1. ISSI Workshop on Mercury, 26–30 June, 2006, Bern Substorm, reconnection, magnetotail in Mercury Rumi Nakamura Space 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

2. 2 Mercury 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 MercuryEarth IMF 21-46 6 nT Strong 16020 nT Vsw 430430 km/s Tp13-17810 4 K Np73-327/cc

3. 3 Mercury 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)  Messenger Polar cap, tail lobe Near-Earth plasma sheet (Solar wind, Magnetosheath)  MPO Polar cap, Inner tail (  12 RE)  MMO Plasma mantle, lobe Midtail plasma sheet (  42 RE) (Magnetosheath) Mariner 10 Orbit III, 17 min [12 h] 200km x 15,000km

4. 4 Time scales Magnetospheric 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) X Y Z Mercury Earth Tail response 1 min 20 min Substorm/Convection 1-2 min30-60 min [Siscoe et al., 1975] Nightside/dayside balanced merging is not happening in the Earth

5. 5 -6<y<3 R E N: flux tube content S: PV 5/3 [Kaufmann et al., 2004] Need for near-Earth reconnection Magnetotail at Earth cannot maintain adiabatic convection: dpV  /dt =0 force balance: p=B 2 /2   simultaneously (Pressure Crisis) Flux tube volume shrinks too steep inward. N (flux tube content) decreases 70% PV  decreases 85% from 30R E to 10R E Near-Earth reconnection (Substorm)  plasma loss through plasmoid  flux tube volume decrease Plasma bubble (Interchange inst.)  Earthward transport of low V low N <15R E 20–30 R E 100 R E Dipolar field Tail-like field NENL DNL How is for Mercury tail ?

6. 6 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 change due to enhanced IMF Bz  Instead of injection: particle entry via open field line Observation Model IMF reconstructed BUT

7. 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 (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)

8. 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. 9 Substorm with Psw increase ObservationModel  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

10. 10 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 ObservationModel Geotail X= -37, Y = 5, Z = -3 RE Mercury: X= -5 RM  MMO 2min

11. 11 Substorm (spontaneous onset) ObservationModel 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 ?

12. 12 Dayside/Nightside Reconnection (Nakamura et al.,1999) dF/dt =  d   n Midtail magnetic flux Day-side reconnection voltage Nigh-side reconnection voltage midtail substorm convection substorm  Midtail flux transport is governed by convection and 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

13. 13 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  D Bx: 80 nT  Spacecraft motion (3.7 km/s) along Z: ~150 km (0.06 R M )  Current sheet center: Z=75 km  Current sheet thickness: D = 150 km ObservationModel DQO (dipole+quadrupole+octupole) + current sheet model dipolarization FAC closest approach

14. 14 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 [Runov et al, 2005] A B C Cluster obs.Mariner 10 Bx

15. 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. 16 Strong 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

17. 17 Fast flow & Dipolarization  Bursty fast flows accompanied by dipolarization  Earthward convection by bursty bulk flows  Fast flow stops near 10 R E by dipolar field (Schödel et al., 2001)  Current diversion through ionosphere associated with dipolarization  Substorm current wedge not the same in Mercury

18. 18 Field aligned current Strong field aligned current observed at dawnside magenetosphere (Slavin et al., 1997)  Field aligned current flowing toward Mercury ( D B=60 nT, D t = 23s)  Reasonable scales expected from Earth substorm Geotail&EquatorS ( D B=30-40 nT, D t = 300-360s) ObservationModel [Nakamura et al., 1999]

19. 19 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 n A/ m 2  Motion of the current sheet/structure are essential to discuss the spatial scale and therefore underlying processes 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)  J ~ 50 mA/m  j ~ 700 n A/ m 2 (taking into account the spacecraft motion ~3km/s)  J ~ 30 mA/m, j ~ 3 nA/m 2 (taking into account the plasma sheet motion)

20. 20 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. Expected useful observations in future mission to enhance our understanding of magnetotail processes