In-situ Observations of Collisionless Reconnection in the Magnetosphere

In-situ Observations of Collisionless Reconnection in the Magnetosphere Tai Phan (UC Berkeley) • Basic signatures of reconnection • 2. Topics: • Bursty (explosive) versus quasi-steady reconnection • Conditions for the onset of reconnection • Particle energization • Extent of X-line

Locations of reconnection in the magnetosphere t1 t2 t3 Dungey [1961] jet jet jet jet • 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

In-situ observations jet jet • 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

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

1. Signatures of reconnection 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 inflow Diffusion region 50 km t1 t2 jet t3

Reconnection signature: Alfvenic outflow jets [Paschmann et al., Nature, 1979] BLMN (nT) BL jet |B| |B| ↓ (nT) spacecraft predicted Vp jet (km/s) density compression Np Diffusion region (cm-3) heating Tp Tp (eV) Tp|| UT DVpredicted = ± DB/(μ0ρ1)1/2

Geotail and Equator-S detections of bi-directional reconnection jets [Phan et al., Nature, 2000]

Bursty (Explosive) versus Quasi-Steady Reconnection 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 Usually thick current sheet (no reconnection) Thin current sheet

Magnetotail: Bursty reconnection Reconnection jets Vx (km/s)

Thin current sheet (~ 1 ion skin depth) • Reconnection occurrence depends also on plasma b and magnetic shear Conditions for the onset of Reconnection 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 b (< 2 )

Reconnection occurrence dependence on b and magnetic shearin asymmetric reconnection [Swisdak et al., ApJ. 2003, 2010] L = li reconnection Magnetic Shear q (degrees) L no reconnection b2 b1 Diffusion region Db = b2-b1 density gradient scale Db < 2 tan(q/2) (L/li) Physics: Diamagnetic drift of X-line prevents reconnection if drift speed > VA Introduction

Occurrence of solar wind reconnection vs. Db and magnetic shear Phan et al. [ApJL, 2010] 197 reconnection events Wind L b1 b2 Diffusion region • At Db=0.1, reconnection can occur for magnetic shear down to 10o • At Db=2, reconnection requires magnetic shear >100o

(Collisionless) Reconnection requires: • Thin current sheet (~ 1 ion skin depth) • Satisfies b and magnetic shear condition: • Low b allows low magnetic shear • High b requires large magnetic shear • Tangential velocity shear across the current sheet < VA • Other conditions? With all these strict conditions, triggering reconnection is not easy !

Particle Energization by Reconnection(mechanisms still not well understood) jet Magnetic energy => Particle energy Alfvenic ion jet thermal heating non-thermal heating inflow inflow Diffusion region 400 km/s = 1 keV up to 300 keV electrons t1 t2 jet t3

f E-k near diffusion region center k=4.8 outflow k=5.3 Maxwellian An example of electron acceleration to 300 keV [Øieroset et al., Nature, 2001] • Energy densities near X-line: • Ion jet: 95% • Thermal ions+electrons: 4% • Electron power law tail: 1% [Øieroset et al., PRL, 2002] Wind f (electrons) (s3m-3) VX (km s-1)

Betatron and Fermi accelerations far downstream of the reconnection site Hara and Nishida [1981] 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

How extended is the reconnection X-line?

Extremely extended (104- 105li) X-lines in Solar Wind Stereo-B X-line up to 600 RE (105li) Stereo-A Phan et al. [Nature, 2006]: 390 Earth radii Gosling et al. [GRL, 2008]: 600 RE

The 390 RE (3x104li) X-line event To Sun current sheet ACE 220 RE 331 RE Cluster Wind • All 3 spacecraft encountered the same solar wind current sheet • All 3 spacecraft detected reconnection jets in the current sheet

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 b (<1) allows strong guide field reconnection. High b 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. Summary

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 2 VA Inside right left left Inside spacecraft Diffusion region right Inside Gosling et al. [2005]

In-situ Observations of Collisionless Reconnection in the Magnetosphere