Alessandro Retinò, F. Sahraoui, G. Belmont Laboratoire de Physique des Plasmas - CNRS, St.-Maur-des-Fossés, France A. Vaivads, Y. Khotyaintsev Swedish Institute of Space Physics, Uppsala, Sweden R. Nakamura, B. Zieger, W. Baumjohann Space Research Institute, Graz, Austria D. Sundkvist, S. Bale, F. S. Mozer Space Sciences Laboratory, University of California, Berkeley, USA M. Fujimoto, K. Tanaka ISAS-JAXA, Sagamihara, Japan In situ observations of magnetic reconnection in solar system plasma Vlasov-Maxwell kinetics: theory, simulations and observations in space plasmas Wolfgang Pauli Institute– Wien

Outline  Magnetic reconnection  In situ spacecraft observations of reconnectionin near-Earth space  Some key open issues:  microphysics  particle acceleration  reconnection & turbulence  Current & future spacecraft data relevant for reconnection  Summary

Magnetic reconnection  Violation of the frozen-in condition in thin boundaries (current sheets )  Effects:  magnetic topology change (E || )  plasma transport across boundaries  plasma acceleration (alfvenic)  plasma heating  particle acceleration (non-thermal)  Importance of scales (collisionless): [ adopted from Paschmann, Nature, 2006] d_ MHD ( >>  i ) ~ 10 3 km d_ ion ( ~  i ) ~ 50 km d_ electron ( ~  e ) ~ 1 km Hall electron pressure electron inertia * MHD anomalous conductivity E' = E+u x B = 0 E||=0 E' = E+u x B =J/  E||≠0 d D  d  L  D

Reconnection in the plasma Universe Laboratory plasma [Intrator et al., Nature Physics, 2009] Solar corona [Yokoyama et. al., ApJ Lett., 2001] Near-Earth space [Paschmann, 2008] Radio galaxy lobes [Kronberg et al., ApJ, 2004] L ~ m L ~ 10 7 m L ~ 10 8 mL ~ m (?)

Near-Earth space as laboratory 5 LABNEAR-EARTHSUNASTRO Direct measur. of E & B yesyes (high res)nono Direct measur. of f(v)noyes (high res)nono Imagingnonoyes (high res)yes Boundary conditionsartificialnaturalnaturalnatural Repeatabilityyesnonono Number of objectsa fewoneonemany [Vaivads et al., Plasma Phys. Contr. Fus., 2009] Solar system plasma (very often) are:  fully ionized  mainly H +, e -  not relativistic (Va<<c)  collisionless

Collisonless reconnection in near-Earth space solar wind: Gosling et al., JGR,2005; Phan et al., Nature, 2006; magnetopause: Paschmann et al., Nature, 1979; Sonnerup et al, JGR, 1981; Mozer et al., PRL, 2002; Vaivads et al., PRL, 2004; magnetosheath: Retinò et al., Nature Physics, 2007; Phan et al., PRL, 2007 KH- vortexes: Nykiri et al., Ann. Geophsy., 2006; Hasegawa et al.,, JGR, 2009 magnetotail: Hones, GRL, 1984; Nagai, JGR, 2001; Øieroset, Nature, 2001; Runov et al., GRL, 2002

ESA-Cluster spacecraft 7  first 4 spacecraft mission  distinguish temporal/spatial variations  measurement of 3D quantities: J=(1/μ 0 )  xB,  B = 0, E  J, etc.  tetrahedrical configuration with changeable spacecraft separation km -> measurements at different scales 4 sets of 11 identical instruments to measure:  DC magnetic field  DC electric field  waves  thermal particle distribution functions  suprathermal particle distribution functions DC magnetometer [

In situ spacecraft observations of reconnection 8 [adopted from Baumjohann & Treumann, 1996] [Phan et al., Ann.Geophys., 2004] [Vaivadset al., PRL., 2004] Alfvenic jets Current sheet Hall physics  L ~ 10 7 km >> ρi

Some key open issues (to be addressed by in situ obs – simulations synergy) 9 I.Microphysics i.e. physics at ion scales and below II.Particle acceleration i.e. ion & electron acceleration at non-thermal energies III.Relationship between reconnection and turbulence

Microphysics 10  What is the structure and dynamics of the diffusion regions (ion & electron)?  How does reconnection start in the electron diffusion region (onset)?  Is (collisionless) reconnection always fast?  How ions and electrons are heated/accelerated?  What is the role of the separatrix region? ...

[Mozer et al., PRL, 2002] Textbook example (rare !):  antiparallel reconnection  Hall fields  Reconnection electric field  Reconnection rate ~ 0.1 also Cluster [Runov, et al., GRL, 2002; Vaivads et al., PRL, 2004 Diffusion regions

Cluster multi-scale orbits in  C1, C2, C3/C4 at fluid/MHD scales ~ 1000 km  C3, C4 at sub-ion scales ~ 20 km  subsolar magnetopause crossed ~ 10 Re  important for MMS preparation!

13  guide field + asymmetric reconnection  reconnection jets in the MP/BL V L ~ 200 km/s ~ 2*V A [N msh ~15cc, B L,msh ~ 20 nT].  VL 0 for C1 as expected. Jet reversal indicates vicinity to the X-line.  rec. rate = /V A ~ 0.1 (but large errors)  electron par-perp anisotropy within MP  timing C1 – C3 not possible (too large separation) -> MP thickness?  multi-scale coupling MP crossing - fluid scales MSH MSP [Retinò et al., in preparation, 2011]

14  comparison of BL between C3-C4 -> MP thickness ~ 20 km ~ 10  e. MP basically standing V N,M P ~ 1 km/s ~ V C3, C4 (temporal variations = spatial variations)  thin MP stable over ~ 15s ~ many ion gyroperiods  i -1  C3, C4 at different locations within MP -> correl. EX, BL proxy of distance from center of MP  strong parallel current J M ~ 100 nA/m 2 and field-aligned (parallel) heating  strong wave turbulence (not shown)  evidence of electron diffusion region ? MP crossing – sub-ion scales MSP MSH

[Retinò et al., GRL, 2006] -strong activity also away from the X-line - ion acceleration (jet) and non-thermal electron acceleration in the separatrix region Separatrix region also [Wygant et al., JGR, 2005; Cattell. et al., JGR, 2005; Khotyaintsev et al., PRL, 2006]

Particle acceleration  Is reconnection always efficient for particle acceleration?  How are particles accelerated around the diffusion region (reconnection electric field vs multi-step acceleration)?  How are particle accelerated away from the diffusion region (dipolarization fronts, flow braking region, etc.)? ...

Non-thermal electron acceleration Acceleration in contracting magnetic islands [Drake 2006, Chen 2008] X-line acceleration [Pritchett 2006,Øieroset 2002, Retinò 2008] Acceleration at magnetic flux pile-up in outflow region [Hoshino 2001, Imada 2007] Strongest acceleration during unsteady reconnection in thin current sheets

Electron acceleration in thin CS  Magnetotail reconnection  Alfvénic plasma outflows  Highest flux increase associated with thin CS embedded in outflow [Retinò et al., JGR, 2008] Electron acceleration in thin current sheet

Electron acceleration in thin CS  direct X-line acceleration by Ey ~ 7 mV/m (unsteady reconnection)  further acceleration within flux rope by betatron + pitch-angle scattering (’gyrorelaxation’) sub-spin time resolution measurements crucial ! Acceleration mechanisms

The flow (jet) braking region flow braking region X-line / microphysics (sub-ion scales) [Nakamura2009, Retinò2010 submitted, Zieger2011 in preparation] / particle acceleration [Asano2010, Retinò2010, Zieger2011] [adopted from Birn2005]

Cluster multi-scale orbits in 2007  C1, C2, C3/C4 at fluid/MHD scales ~ 1000 km  C3, C4 sub-ion scales ~ 20 km  near-Earth plasma sheet crossed ~ 10 R E  important for MMS preparation!

Electron acceleration in the flow braking region / flow braking from two-point measurements C1-C4 (MHD/fluid scale) /large-amplitude magnetic field fluctuations /strong lower hybrid and whistler waves /supra-thermal particle acceleration /multi-scale coupling Vx=Ey/Bz H+ k B Ti k B Te energetic e- e- waves flow mag

/ thickness from two-point measurements C3-C4 /Hall physics E x ~(J y xB z )/N e /strong Ey and lower-hybrid waves /electron acceleration up to ~400 keV  x~70 km ~ several  e Acceleration in thin current layers

Reconnection & turbulence Large-scale laminar vs small-scale turbulent current sheets 24 [Phan et al., Nature, 2006] L ~ 3 ·10 6 km ~ Ls Coronal loop observed by NASA/TRACE (UV ~10 6 K) L ~10 5 km ~ Ls [Dmitruk & Matthaeus, Phys. Plasmas, 2006] L << Ls Ls [Shibata et al., Science, 2007] Ca II image from Hinode - SOT L ~ 10 3 km << L s L |B| Hall MHD

Reconnection & turbulence Small-scale current sheets in turbulence [Matthaeus & Lamkin, Phys. Fluids,1986; Dmitruk & Matthaeus, Phys; Plasmas, 2006; Servidio et al., Phys. Plasmas, 2010] Turbulent current sheet [Lazarian & Vishniac, ApJ, 1999; Loureiro et al., MNRAS, 2009] Turbulence/waves in laminar current sheet [Belmont & Rezeau, JGR, 2001; Bale et al;, GRL, 2002; Vaivads et al., GRL, 2004; Khotyaintsev et al., Ann. Geophys., 2004; Retinò et al., GRL, 2006; Eastwood et al.; PRL, 2009; Huang et al., JGR, 2010] D d  << D

26  How do small-scale current sheets form in turbulence ?  Is reconnection occurring in such current sheets ?  Is reconnection in turbulent plasma faster than laminar reconnection ? (reconnection rate)  What is the role of small-scale reconnecting current sheets for energy dissipation in turbulent plasma ?  Is reconnection in turbulent plasma efficient for accelerating particles to non-thermal energies? ... Reconnection & turbulence

In situ evidence of reconnection in turbulent plasma (I) quasi-|| quasi-  cartoon of small-scale current sheets formation in turbulent plasma reconnecting current sheets [Retinò et al., Nature Physics, 2007] further evidence in fast SW [Gosling et al., ApJLett, 2007]  N/N ~ 1  B/N ~ 1 Energetic ions ~ d

In situ evidence of reconnection in turbulent plasma (II) [Retinò et al., Nature Physics, 2007] further evidence in fast SW [Gosling et al., ApJLett, 2007] 4 spacecraft crucial to determine the thickness d~ i of the current sheet current sheet energy dissipation electron heating plasma acceleration rate ~ 0.1 (fast) LH turbulence topology change Hall field

Turbulence properties inertial range dissip/disp. range B E' alfvenic turbulence  Alfvenic turbulence close to -5/3 (inertial range)  Intermittency at scales of a few ρ i and smaller ( close to dissip./disp. range) -> presence of coherent structures  dissipation in current sheets with d~ i comparable to wave damping around  ci -> turbulent reconnection competing mechanism for energy dissipation at i scales Intermittency Gaussian ii i [Sundkvist et al., PRL, 2007]

Possible applications of results from in situ observations (with caution!)  Sawtooth oscillations in tokamaks  Coronal heating  Particle acceleration in solar flares  Dissipation in accretion disks  Cosmic rays acceleration Radio galaxy [adopted from ] [Mann et al., A&A, 2009]

Current & future spacecraft data relevant for reconnection (and with LPP involvement) ESA/Cluster [ (2014) -- near-Earth space NASA/Themis [ near-Earth space NASA/MMS [ ]: near-Earth space Goal: the physics of reconnection at electron scales (also turbulence, particle acceleration) ESA/SolarOrbiter [ near-Sun corona (62 Rs). Goals: solar wind acceleration, coronal heating, production of energetic particles (turbulence, reconnection) ESA/SolarProbePlus [ near-Sun corona (8.5 Rs). Similar goals to SolarOrbiter

Summary (I)  Reconnection universal process responsible for mayor plasma transport, plasma acceleration / heating and non-thermal particle acceleration  Near-Earth space excellent laboratory to study the physics of reconnection through in situ measurements (Cluster first multi- point)  Microphysics of reconnection:  Observations at sub-ion scales  Structure of separatix regiuon  Particle acceleration:  Electron acceleration mechanisms in thin current sheet  Electron acceleration mechanisms in the flow braking region  Reconnection and turbulence:  Evidence of reconnection in turbulent plasma in small-scale current sheets.  Turbulent reconnection can be efficient mechanism for energy dissipation

Summary (II)  Possible applications of results from in situ obs: sawtooth oscillations in tokamaks, coronal heating, particle acceleration in flares, dissipation in accretion disks, cosmic ray acceleration etc.  Future missions will (hopefully) improve our understanding of reconnection at electron scales, particle acceleration and turbulent reconnection. Current missions (Cluster, Themis) very important for preparation!  Synergy between in situ ibs – simulations very important:  PIC/Vlasov: electron scales  PIC/Vlasov+ hybrid: particle acceleration  PIC/Vlasov + hybrid + MHD: turbulent reconnection