Issi workshop on mercury 26 30 june 2006 bern
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ISSI Workshop on Mercury, 26–30 June, 2006, Bern. Substorm, reconnection, magnetotail in Mercury Rumi Nakamura Space Research Institute, Austrian Academy of Sciences Magnetotail response to solar wind change Substorm relevant current dynamics.

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

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Issi workshop on mercury 26 30 june 2006 bern

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

Substorm, reconnection, magnetotailin Mercury

Rumi Nakamura

Space Research Institute, Austrian Academy of Sciences

Magnetotail response to solar wind change

Substorm relevant current dynamics

Unknowns in Mercury based on Mercury-Earth comparison

Discuss how the planned Mercury mission will enhance our understandings


Mercury magnetosphere

Mercury magnetosphere

Solar wind condition

MercuryEarth

IMF 21-46 6 nT

Strong 16020 nT

Vsw 430430 km/s

Tp13-178104 K

Np73-327/cc

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

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


Mercury night side observation

Mariner 10

Orbit III, 17 min

[12 h]200km x 15,000km

Mercury night-side observation

previous and future planned mission

  • 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)

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

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


Time scales

Z

X

Y

Time scales

Magnetospheric flux transport driven by solar wind.

Mercury Earth

Tail response 1 min 20 min

Substorm/Convection 1-2 min30-60 min

[Siscoe et al., 1975]

  • Substorm/Convection time scale:

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

Nightside/dayside balanced merging is not happening in the Earth


Need for near earth reconnection

[Kaufmann et al., 2004]

-6<y<3 RE

Tail-like field

Dipolar field

NENL DNL

<15RE 20–30 RE 100 RE

N: flux tube contentS: PV5/3

Need for near-Earth reconnection

  • Magnetotail at Earth cannot maintain

  • adiabatic convection: dpVg/dt =0

  • force balance: p=B2/2m0

  • simultaneously (Pressure Crisis)

  • Flux tube volume shrinkstoo steep inward.

  • N (flux tube content) decreases 70%PV5/3 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

How is for Mercury tail ?


Substorm or driven disturbance

Substorm or driven disturbance ?

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

IMF reconstructed

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

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

  • Instead of dipolarization: Configuration changedue to enhanced IMF Bz

  • Instead of injection: particle entry via open field line

Model

Observation

  • 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

BUT


Expected disturbance at mercury tail

Expected 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

  • Geotail data from midtail(period with substorm)

  • MODEL driven response

  • 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)


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


Substorm with psw increase

2min

Substorm with Psw increase

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

Mercury: X= -7 RM  MMO

  • 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

Observation

Model


Substorm imf triggered onset

2min

Substorm (IMF triggered onset)

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

Mercury: X= -5 RM  MMO

  • 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

Observation

Model

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


Substorm spontaneous onset

2min

Substorm (spontaneous onset)

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

Mercury: X= -3.4 RM  MESSENGER ?

  • 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

Observation

Model

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


Dayside nightside reconnection

Dayside, nightside reconnection are unbalanced

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

midtail

substorm

convection

substorm

Dayside/Nightside Reconnection

dF/dt = DFd- DFn

Dayside observation

Day-side reconnection

voltage

Magnetotail observation

Midtail magnetic flux

Observed value

Nigh-side reconnection voltage

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

If convection only

(Nakamura et al.,1999)


Thin current sheet crossing

Thin current sheet crossing ?

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

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

  • Time scale: 40 s

  • DBx: 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

closestapproach

FAC

Observation

Model

dipolarization

  • 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 ?


Current sheet structure

A B C

Current 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

Mariner 10

Cluster obs.

[Runov et al, 2005]

Bx


Heavy ions and thin current shet

Heavy 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]


Strong north south asymmetry

Strong North-south asymmetry

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

Solar wind proton density and field configuration from a hybrid model

IMF [32,10,0] nT

  • 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 ?


Fast flow dipolarization

Fast 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


Field aligned current

Field aligned current

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

Observation

Model

  • Field aligned current flowing toward Mercury(DB=60 nT, Dt = 23s)

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

[Nakamura et al., 1999]


Substorm current wedge

Substorm current wedge ?

Intense field aligned current at Mercury without ionosphere

  • J ~ 50 mA/m

  • j ~ 700 nA/m2 (taking into account the spacecraft motion ~3km/s)

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

Earth-example of plasma sheet expansion associated with

field aligned current and dipoliarzation

plasma sheet expansion

speed ~30km/s(980425 case)

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

  • Taking into account the plasma sheet motion, field aligned current density may be smaller (at least x 10-1?) than 700 nA/m2

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


Summary

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

Summary

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.


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