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Magnetotail global dynamical structure and stability
A.A.Petrukovich & L.M.Zelenyi
Space Research Institute, Moscow, Russia
Introduction to magnetosphere dynamics
Large-scale magnetotail dynamics: substorms and convection
Magnetotail plasma sheet local structure
In memory of Prof. Galperin
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History of Magnetosphere:
complements one in Joe Allen’s presentation.,.
Carrington (1859): Geomagnetic activity is related to Sun
Birkeland (1867-1917):Aurora caused by electrically charged particlesionospheric & field-aligned currents
Chapman and Ferraro (1930): Compressed closed magnetosphere(currents are due to atmospheric motion)
Axford and Hines (1961)Closed magnetosphere withviscous interactionbetween solar wind and Earth plasma
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Dungey,1961 Open magnetosphere
Magnetosphere is shaped by solar wind flow, but is controlled by Interplanetary magnetic field (IMF)
IMF is just 2% of solar wind flow energy.
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Confirmed also with local plasma observations in the magnetosphere:One of primary mechanisms providing particle acceleration andconversion of magnetic energy in thermal one
Reconnection or merging paradigm: Extension of ideal MHD, allowing breaking of the “frosen-in” approximation in certain X points
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Illustration: AL geomagnetic index vs IMF north – south component
IMF north
IMF south
AL ~ VBZ
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Illustration: AL geomagnetic index vs solar wind densityfor southward IMF, density changes 1 – 50 cm-3
Reflects mainly magnetospheric (and magnetic field) compression.
AL ~ (nV2)1/6
VBz = 1 mV/m
VBz = 3 mV/m
VBz = 6 mV/m
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Northward Bz geomagnetic activity ? - scattered examples in literature
statistics of 66 small substorms
Actually IMF |By| > Bz
Azimuthal IMF corresponds to
skewed reconnection scheme.
Petrukovich et al, 2000
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Summary
1. Magnetosphere is controlled by IMF
Why the magnetosphere is relevant to CAWSES ?
It is between Sun and Earth
Controls energy deposition to ionosphere and atmosphere
Particle precipitation Ionospheric currents Magnetospheric topology
Magnetotail is the key region of the magnetosphere
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Auroral oval and polar cap depend on magnetosperic state
Magnetospheric particles precipitate in auroral ovalSolar cosmic rays penetrate in near-polar cap area
Normal auroral oval (65-70 Gmlat) Extended oval ~ 40-50 GmlatDuring strong storms.
Polar cap Auroral oval
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Diversion of magnetospheric currents to the ionosphere createsSubstorm current wedge and ionospheric electrojets
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Magnetospheric convection (Dungey scheme)
Dayside: stable reconnection rate providing open magnetic fluxNight-side: Magnetotail reconnection returns open flux to closed field lines,
generally does not balance the day-side: special substorm dynamicsInner magnetosphere: magnetic flux and particles provided from the magnetotail
generate geomagnetic stormin case of prolonged southward IMF (>3 hours of Bz = –10 nT)
Stable inputVariable conversion
Geomagnetic storm
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Magnetotail – energy reservoir of the magnetosphere
Lobe (polar cap): open field lines, low beta plasma (only magnetic pressure)merely a container of magnetic flux
Plasma sheet (auroral zone): closed field lines, hot plasma, contains cross-tail current, complicated internal dynamics determines stability
Plasma sheet
lobes
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Typical (Perfect) Substorm
IMF Bz southward
growth phaseMagnetic flux accumulation in lobenightside reconnection is supressed
Lobe magnetic pressure maximum
onset of expansion phaseExplosion of night side reconnectiondischarge of extra magnetic flux.X ground magnetic disturbanceburst of auroral current at onset
growth onset
Lobe magnetic field: robust indicator of the large-scale magnetotail state
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Substorm ScenarioPlasmoid dropout at onset, Earthward transport
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Growth phase duration dependence on solar wind and IMF[Petrukovich, 2004]
( )22sin22 θ+= zByBswVyE
Small substormsNormal substormsStorm-time substorms
duration
There is no threshold for energy accumulation during growth phase(magnetotail stability): larger inputs generate stronger (storm – time) substorms
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Where ? current disruption, boundary layer, solar wind trigger
Substorm onset: energy discharge in the magnetotailOne of major magnetospheric puzzles
Balooning
Sausage
Tearing
LHD
Drift-kinkHow ? Plasma instability disrupting current sheet
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Stable pressure at convection period
Second option for global tail mode: enhanced convection [Sergeev,1996]Day-side and night-side reconnection are balanced.Low open magnetic flux – small polar cap, extended auroral oval
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Storm level southward IMF
Geomagnetic storm:Slow 10-hours time scale
Intermittent substorms and enhanced convectionat ~1-hour scale
Lobe magnetic pressure
Auroral ground magnetic disturbance
substorms vs convection50 / 50
Southward IMF
Storm SYM-H (Dst) –100 nT
substorms convection
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Summary
1. Magnetosphere is controlled by IMF
2. Magnetotailbalances the day-side input with substorms and convection supplies material to inner magnetospherecontrols particle precipitation via magnetic topology
3. Necessary to take into account the magnetotail state if one wants resolution of ~1 hour
4. Substorm onset problem and choice of substorm/convection will feed the next generation of magnetospheric physicists
Couple of novel approaches to description of local processes:the solution to problems of where and how ?
Fractional descriptionLocal current sheet structure
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The brightest evidence of multi-scale structure of magnetotail dynamics:discrete aurora
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Non-equilibrium Quasi-stationary states of Current Sheets
recall: plasma sheet carries cross-tail current and is responsible for stability
Laminar current sheetTurbulent during substorms
Always turbulent current sheet
Bn ≠ 0 δB ≠ 0
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Properties of turbulence
MHD turbulence (fluid with frosen-in B)
Fourier spectra of magnetic and velocity variations with power law slopes
αV = αB = 5/3
MHD approach breaks at certain scale lengths
Non-MHD kinetic model (fractal field) [Zelenyi & Milovanov, 1996,1998]
αB = 7/3 αV =5/3
Borovsky, 1997
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Structural scaling of current network
x
y
Knotted web-like percolating network self-consistently “pulled” on the multiscale magnetic field clumps δj ↔ δBz
Average transverse current supports global field reversal : ⟨jy⟩ ↔ Bx
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From point of view of standard “instability”approach:
Non-Equilibrium
Steady State
Linear approximation Multiscale self-similar fractal structuringBalooning Tearing
LHDSausage
Drift-kink
Nonlinear interaction of many unstable nodes. Saturation of fluctuation growth
B(t)
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Structural changes prior to substorm onset:
Simplification of current network to accommodate increasing cross-tail current.
Weakly branched current network is
TOPOLOGICALLY UNSTABLE
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Cascade to lower frequencies prior to substorm onset
[Lui et al, 1997]
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Finally – current wedge formationand substorm.
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Cross-tail current sheet
X, RE
Z, RE
T95
X
Z
Y
• Detected by spacecraft as sign change of Bx.
• Horizontal and slowly moving up and down ?
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Cluster project (4 spacecraft – measure local 3D structure !)
results: wave normals indicate tilted current sheet
[Sergeev et al., 2003]
Nominalnormal
Wave train[Zhang et al., 2002]
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Non-horizontal current sheet and local magnetic field geometry[Petrukovich et al, in press]
Spacecraft trajectory
Bend-type deformation:Magnetic field direction follows the normal.
Flux tubes rotate
ZgsmPlanar tail magnetic configuration: Ygsm
Shear (Slip) - type deformation:Magnetic field direction constant.
Flux tubes shift vertically
J
B
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Embedded thin dynamical sheetEmbedded thin varying current sheet J supports only dynamic ±B0 layer Global ∆BL is supported by much wider current JL
+BL
-BL
+B0
-B0
J0JL
Golovchanskaya & Maltsev, 2005
New instability mode in the plasma sheet
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Summary
1. Magnetosphere is controlled by IMF
2. Magnetotail is an important part of solar-terrestrial linkbalances the day-side input with substorms and convection supplies and accelerates particles for inner magnetospherecontrols particle precipitation via magnetic topology
3. Fractional topological description and unexpected types ofplasma sheet deformation may provide a key to ever standingproblem of substorm onset problem
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In memory of
Prof. Yuri Galperin 1932-2001
member of SCOSTEP bureau
Since 1967 Head of lab for auroral physics at Space Research Institute, Moscow
The head of first international USSR-French space projects Aureol and Arcad
Advised in creation of several space-geophysics related institutions across USSR
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International Geophysical Year Yu. Galperin with S. Chapman and photometer, 1958
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At Loparskaya polar station, 1957
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In 1959-1960 at the age of 27 he was visiting Chinato give lectures on aeronomy observations
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A special memorialConference was heldin Moscow, 2003
Published as the first CAWSES handbook
Is available on request.
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Thank you!
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In a row of other magnetospheres…
Mercury, Venus, Mars, comets –weak or absent magnetospheres, fully controlled by solar windJupiter, Saturn, Uranus, Neptune –strong dipoles, dominated by internal dynamicsEarth’s magnetosphere -both internal dynamics and solar wind are important – rich variety of features.
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Magnetospheric precipitationOriginates in the magnetotail
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Visualization of Tsyganenko modelShape and size are controlled by solar wind flowEnergy content in the magnetotail - by reconnection (density of white lines)
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История вопроса
• Концепция открытой магнитосферы Dungey 60-ые гг
1960-1970ies Explorer, IMP, OGO mapping and General dynamics
1980ies ISEE, AMPTE details: Bursty transport, current disruption
1995-2000 Interball, Geotail, Polar, WindFirst time full solar wind coverage statistical analyses of hypothesesExtensive global auroral imaging multipoint measurements real timingQuantitative plasma data from tail. Input for global modeling as a tool
2000-2005 Cluster - local 3D structure
2006- Themis – coordinated global 3d
Both small scale and large scale output…