Magnetotail global dynamical structure and stability...Auroral oval and polar cap depend on...

Space Research InstituteSpace Research Institute

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


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


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.


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


Illustration: AL geomagnetic index vs IMF north – south component

IMF north

IMF south

AL ~ VBZ


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


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


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


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


Diversion of magnetospheric currents to the ionosphere createsSubstorm current wedge and ionospheric electrojets


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


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


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


Substorm ScenarioPlasmoid dropout at onset, Earthward transport


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


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


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


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


Summary


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


The brightest evidence of multi-scale structure of magnetotail dynamics:discrete aurora


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


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


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


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)


Structural changes prior to substorm onset:

Simplification of current network to accommodate increasing cross-tail current.

Weakly branched current network is

TOPOLOGICALLY UNSTABLE


Cascade to lower frequencies prior to substorm onset

[Lui et al, 1997]


Finally – current wedge formationand substorm.


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 ?


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]


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


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


Summary


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


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


International Geophysical Year Yu. Galperin with S. Chapman and photometer, 1958


At Loparskaya polar station, 1957


In 1959-1960 at the age of 27 he was visiting Chinato give lectures on aeronomy observations


A special memorialConference was heldin Moscow, 2003

Published as the first CAWSES handbook

Is available on request.


Thank you!


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.

Space Research InstituteSpace Research InstituteИнститутИнститут космических исследованийкосмических исследований

Magnetospheric precipitationOriginates in the magnetotail


Visualization of Tsyganenko modelShape and size are controlled by solar wind flowEnergy content in the magnetotail - by reconnection (density of white lines)


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…

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Magnetotail global dynamical structure and stability...Auroral oval and polar cap depend on...

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