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 reconnection b.Conditions for the onset of reconnection c.Particle energization d.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~ G, Density~ 1-10 cm -3, Energy~ keV jet Locations of reconnection in the magnetosphere t1t1 t2t2 t3t3 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

Outline 1.Basic signatures of reconnection 2.Topics: a.Bursty (explosive) versus quasi-steady reconnection b.Conditions for the onset of reconnection c.Particle energization d.Extent of X-line

1. Signatures of reconnection Diffusion region 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 t1t1 t2t2 t3t3 jet 50 km

Reconnection signature: Alfvenic outflow jets B LMN (nT) |B||B| VpVp (km/s) NpNp (cm -3 ) TpTp ( eV ) predicted Diffusion region jet spacecraft  V predicted = ±  B/(μ 0 ρ 1 ) 1/2 [Paschmann et al., Nature, 1979] density compression heating T p|| TpTp |B| ↓ BLBL 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 reconnection b.Conditions for the onset of reconnection c.Particle energization d.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 reconnection b.Conditions for the onset of reconnection c.Particle energization d.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] Magnetic Shear  (degrees)  < 2 tan(  /2) (L/ i ) density gradient scale reconnection no reconnection L = i Physics: Diamagnetic drift of X-line prevents reconnection if drift speed > V A Diffusion region Introduction L        

Occurrence of solar wind reconnection vs.  and magnetic shear Wind 197 reconnection events Phan et al. [ApJL, 2010] - At  reconnection can occur for magnetic shear down to 10 o - At  reconnection requires magnetic shear >100 o Diffusion region 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 < V A 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 reconnection b.Conditions for the onset of reconnection c.Particle energization d.Extent of X-line

Particle Energization by Reconnection (mechanisms still not well understood) Diffusion region jet Magnetic energy => Particle energy Alfvenic ion jet thermal heating non-thermal heating t1t1 t2t2 t3t3 jet inflow 400 km/s = 1 keVup to 300 keV electrons

f (electrons) (s 3 m -3 ) V X (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 density of 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 reconnection b.Conditions for the onset of reconnection c.Particle energization d.Extent of X-line

How extended is the reconnection X-line?

Extremely extended Extremely extended ( i ) X-lines in Solar Wind X-line up to 600 R E (10 5 i ) Phan et al. [Nature, 2006]: 390 Earth radii Gosling et al. [GRL, 2008]: 600 R E 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 ClusterWind 220 R E 331 R E current sheet The 390 R E (3x10 4 i ) X-line event

Summary 1. X-line can be extremely extended (> 10 5 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 energies Isotropic at higher energies ( > 6 keV)

Interpenetrating ion beams as further evidence for reconnection left right Inside Gosling et al. [2005] Diffusion region spacecraft 2 V A left rightInside