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In situ observations of magnetic
reconnection in solar system plasma
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
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
25.05.17
[email protected]
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)
E' = E+u x B = 0
E||=0
 Importance of scales (collisionless):
Dd
d
*
MHD
anomalous Hall
conductivity
LD
electron
pressure
d_MHD ( >> i) ~ 103 km
d_ion ( ~ i ) ~ 50 km
d_electron ( ~ e) ~ 1 km
electron
inertia
E' = E+u x B =J/
E||≠0
[ adopted from Paschmann, Nature, 2006]
Reconnection in the plasma Universe
L ~ 107 m
Laboratory plasma
[Intrator et al., Nature Physics, 2009]
Near-Earth space
[Paschmann, 2008]
L ~ 10-2 m
L ~ 108 m
Solar corona
[Yokoyama et. al., ApJ Lett., 2001]
L ~ 1016 m (?)
Radio galaxy lobes
[Kronberg et al., ApJ, 2004]
Near-Earth space as laboratory
LAB
NEAR-EARTH
SUN
ASTRO
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
one
many
Solar system plasma (very often) are:
 fully ionized
 mainly H+, e not relativistic (Va<<c)
 collisionless
[Vaivads et al., Plasma Phys. Contr. Fus., 2009]
5
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
 first 4 spacecraft mission
 distinguish temporal/spatial variations
 measurement of 3D quantities: J=(1/μ0) xB,
B = 0, EJ, etc.
 tetrahedrical configuration with changeable
spacecraft separation 100-10000 km ->
measurements at different scales
[http://sci.esa.int/sciencee/www/area/index.cfm?fareaid=8]
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
7
In situ spacecraft observations of reconnection
Alfvenic jets
Current sheet
 L ~ 107 km >> ρi
[Phan et al., Ann.Geophys., 2004]
Current sheet
Hall physics
[adopted from Baumjohann & Treumann, 1996]
[Vaivadset al., PRL., 2004]
8
Some key open issues
(to be addressed by in situ obs – simulations synergy)
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
9
Microphysics
 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?
 ...
10
Diffusion regions
Textbook example (rare !):




[Mozer et al., PRL, 2002]
antiparallel reconnection
Hall fields
Reconnection electric field
Reconnection rate ~ 0.1
also Cluster [Runov, et al., GRL,
2002; Vaivads et al., PRL, 2004
Cluster multi-scale orbits in 2008
 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!
12
MP crossing - fluid scales
MSP
MSH
 guide field + asymmetric reconnection
<VN>
 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
[Retinò et al., in preparation, 2011]
13
MP crossing – sub-ion scales
 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)
MSH
MSP
 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 ?
14
Separatrix region
[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
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
X-line acceleration
[Pritchett 2006,Øieroset 2002, Retinò 2008]
Acceleration in contracting
magnetic islands
Acceleration at magnetic flux
pile-up in outflow region
[Drake 2006, Chen 2008]
[Hoshino 2001, Imada 2007]
Strongest acceleration during unsteady reconnection in thin current sheets
Electron
acceleration
in thin
Electron
acceleration
in thin
CS
current sheet



[Retinò et al., JGR, 2008]
Magnetotail reconnection
Alfvénic plasma outflows
Highest flux increase associated
with thin CS embedded in outflow
Electron
acceleration
in thin CS
Acceleration
mechanisms


sub-spin time resolution measurements crucial !
direct X-line acceleration by
Ey ~ 7 mV/m (unsteady
reconnection)
further acceleration within flux
rope by betatron + pitch-angle
scattering (’gyrorelaxation’)
The flow (jet) braking region
flow braking region
X-line
[adopted from Birn2005]
/ microphysics (sub-ion scales) [Nakamura2009, Retinò2010
submitted, Zieger2011 in preparation]
/ particle acceleration [Asano2010, Retinò2010, Zieger2011]
25.05.17
20
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 RE
 important for MMS
preparation!
25.05.17
[email protected]
21
Electron acceleration in the flow braking region
kBTi
H+
/ flow braking from two-point
measurements C1-C4
(MHD/fluid scale)
energetic ekBTe
e-
/large-amplitude magnetic field
fluctuations
/strong lower hybrid and
whistler waves
waves
/supra-thermal particle
acceleration
mag
Vx=Ey/Bz
/multi-scale coupling
flow
25.05.17
[email protected]
22
Acceleration in thin current layers
Dx~70 km ~ several e
/ thickness from two-point
measurements C3-C4
/Hall physics Ex~(JyxBz)/Ne
/strong Ey and lower-hybrid
waves
/electron acceleration up to
~400 keV
25.05.17
[email protected]
23
Reconnection & turbulence
Large-scale laminar vs small-scale turbulent current sheets
L << Ls
L ~ 3 ·106 km ~ Ls
L
Ls
|B|
Hall MHD
[Phan et al., Nature, 2006]
[Dmitruk & Matthaeus, Phys. Plasmas, 2006]
L ~105 km ~ Ls
Ca II image from Hinode - SOT
L ~ 103 km << Ls
Coronal loop observed by NASA/TRACE (UV ~106 K)
[Shibata et al., Science, 2007]
24
Reconnection & turbulence
Small-scale current sheets in turbulence
[Matthaeus & Lamkin, Phys. Fluids,1986; Dmitruk &
Matthaeus, Phys; Plasmas, 2006; Servidio et al., Phys.
Plasmas, 2010]
D
d
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
Reconnection & turbulence
 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?
 ...
26
In situ evidence of reconnection in turbulent plasma (I)
quasi-||
quasi-
Energetic ions
dN/N ~ 1
~d
cartoon of small-scale current sheets
formation in turbulent plasma
dB/N ~ 1
reconnecting current sheets
[Retinò et al., Nature Physics, 2007]
further evidence in fast SW [Gosling et al., ApJLett, 2007]
In situ evidence of reconnection in turbulent plasma (II)
current sheet
Hall field
LH turbulence
rate ~ 0.1 (fast)
topology change
plasma acceleration
electron heating
energy dissipation
4 spacecraft crucial to determine the
thickness d~i of the current sheet
[Retinò et al., Nature Physics, 2007]
further evidence in fast SW [Gosling et al., ApJLett, 2007]
Turbulence properties
dissip/disp. range
B
 Alfvenic turbulence close to -5/3
(inertial range)
inertial range
E'
 Intermittency at scales of a few ρi and
smaller ( close to dissip./disp. range) ->
presence of coherent structures
alfvenic turbulence
 dissipation in current sheets with d~ i
comparable to wave damping around
wci -> turbulent reconnection competing
mechanism for energy dissipation at i
scales
Intermittency
Gaussian
i
[Sundkvist et al., PRL, 2007]
i
Possible applications of results from in situ
observations (with caution!)
 Sawtooth oscillations in
tokamaks
 Coronal heating
 Particle acceleration in solar
flares
[Mann et al., A&A, 2009]
 Dissipation in accretion disks
 Cosmic rays acceleration
Radio galaxy [adopted from
http://www.ece.unm.edu/~plasma/Space/jets.htm
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 space
Goal: 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 multipoint)
 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