In-situ Observations of Collisionless Reconnection in the Magnetosphere

Tai Phan (UC Berkeley)

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

- Reconnection occurs at the dayside magnetopause and in the magnetotail

- The properties of reconnection are vastly different in the two regions

- Parameter regime: B~ 10-4-10-3 G, Density~ 1-10 cm-3, Energy~ 1-300 keV

jet

jet

jet

jet

Locations of reconnection in the magnetospheret1 t2 t3 Dungey [1961]

In-situ measurements of B, E, and particle distributions (Density, T, V)

Advantages with in-situ observations:- Conclusive evidence for reconnection- Detailed properties of reconnection - Quantitative comparisons with theory

Disadvantage: Global context and consequences often not known

In-situ observations

jet

jet

Outline

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

1. Signatures of reconnection

Diff

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gion

jet

I. Exhaust (outflow region): > 99% of reconnection encounters

- Ion (Alfvenic) jets: most basic and universal signature

II. Diffusion regions: Rare encounters

- Ions and electrons decoupled from the magnetic field

inflow

t1 t2

t3

inflow

jet

50 km

Reconnection signature: Alfvenic outflow jets

BLMN

(nT)

|B|(nT)

Vp

(km/s)

Np

(cm-3)

Tp

(eV)

predicted

Diff

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gion

jet

jet

spacecraft

Vpredicted = ± B/(μ0ρ1)1/2

[Paschmann et al., Nature, 1979]

density compression

heating

Tp||

Tp

|B| ↓

BL

UT

Geotail and Equator-S detections of bi-directional reconnection jets

[Phan et al., Nature, 2000]

Outline

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

Dayside Magnetopause: - Can be quasi-steady - Maintain thin (ion skin depth) current sheet due to constant solar wind compression

Magnetotail: - Always bursty: Storing and releasing magnetic energy (similar to solar flares) - Generally thick current sheet (of many ion skin depths thick) - Requires accumulation of magnetic flux to compress current sheet

Bursty (Explosive) versus Quasi-Steady Reconnection

Thin current sheet Usually thick current sheet (no reconnection)

Magnetotail: Bursty reconnection

Vx(km/s)

Reconnection jets

Outline

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

Conditions for the onset of Reconnection

• Thin current sheet (~ 1 ion skin depth)

• Reconnection occurrence depends also on plasma and magnetic shear

Reconnection jet not always seen at the magnetopause

=> Thin current sheet is a necessary but not sufficient condition for reconnection

Paschmann [1996] found that reconnection events tend to occur for low

Reconnection occurrence dependence on and magnetic shear in asymmetric reconnection [Swisdak et al., ApJ. 2003, 2010]

Mag

netic

She

ar

(deg

rees

)

< 2 tan(/2) (L/i)

density gradient scale

reconnection

no reconnection

L = i

Physics: Diamagnetic drift of X-line prevents reconnection if drift speed > VA

Diff

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n re

gion

L

Occurrence of solar wind reconnection vs. and magnetic shear

Wind197 reconnection events

Phan et al. [ApJL, 2010]

- At reconnection can occur for magnetic shear down to 10o

- At reconnection requires magnetic shear >100o

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L

(Collisionless) Reconnection requires:

• Thin current sheet (~ 1 ion skin depth)

• Satisfies and magnetic shear condition:- Low allows low magnetic shear- High requires large magnetic shear

• Tangential velocity shear across the current sheet < VA

• Other conditions?

With all these strict conditions, triggering reconnection is not easy !

Outline

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

Particle Energization by Reconnection(mechanisms still not well understood)

Diff

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gion

jet

Magnetic energy => Particle energy

Alfvenic ion jet thermal heating non-thermal heating

t1 t2 t3jet

inflow inflow400 km/s = 1 keV up to 300 keV

electrons

f (el

ectr

ons)

(s3 m

-3)

VX

(km s-1)

Maxwellian

f E-k

near diffusion region center

k=4.8

outflow k=5.3

Energy densities near X-line:

- Ion jet: 95%- Thermal ions+electrons: 4%- Electron power law tail: 1%

[Øieroset et al., Nature, 2001]

[Øieroset et al., PRL, 2002]

An example of electron acceleration to 300 keV

Wind

Betatron and Fermi accelerations far downstream of the reconnection site

In flow breaking region: substantial energy density in the power law tail

Conclusions:

- Electrons are accelerated to hundreds of keV near the X-line, but the energy densityof the energetic electron population is low compared to the ion jet

- However, additional energization occurs at flow breaking

Hara and Nishida [1981]

Outline

1. Basic signatures of reconnection

2. Topics:a. Bursty (explosive) versus quasi-steady reconnectionb. Conditions for the onset of reconnectionc. Particle energizationd. Extent of X-line

How extended is the reconnection X-line?

Extremely extended Extremely extended (104- 105 i) X-lines in Solar Wind

X-line up to 600 RE (105 i)

Phan et al. [Nature, 2006]: 390 Earth radii Gosling et al. [GRL, 2008]: 600 RE

Stereo-A

Stereo-B

• All 3 spacecraft encountered the same solar wind current sheetAll 3 spacecraft encountered the same solar wind current sheet• All 3 spacecraft detected reconnection jets in the current sheetAll 3 spacecraft detected reconnection jets in the current sheet

To Sun

ACE

Cluster Wind

220 RE

331 RE

current sheet

The 390 RE (3x104 i) X-line event

Summary

1. X-line can be extremely extended (> 105 ion skin depths)

2. Both bursty and quasi-steady behaviors have been seen - Quasi-steady requires maintaining thin current sheet.

3. Not easy to trigger reconnection in current sheets. Requirements:- Thin (ion skin depth scale) current sheet- Low (<1) allows strong guide field reconnection. High reconnection requires large magnetic shear (small guide field).

4. Reconnection can accelerate electrons to non-thermal energies, but the additional obstacle downstream helps energize electrons further.

Pitch angle spectrum near diffusion region center

Counter-streaming at low energiesIsotropic at higher energies ( > 6 keV)

Interpenetrating ion beams as further evidence for reconnection

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right

Inside

Inside

Gosling et al. [2005]

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2 VA

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rightInside