magnetotail current sheet iki26-30 september 2011 magnetic reconnection in the current sheet:

Magnetotail Current Sheet IKI 26-30 September 2011

Magnetic Reconnection in the Current Sheet: Geotail Observations

Wednesday 28 September 2011, 1220-1300

T. Nagai Tokyo Institute of Technology

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The basic physical components on the magnetic reconnection site

1.The central electron current layer – the (electron) diffusion region

2.The ion-electron decoupling region – the Hall physics region

3.Inflows and Outflows – the MHD region

MHD Picture of Magnetic Reconnection

inflow

outflowBz > 0 Bz < 0

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Ion flows

Electron flows

unit ViA

unit VeA

Particle Picture of Magnetic Reconnection

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Hall Magnetic Fields

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Ve Ve

Vi Vi

Outflows

Bz > 0 　　　　 Bz < 0

MHD Ion-electron decoupling MHD

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- Vey

electron current layer

Bz > 0 　　　　 Bz < 0

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The basic physical components on the magnetic reconnection site

1.The central electron current layer – the (electron) diffusion region

2.The ion-electron decoupling region – the Hall physics region

3.Inflows and Outflows – the MHD region

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0700 UT

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electron Vx

ion Vx

ion Vy

electron Vy

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electron Vx

ion Vx

ion Vy

electron Vy

electron Vperp x

electron Vperp y

10 nA/m2

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0630 UT

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electron Vx

electron Vy

ion Vx

ion Vy

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electron Vx

electron Vy

electron Vperp x

electron Vperp y

20 nA/m2

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1055 UT

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electron Vx

ion Vx

ion Vy

electron Vy

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Geotail Observationsin the Plasma Sheet(plasma > 1)

in 1994-2010 X = -18 to -30 RE

Y = -20 to +20 RE

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electron Vxion Vx

electron Vyion Vy

MHD flows

Vi = Ve

Errors are smallwhen energetic electrons are rich.

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Tailward Flows with Bz < 0

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Tailward flows with Bz < 0

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Reconnection events

Physical Size ofthe Magnetic Reconnection Site

in the X direction

in the Y direction

?

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15 May 2003

near 1055 UT

the spacecraftGeotail at 28 RE

in situ observations ofmagnetic reconnection

Geotail

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Tailward flows

Earthward flows

Bz < 0 Bz > 0

Tailward flows

Earthward flows

Ion Et

Bz > 0 　　　　 Bz < 0

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Main Targets of This Paper

1.Detect the central intense electron current layer2.Get scales of magnetic reconnection

with Geotail observations

VA = 2,200 km/s i = 1,200 km (ion inertial length)

Geotail MGF 16 vectors /sLEP 12 s

electron g-factor 4x10 T 5.5x10

-4

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Tailward flows

Earthward flows

Bz < 0 Bz > 0

Tailward flows

Earthward flows

Ion Et

5 minutesBz > 0 　　　　 Bz < 0

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Bz < 0 Bz > 0

Tailward flows

Earthward flows1000 km/s ion flow

VA = 2200 km/s

Bz > 0 　　　　 Bz < 0

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Ion observations

Bz < 0 Bz > 0

Vx > 3000 km/s electron flows

Vy > 6000 km/s electron flows

Ve

Vi

Ve

Bz > 0 　　　　 Bz < 0

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Ion flows vs. electron flows

Ion Et

Vx > 2000 km/s electron flows

Vy > 3000 km/s electron flows

VBz < 0 Bz > 0

Ve

Ve

Bz > 0 　　　　 Bz < 0

Tailward flows

Earthward flows

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Ion and electron flows

perpendicular to the magnetic field

Vx > 2000 km/s electron flows

Vy > 3000 km/s electron flows

VBz < 0 Bz > 0

Ve

Ve

Bz > 0 　　　　 Bz < 0

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Electron observations

acceleration and heating

electron Et

time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

1057:44 1053:44 UT

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> 8.47 keV electron directional fluxes

Earth tail

time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

Vy < 0 dawnward

Vy > 0 duskward

electron velocitydistribution function

In the equatorial plane

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Earth tail

time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

Hall electrons

electron velocitydistribution function

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Earth tail

time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

ion velocitydistribution functions

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Earth tail

time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

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time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

counter-streaming inflows

ion velocitydistribution functions

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ion-electron decoupling Ve >> Vi

intense electron current layer

large Vey

spatial scales?

Important Questions

MHD MHD

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+450 km/s -650 km/s

Earthward flow speed tailward flow speed

MHD flows

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Asymmetric outflows

+550 km/s -550 km/s(+450 km/s) (-650 km/s) -100 km/s

MHD flows

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time

Bz > 0 　　　　 Bz < 0

Tailward flowsEarthward flows

Bz > 0 　　　　 Bz < 0

counter-streaming inflows

ion velocitydistribution functions

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Vx = +64 km/s Vx= -269 km/s

inflows

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Asymmetric inflows

Vx = +164 km/s Vx= -169 km/s (+264 km/s -269 km/s)

-100 km/s

inflows

Cluster results

-100 km/s Baker et al. 2002Imada et al. 2007

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ion-electron decoupling region

8 i

central electron current layer

1 iMHD MHD

Vx peakFlux peak

Ion ele ele ion

Earth tail

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PIC simulation results

the northern earthward sideof the reconnection site(upper-left quadrant)

the X-line position

observation simulation at TΩi = 35

Vix / Vex 0.2-0.3 0.1

Vey / Vex 1.25 1.4

Vix 0.5 VA 0.3 VA

the full extent of the central 1 i 1 ielectron current layer

the full extent of 8 i 8 ithe ion-electron decouplingregion

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Geotail Survey ofTailward Flows with Bz < 0 in 1995-2003

209 events

Nagai et al., 200504/22/23 01:21 49

8 i = 1.5 RE ion-electron decoupling region

Geotail Reconnection Events= Observations of Ion-Electron Decoupling Region

34 events 34/208 = 0.16

reconnection events at 20-30 RE

Nagai et al., 1998; 200104/22/23 01:21 50

Most of Events are MHD FlowsReconnection Events 34 events

34/208 = 0.16

reconnection events at 20-30 RE 10 RE x 0.16 = 1.6 RE

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ion-electron decoupling region

8 i

central electron current layer

1 iMHD MHD

Vx peakFlux peak

Ion ele ele ion

Earth tail

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Geotail Observationsin the Plasma Sheet(plasma > 1)

in 1994-2010 X = -18 to -30 RE

Y = -20 to +20 RE

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Main conclusions

In the magnetic reconnection site of the near-Earth magnetotail

1.the central intense electron current layer 1 i2.ion-electron decoupling region 8 i3.MHD regions outside the i-e decoupling region

Nagai, T., I. Shinohara, M. Fujimoto, A. Matsuoka, T. Saito, and T. Mukai, J. Geophys. Res., 116, A04222, 2010JA016283, 2011.

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The simulation box size [−Lx/2,+Lx/2]×[−Lz/2,+Lz/2] Lx = 48D Lz = 24D

initial current sheet thickness D = 0.5λiΔ is equal to the Debye λi = 200Δ

The number of simulation grids 4800×2400

Particle number 1.5×10 particles for each species nCS Ti;CS/Te;CS = 5nBK = nCS Ti;BK = Te;BK = Te;CS

ion to electron mass ratio mi/me = 400

frequency ratio ωpe/Ωe = 4,ωpe ≡√4πnCSe2/me Ωe ≡ eB0/mec λi ≡ c/ωpi = c/√4πnCSe2/mi

The initial magnetic field the Harris sheet Bx(z) = B0 tanh(z/D) B0 the asymptotic magnetic field

D the current sheet half-thicknessThe perturbed magnetic flux function ψ(x, z) = ψ0 sin(2πx/Lx) cos(2πz/Lz)

B (x, z) = eˆy × ψ∇ (x, z)

at TΩi = 35 Vi x ∼ 0.3VA VA the Alfven speed B0/√4πminCS

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1. What are the major latest achievements in the topic of the section?

2. What major results are still expected/possible given the available data (ongoing projects) ?

3. What are major questions to be answered by future missions?

Some comments:

Location of magnetic reconnection

A mechanism (or mechanisms) of the current sheet thinning

Location of Magnetic Reconnection

Aurora Onset Location

Morning

Midnight

Evening

X = -20 to -30 RE and Y = -5 to +10 RE

Invariant Latitude degrees 　 MLT

Grocott et al. 2009

High Occurrence

Nagai et al., 1998a

Magnetic reconnection at a substorm onset 　

Fast Tailward Flows

Bz < 0

Fast tailward Flowswith Bz < 0

Fast Earthward Flowswith Bz > 0

Ieda et al. 2008

Evidence of Magnetic Reconnection

１． Acceleration of electrons２． Hall current system

Ions

Electrons

Accelerated electrons

1530 1540 UT February 18, 1996

High speed ion flows & Bz <0

Nagai et al., 2001

Geotail observations at 25 RE

1996/02/18

strong acceleration of electrons

strong acceleration of electrons

thermal

accelerated

electron energy spectra

Flux

Energy

1530 1540 UT

> 2000 km/s tailward flowing ions

Geotail observations at 25 RE

1996/02/18

strong acceleration of electrons

1530 1540 UT

> 2000 km/s tailward flowing ions

48 sec

Time scale ofclosed field line reconnection open field lines reconnection

12 sec

X-line tailward motion

Tailward motion of reconnection site

Hones et al., 1973

Substorm models

current continuity

current continuity?

Structure of the cross-tail current system

intense electron current layer

Location of Magnetic Reconnection

Aurora Onset Location

Morning

Midnight

Evening

X = -20 to -30 RE and Y = -5 to +10 RE

Invariant Latitude degrees 　 MLT

Grocott et al. 2009

High Occurrence

Nagai et al., 1998a

An initial onset in the pre-midnight sector

East-wst expansion of the onset region

at Geosynchronous orbit (6.6 Re)

Nagai JGR 1982

NagaiJGR 1987

Nagai JGR 1987

Location of Magnetic Reconnection

Aurora Onset Location

Morning

Midnight

Evening

X = -20 to -30 RE and Y = -5 to +10 RE

Invariant Latitude degrees 　 MLT

Grocott et al. 2009

High Occurrence

Nagai et al., 1998a

IMF Bz prior to magnetic reconnection onset

The site of magnetic reconnection

Near-Earth X = -15 to -25

Middle X = -25 to -31

Typical growth phase 4 minnorthward turning(IMF-triggered 60%)

Nagai et al., 2005Hsu and McPherron, 2003

Nagai et al., 2005

threshold

Solar wind electric field

E = V x Bs Flux accumulation

Near-Tail

Midtail

The solar wind energy input controls the magnetic reconnection site.

Solar wind Electric field

E = V x Bs

Nagai et al., 2005

Position of magnetic reconnection 　　 tailward edge of thin current sheet

thin current sheet

substorm onset

Asano et al., JGR 2004

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IMF Bz southward turning

More taillike configuration(the growth phase)

The thinning

Geotail observations in the plasma sheetIn the growth phase

Nagai GRL 1997

No drastic changes

plasma sheet structureprior to an onset

Near the equatorial plane　　　　 Bx = 0

Increase of number density

Decrease of temperature

Increase of total pressure

Bx

By

Bz

Bt

Vx

ion

electron

density

Ti

Pt Pp

magnetic reconnection

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A mechanism (or mechanisms) of the current sheet thinning

Miho Saito, D. Fairfield, G. Le, L.-N. Hau, V. Angelopoulos, J. McFadden, H.-U. Auster, J. Bonnell, and D. Larson (2011),

Structure, force balance, and evolution of incompressible cross-tail current sheet thinning,

J. Geophys. Res., doi:10.1029/2011JA016654, in press.

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P5 THA

P2 THC

P3 THD

P4 THE

IMF Bz southward turning

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Z=0.20Z=0.15Z=0.10Z=0.05Z=0.00

X=0.0X=0.4X=0.8X=1.2

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[−Lx/2,+Lx/2]×[−Lz/2,+Lz/2] 　 Lx = 48D Lz = 24D D = 0.5λi　　　　　　　　　　　　　　　　　　　　　 λi = 200Δ 　 Debye length simulation grids is 4800×2400 1.5×109 particles

mi/me = 400 ωpe/Ωe = 4

Ti;CS/Te;CS = 5

Initial Current Thickness 0.5 i (Harris Current Sheet) Bx(z) = B0 tanh(z/D)

Results at time i t = 35 Vi x 0.3VA ∼

2D Full Particle Simulations

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Typical Plasma Parameters

Alfven velocity 2200 km/sBlobe = 20 nTDensity = 0.04 cm-3(plasma sheet)

ion inertial length 1200 km Density = 0.04 cm-3

i = V / i 　 = c / pi

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The initial magnetic field the Harris sheet Bx(z) = B0 tanh(z/D) B0 the asymptotic magnetic field

D the current sheet half-thickness

nCS the current sheet has number densityTi;CS/Te;CS = 5 ion to electron temperature ratio

nBK = nCS a uniform background plasmaTi;BK = Te;BK = Te;CS

The perturbed magnetic flux function ψ(x, z) = ψ0 sin(2πx/Lx) cos(2πz/Lz)B (x, z) = eˆy × ψ∇ (x, z)

nCS the current sheet has number densityTi;CS/Te;CS = 5 ion to electron temperature ratio

nBK = nCS a uniform background plasmaTi;BK = Te;BK = Te;CS

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The simulation box size is [−Lx/2,+Lx/2]×[−Lz/2,+Lz/2] Lx = 48D and Lz = 24D

Periodic boundary conditions are imposed in the x directionconducting walls are set at the z boundaries

ion to electron mass ratio mi/me = 400

frequency ratio ωpe/Ωe = 4,initial current sheet thickness D = 0.5λiωpe ≡√4πnCSe2/me, Ωe ≡ eB0/mec, and λi ≡ c/ωpi = c/√4πnCSe2/mi

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The spatial grid size Δ is equal to the Debye length of the background plasma, and λi = 200Δ

The number of simulation grids is 4800×2400

1.5×109 particles for each species

Vi x ∼ 0.3VA at TΩi = 35

VA the Alfven speed is defined by B0/√4πminCS

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The simulation box size [−Lx/2,+Lx/2]×[−Lz/2,+Lz/2] Lx = 48D Lz = 24D

initial current sheet thickness D = 0.5λiΔ is equal to the Debye λi = 200Δ

The number of simulation grids 4800×2400

Particle number 1.5×10 particles for each species

ion to electron mass ratio mi/me = 400

frequency ratio ωpe/Ωe = 4,

ωpe ≡√4πnCSe2/me Ωe ≡ eB0/mec λi ≡ c/ωpi = c/√4πnCSe2/mi

The initial magnetic field the Harris sheet Bx(z) = B0 tanh(z/D) B0 the asymptotic magnetic field

D the current sheet half-thicknessThe perturbed magnetic flux function ψ(x, z) = ψ0 sin(2πx/Lx) cos(2πz/Lz)

B (x, z) = eˆy × ψ∇ (x, z)

at TΩi = 35 Vi x ∼ 0.3VA VA the Alfven speed B0/√4πminCS

9

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