraft– A demagnetized electron-scale layer downstream at δ = 4.5 c/pe

• Chen et al. (08) by Cluster spacecraft– An demagnetized electron layer between islands at δ = 4 c/pe

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Cluster

Observations

:

Electron-scale

Layers

Embedded in Ion-scale

Layers

Phan et al. (‘07)

δ = 3-5 c/pe δ = 4.5 c/pe

Wygant et al. (‘05)

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Limited Observations of Electron Layer in Space

• Scudder et al. (‘02) by Polar spacecraft– Reported electron-layer like events including first signatures of

electron nongyrotropic pressure

• Mozer (‘05) by Polar spacecraft– Documented many electron-scale layers but most of them are

magnetized

• Wygant et al. (05) by Cluster spacecraft– A demagnetized electron-scale layer at δ = 3-5 c/pe

• Phan et al. (07) by Cluster spacecraft– A demagnetized electron-scale layer downstream at δ = 4.5 c/pe

• Chen et al. (08) by Cluster spacecraft– An demagnetized electron layer between islands at δ = 4 c/pe

Dissipation processes and relative location to the X-line difficult to determine

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First Detection of Electron Layer in Laboratory

Electron layer

Ren et al. PRL (‘08)

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Sizes of Electron Layer Are Independent of Ion Mass

Width: Length:

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2D PIC Simulations in Geometry Similar to MRX

• Driven by currents in PF coils

• Flux core surface either absorbing or reflecting

• No toroidal effects

• Box boundary either conducting or insulating

• Small numbers of (macro) particles pe/ ce = a few, compared to ~100 in MRX

• Artificially heavy electrons– mi/me=10-400, compared to large mass ratios in H,

D, and He plasmas Dorfman et al. PoP (‘08)

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All Features in Ion Scales Are Reproduced by 2D PIC Simulations

MRX 2D PICJi et al. GRL (‘08)

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… But NOT in Electron Scales:δ = 8c/ωpe vs δ = 1.5-2 c/ωpe

Independent of ion mass

conducting field, reflecting particle B.C.

absorbing particle B.C.

collisionless with collisions

135 140 145 150 155 160 135 140 145 150 155 160

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0

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Residual Collisions Can Broaden Layer, But Still Not Enough

• Width from 2D PIC increases by about (50-70)% to 2.5-3.5 c/ωpe

• Width from MRX: 5.5-7.5 c/ωpe with probe corrections

• A factor of 2-3 difference: non-gyrotropic pressure unimportant

3D effects?

Roytershteyn et al. (‘09)

Electrostatic Fluctuations Observed at CS Edge: Not Directly Important To Reconnection

EM

ES

Bale et al. GRL (’02)Vaivads et al. GRL (’04)

MRX Polar Cluster

Identified as LHDW

Carter et al. PRL (’02)

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Electromagnetic Fluctuations Observed at Current Sheet Center in

MRX

Ji et al. PRL (’04)

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Electromagnetic Fluctuations Also Observed At High-β Areas By Cluster

Phan et al. (’03) Zhou et al. JGR (’09)

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Fluctuations Correlated with Large Reconnection Electric Field and Large Current Density

(Preliminary)

b:z, g:r, r:t, c:z2

• Suggestive of anomalous resistivity due to waves?

Dorfman et al. (’09)

Fluctuations Correlated with Large Reconnection Electric Field and Large Current Density

(Preliminary) Dorfman et al. (’09)

Larger E, J Smaller E, J

Fluctuations Correlated with 3D Global Structures (Preliminary)

Dorfman et al. (’09)

Larger E, J Smaller E, J

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Summary (I)• Laboratory experiments can be, should be, and is already part of

plasma space and astrophysics.– A growing field of laboratory plasma space/astrophysics

• First one-to-one comparisons attempted between experiments and PIC simulations• All ion scale features reproduced by 2D PIC simulations• However, the electron layers are 2-3 times thicker than

simulations: something is missing in 2D PIC models

MRX 2D PIC Cluster

δ (c/pe) 5.5-7.5 1.5-2 3-5

δ (c/pe) (collisionless)

3-5 1.5-2 3-5

δ (c/pe) (weakly coll.)

5.5-7.5 2.5-3.5 4.5-8.5

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Summary (II)• Electrostatic waves at current sheet edge identified as LHDW in

MRX, simulations and space observations: not directly important.• Electromagnetic waves in LH frequency range are less well

understood• Propagate perpendicularly to magnetic field (MRX, PIC, Cluster)• Consistent with a theory on EM LHDW by Wang et al, PoP (‘08)• Preliminary evidence of correlations between EM waves and

locally fast reconnection associated with globally 3D structures

• Current focuses: 4-way close collaborations

(1) MRX: global and local 3D effects

(2) Simulations: 2D with real mass ratios and collisions, and 3D

(3) Space: electron layer structures, and EM waves

(4) Theory: EM wave linear and nonlinear dynamics