ISSI Workshop on Mercury, 26–30 June, 2006, BernISSI Workshop on Mercury, 26–30 June, 2006, Bern

Substorm, reconnection, Substorm, reconnection, magnetotailmagnetotailin Mercury in Mercury

Rumi NakamuraSpace Research Institute, Austrian Academy of Sciences

1. Magnetotail response to solar wind change

2. Substorm relevant current dynamics

Unknowns in Mercury based on Mercury-Earth comparison

Discuss how the planned Mercury mission will enhance our understandings

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Mercury magnetosphereMercury magnetosphere

Spatial scale Earth:Mercury 7 : 1 [Siscoe et al. 1975] (Based on Solar wind and dipole moment)

... but not only a mini-Earth magnetosphere ...

Solar wind condition

Mercury Earth

IMF 21-46 6 nT

Strong 160 20 nT

Vsw 430 430 km/s

Tp 13-17 8 104 K

Np 73-32 7 /cc

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Mercury night-side observationMercury night-side observation

previous and future planned mission

Near-Earth reconnection plasma loss through plasmoid flux tube volume decrease

Plasma bubble (Interchange inst.) Earthward transport of low V low N

Mariner 10 (Orbit III)Inner tail (13 RE)

MessengerPolar cap, tail lobe Near-Earth plasma sheet(Solar wind, Magnetosheath)

MPOPolar cap, Inner tail (12 RE)

MMOPlasma mantle, lobeMidtail plasma sheet ( 42 RE)(Magnetosheath)

Mariner 10 Orbit III, 17 min

[12 h]200km x 15,000km

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Time scalesTime scalesMagnetospheric flux transport driven by solar wind.

Substorm/Convection time scale:

Time to cycle the magnetic flux in the tail under the electric field potential across magnetosphere (due to merging)

XY

Z

Mercury Earth

Tail response 1 min 20 min

Substorm/Convection 1-2 min 30-60 min

[Siscoe et al., 1975]

Nightside/dayside balanced merging is not happening in the Earth

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-6<y<3 RE

N: flux tube contentS: PV5/3

[Kaufmann et al., 2004]

Need for near-Earth reconnectionNeed for near-Earth reconnection

Magnetotail at Earth cannot maintain•adiabatic convection: dpV/dt =0 •force balance: p=B2/2

simultaneously (Pressure Crisis)

Flux tube volume shrinks too steep inward.N (flux tube content) decreases 70%

PV decreases 85% from 30RE to 10RE

Near-Earth reconnection (Substorm) plasma loss through plasmoid flux tube volume decrease

Plasma bubble (Interchange inst.) Earthward transport of low V low N

<15RE 20–30 RE 100 RE

Dipolar field

Tail-like field

NENL DNL

How is for Mercury tail ?

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Substorm or driven disturbance ?Substorm or driven disturbance ?

Fitting Mariner 10 observation to model field (Luhmann et al. , 1998)

Near-Earth reconnection plasma loss through plasmoid flux tube volume decrease

Plasma bubble (Interchange inst.) Earthward transport of low V low N

Transient, current sheet crossing, Bz & Bx disturbances not reproduced

Large By disturbance (field aligned current) not reproduced

No way to check the real IMF or Psw

Instead of dipolarization: Configuration changedue to enhanced IMF Bz

Instead of injection: particle entry via open field line

Observation Model

IMF reconstructed

BUT

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Expected disturbance at Mercury tailExpected disturbance at Mercury tail

Examine expected disturbance at MMO/Messenger based on Geotail data and model fields using IMF data

DATA (Earth) substorm and driven response

(1) Geotail data from midtail (period with substorm)

MODEL driven response

(2) Empirical model [Fairfield and Jones, 1996] pressure balance using hourly average B function of X,Psw,IMFBy,Bz

(3) Dipole+Tsyganenko 96 [Tsyganenko et al, 1996] Model of currents, empirically depending on: Psw,IMFBy,Bz,Dst

(4) Dipole+modified Tsyganenko 96 [Luhmann et al, 1998](T96 without ring current and R1,R2 current)

All output scaled to Mercury: x 2 (for B), x 7-1(for distance)

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Magnetic flux in the tail Magnetic flux in the tail

Global parameter (magnetic flux in the tail) based on local measurement

B*R*R

IMF Bz south increase flaring, R , B

Psw decrease flaring, R, increase B

midtail: change in R not significant (< 7 % )

Using pressure balance B (lobe B) can be monitored from Ptotal (plasma pressure + magnetic pressure) both at plasma sheet and lobe.

Mariner 10 observed pressure balance-like behaviour

Compare response of B (or P) from insitu magnetotail observation and that expected from solar wind direct response

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Substorm with Psw increaseSubstorm with Psw increase

Observation Model

Driven response: Flux level high due to enhanced Psw and IMF Bz south

Geotail:

Compression and substorm response: Profile of enhanced pressure + Flux pileup after IMFBz south and decrease associated with onset

Geotail X = -47, Y = -5, Z = -5 RE

Mercury: X= -7 RM MMO

2min

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Substorm (IMF triggered onset) Substorm (IMF triggered onset)

Driven response: Flux level high due to enhanced IMF Bz south

Geotail:

Substorm response: Flux pileup associated with IMF Bz south. Rapid decrease around northward turning

Steady magnetospheric convection: Flux level does not increase during IMF Bz south interval

Tail reconnection rate changes differently from that expected from IMF Bz change

Observation Model

Geotail X= -37, Y = 5, Z = -3 RE

Mercury: X= -5 RM MMO

2min

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Substorm (spontaneous onset)Substorm (spontaneous onset)

Observation Model

2min

Driven response: Flux level enhance due to enhanced IMF Bz south (during P decrease)

Geotail:

Substorm response: Flux pileup associated with IMF Bz south. Rapid decrease at onset (still during IMF Bz south)

Continued magnetospheric convection: Flux level does not increase during IMF Bz south interval

Tail reconnection rate changes differently from that expected from IMF Bz change

Geotail X= -24, Y = -1, Z = -3 RE

Mercury: X= -3.4 RM MESSENGER ?

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Dayside/Nightside ReconnectionDayside/Nightside Reconnection

(Nakamura et al.,1999)

dF/dt = dn

Midtail magnetic flux

Day-side reconnectionvoltage

Nigh-side reconnection voltage

midtail

substormconvection

substorm

Midtail flux transport is governed by convectionand by substorms How is Mercury response?

Dayside, nightside reconnection are unbalanced

(timescale of several hours: Earth several-10min: Mercury)

If convection only

Magnetotail observation

Dayside observation

Observed value

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Thin current sheet crossing ?Thin current sheet crossing ?

Mariner 10 tail current sheet crossing (Whang et al., 1977)

Larmor radius for 2 keV proton: ~1000 km (B=5nT) , ~100 km! (B=40 nT)

proton (n=1/cc) inertia length: 230 km

Is this a thin current sheet before substorm ?

Time scale: 40 s

Bx: 80 nT

Spacecraft motion (3.7 km/s) along Z:

~150 km (0.06 RM)

Current sheet center: Z=75 km

Current sheet thickness:D = 150 km

Observation Model

DQO (dipole+quadrupole+octupole) + current sheet model

dipolarization

FAC

closestapproach

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Current sheet structureCurrent sheet structure

Earth’s tail current sheet is very dynamic (Cluster observation) Bifurcated current sheet, off-equatorial current sheet (Mercury, too?)

Current sheet motion: several tens - hundred km/s Quiet current sheet motion: 10-20 km/s V_E x 1/7 (spatial scale diff.) x 30 (time scale diff.) V_M = 4 V_E ? Current sheet motion at Mercury ? (use of “finite ion gyro effect” may help)

Earth’s tail current sheet is very dynamic

[Runov et al, 2005]

A B C

Cluster obs.Mariner 10

Bx

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Heavy ions and thin current shetHeavy ions and thin current shet At Earth, Speiser-type motion of oxygen identified during storm-time substorm reconnection event

O+ dominates in pressure and density

At Mercury, Na+ is sputtered from the surface. Due to small spatial scales non-adiabatic transport features are expected also for H+ based on particle simulation. (Delcourt et al., 2003; 2005)

[Kistler et al., 2005]

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Strong North-south asymmetryStrong North-south asymmetry

Parker spiral IMF case produce substantial asymmetric plasma magnetic field configuration (Kallio and Jahunen, 2003; 2004)

Only few case reported, but can happen also in the Earth’s magnetotail:

Distant tail observation under strong By (Oieroset et al., 2004)

Asymmetric substorm disturbances expected: field-aligned current, current sheet processes, particle acceleration, precipitation etc.. like Mariner 10 ?

Solar wind proton density and field configuration from a hybrid model

IMF [32,10,0] nT

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Fast flow & DipolarizationFast flow & Dipolarization Bursty fast flows

accompanied by dipolarization

Earthward convection by bursty bulk flows

Fast flow stops near 10 RE by dipolar field

(Schödel et al., 2001)

Current diversion through ionosphere associated with dipolarization

Substorm current wedge

not the same in Mercury

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Field aligned currentField aligned current

Strong field aligned current observed at dawnside magenetosphere (Slavin et al. , 1997)

Field aligned current flowing toward Mercury(B=60 nT, t = 23s)

Reasonable scales expected from Earth substorm Geotail&EquatorS (B=30-40 nT, t = 300-360s)

Observation Model

[Nakamura et al., 1999]

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Substorm current wedge ?Substorm current wedge ?

Intense field aligned current at Mercury without ionosphere

Taking into account the plasma sheet motion, field aligned current density may be smaller (at least x 10-1?) than 700 nm2

Motion of the current sheet/structure are essential to discuss the spatial scale and therefore underlying processes

Earth-example of plasma sheet expansion associated withfield aligned current and dipoliarzation

plasma sheet expansionspeed ~30km/s(980425 case)

Higher speed obtained by Cluster(Dewhurst et al., 2002)

J ~ 50 mA/m

j ~ 700 nm2 (taking into account the spacecraft motion ~3km/s)

J ~ 30 mA/m , j ~ 3 nA/m2 (taking into account the plasma sheet motion)

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SummarySummary

MMO-MPO combination, even without a solar wind monitor, we can study:

Solar wind-magnetotail interaction >Magnetotail radial pressure profile >Statistically determine scale of the pressure changes (to compare with solar wind profile) >Magnetosheath-inner tail comparison

With MESSANGER, MMO, MPO we can expect to identify:

“Substorm” evidence >Current sheet profiles >Relationship between midtail and inner magnetosphere >Plasmoid >Dipolarization/acceleration of particles >Field aligned current

Current sheet processes significantly governed by particle dynamics

Need to determine the right spatial/temporal scales of the processes.

Expected useful observations in future mission to enhance our understanding of magnetotail processes