Alessandro Retinò, F. Sahraoui, G. BelmontLaboratoire de Physique des Plasmas - CNRS, St.-Maur-des-Fossés, France

A. Vaivads, Y. KhotyaintsevSwedish Institute of Space Physics, Uppsala, Sweden

R. Nakamura, B. Zieger, W. BaumjohannSpace Research Institute, Graz, Austria

D. Sundkvist, S. Bale, F. S. MozerSpace Sciences Laboratory, University of California, Berkeley, USA

M. Fujimoto, K. TanakaISAS-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

29.03.2011

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

18 avril 2023 alessandro.retino@lpp.polytechnique.fr 2

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) ~ 103 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 ~ 10-2 m

L ~ 107 m

L ~ 108 m L ~ 1016 m (?)

Near-Earth space as laboratory

5

LAB NEAR-EARTH SUNASTRO

Direct measur. of E & B yes yes (high res) no no

Direct measur. of f(v) no yes (high res) no no

Imaging no no yes (high res) yes

Boundary conditions artificial natural natural natural

Repeatability yes no no no

Number of objects a few one onemany

[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, 2007KH- vortexes: Nykiri et al., Ann. Geophsy., 2006; Hasegawa et al.,, JGR, 2009magnetotail: 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/μ0) xB,

B = 0, EJ, etc.

tetrahedrical configuration with changeable spacecraft separation 100-10000 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

[http://sci.esa.int/sciencee/www/area/index.cfm?fareaid=8]

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

Current sheet

Hall physics

L ~ 107 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 2008

12

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 VL~ 200 km/s ~ 2*VA [Nmsh~15cc, BL,msh ~ 20 nT].

VL <0 for C3, VL>0 for C1 as expected. Jet reversal indicates vicinity to the X-line.

rec. rate = <VN>/VA ~ 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]

<VN>

14

comparison of BL between C3-C4 -> MP thickness ~ 20 km ~ 10 e. MP basically standing VN,MP ~ 1 km/s ~ VC3,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 JM ~ 100 nA/m2 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 contractingmagnetic 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

18 avril 2023 20

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

18 avril 2023 21alessandro.retino@lpp.polytechnique.fr

C1, C2, C3/C4 at fluid/MHD scales ~ 1000 km

C3, C4 sub-ion scales ~ 20 km

near-Earth plasma sheet crossed ~ 10 RE

important for MMS

preparation!

Electron acceleration in the flow braking region

18 avril 2023 22alessandro.retino@lpp.polytechnique.fr

/ 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 couplingVx=Ey/Bz

H+

kBTi

kBTe

energetic e-

e-

waves

flow

mag

18 avril 2023 23alessandro.retino@lpp.polytechnique.fr

/ thickness from two-point measurements C3-C4

/Hall physics Ex~(JyxBz)/Ne

/strong Ey and lower-hybrid waves

/electron acceleration up to ~400 keV

x~70 km ~ several e

Acceleration in thin current layers

Reconnection & turbulenceLarge-scale laminar vs small-scale turbulent current sheets

24

[Phan et al., Nature, 2006]

L ~ 3 ·106 km ~ Ls

Coronal loop observed by NASA/TRACE (UV ~106 K)

L ~105 km ~ Ls

[Dmitruk & Matthaeus, Phys. Plasmas, 2006]

L << Ls

Ls

[Shibata et al., Science, 2007]

Ca II image from Hinode - SOT

L ~ 103 km << Ls

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; Vaivadset al., GRL, 2004; Khotyaintsev et al., Ann. Geophys., 2004;Retinò et al., GRL, 2006; Eastwood et al.; PRL, 2009; Huang etal., 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. rangeB

E'

alfvenic turbulence

Alfvenic turbulence close to -5/3 (inertial range)

Intermittency at scales of a few ρi 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

ii

[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 fromhttp://www.ece.unm.edu/~plasma/Space/

jets.htm]

[Mann et al., A&A, 2009]

Current & future spacecraft data relevant for reconnection (and with LPP involvement)

ESA/Cluster [http://sci.esa.int/cluster]: 2000-2012(2014) -- near-Earth space

NASA/Themis [http://themis.ssl.berkeley.edu]: 2007 -- near-Earth space

NASA/MMS [http://mms.gsfc.nasa.gov]: 2014 -- near-Earth spaceGoal: the physics of reconnection at electron scales (also turbulence, particle acceleration)

ESA/SolarOrbiter [http://sci.esa.int/solarorbiter]: 2017 -- near-Sun corona (62 Rs). Goals: solar wind acceleration, coronal heating, production of energetic particles (turbulence, reconnection)

ESA/SolarProbePlus [http://solarprobe.gsfc.nasa.gov]: 2018 -- 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