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

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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  1. 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

  2. Mercury 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 Spatial scale Earth:Mercury 7 : 1 [Siscoe et al. 1975] (Based on Solar wind and dipole moment) ... but not only a mini-Earth magnetosphere ...

  3. 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

  4. Z X Y Time scales Magnetospheric flux transport driven by solar wind. Mercury Earth Tail response 1 min 20 min Substorm/Convection 1-2 min 30-60 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

  5. [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 ?

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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)

  13. 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 ?

  14. 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

  15. 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]

  16. 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 ?

  17. 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

  18. 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]

  19. 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

  20. 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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