Effects of the surface conductivity and IMF strength on dynamics of planetary ions in Mercury s magnetosphere

Transcription

1 1! 5 th SERENA-HEWG workshop (6/16/2014)! Effects of the surface conductivity and IMF strength on dynamics of planetary ions in Mercury s magnetosphere K. Seki 1, M. Yagi 2, Y. Matsumoto 3, N. Terada 4,! D. C. Delcourt 5, F. Leblanc 2, and T. Ogino 1! 1. Solar-Terrestrial Environment Laboratory, Nagoya University,! 2. LPP, CNRS, France, 3. Dept. Physics, Science, Chiba University,! 4. Dept. Geophys., Science, Tohoku University, and! 5. LATMOS/IPSL, CNRS, France!

2 Introduction: SW Comparison at Mercury with at Earth 2 Comparative solar wind conditions at the terrestrial planets R [AU] Mercury np [cm -3 ] B [nt] ( ) V A [km/s] T P [10 3 K] T e [10 3 K] Venus Earth Mars R N B Heliocentric distance in AU Average solar wind proton density in cm-3 Average interplanetary magnete field (I MF) strength in nt Average angle between the IMF and the radial direction in degrees V A Average Altvén speed in krn/s T P Average solar wind proton temperature in 10 3 K T e Average solar wind electron temperature in 10 3 K

3 Introduction: Comparison between Mercury s and Earth s magnetospheres Characteristics of the Mercury s Magnetosphere!! Small distance from the Sun ---> solar wind condition! Small intrinsic magnetic field ---> small magnetosphere! Tenuous atmosphere ---> absence of the ionosphere! ~1/2700 magnetic moments [Anderson+, 2011] ~1/6-1/8 spatial scaling factor (1/20 size) ~1/30-1/50 temporal scale [Siscoe, 1975] ~1/10-1/100 dayside conductivity [Cheng et al., 1987] Earth Mercury 3D MHD simulation Earth Mercury

4 Introduction: Contribution of planetary ions! Important contribution of planetary ions in the magnetosphere! by MESSENGER observations! [Zurbuchen et al., Science, 2011]! Substantial contribution of Na+ pressure compared to H+.!

5 Introduction: Hybrid simulation result! Hybrid simulation results show that distribution of surface conductivity can change the current system in the magnetosphere [Janhunen and Ka lio, Ann. Geophys., 2004]! Change in current system can alter magnetospheric configuration.!

6 Effects of the Surface Conductivity" " [Seki et al., JGR, 2013] 6!

7 Method Used: Systematic trajectory tracing of planetary Na+ ions in MHD fields 7 Trajectory tracing method in MHD fields:!! In order to conserve the magnetic moment by satisfying solenoidal condition of magnetic field (div B=0), cubic spline interpolation [Shimizu and Ugai, 1995] is used for B field.! E field is interpolated so as to satisfy E = B x v locally.! Steady MHD fields after the simulation reached the quasisteady state are used.! Moments of ions in each cubic bin with ΔV=(Δx) 3 = R M. Na + density: n = (Σ i f i N i v i Δt)/ΔV, where f i : ionization rate! Na + energy: ε = (Σ i f i N i v i Δt E i )/(Σ i f i N i v i Δt),!

8 Method Used: Exospheric Na model for input! Exospheric Na model for perihelion [Leblanc and Johnson, 2002] is used.! Photo-stimulated desorption, Micro-meteoric vaporization, and solar-wind sputtering are considered in the Na model.! 8!

9 Effects of Surface Conductivity 9! Dynamics of planetary ions! Analytical model! (rescaled based on Earth)! Precipitation of Na+! In analytical models based on analogy to Earth, some of Na+ return to planet.! (Analytical fields: Superposition of a dipole with a Harris sheet for magnetic field, and Volland formulation for electric field) [Delcourt et al., 2003]! Can these features sustain in the 3-D MHD magnetosphere?!

10 Effects of Surface Conductivity 3 cases of MHD simulation! 10! Vsw=400 km/s, Nsw=35 cm-3 $Analytical model! (rescaled based on Earth)! Subsolar MP at 1.4 RM! $Case 2: MHD_lc, IMF Bz=-5 nt! $Case 1: MHD_lc, IMF Bz=-30 nt! $Case 3: MHD_hc, IMF Bz=-5 nt! 2 types of inner boundary conditions! lc: corresponding to low conductivity! hc: corresponding to high conductivity!

11 Effects of Surface Conductivity 11! Result of systematic trajectory tracing --- Case 1:! MHD_lc, IMF Bz=-30 nt, V sw =400 km/s, N sw =35 1/cc

12 Effects of Surface Conductivity Comparison of Na+ 12! dynamics in empirical model model! $Rescaled analytical model! n!!! $Case 2:MHD_lc, IMF Bz=-5 nt! $Case 1:MHD_lc, IMF Bz=-30 nt! $Case 3:MHD_hc, IMF Bz=-5 nt!

13 Effects of Surface Conductivity 13! Comparison of Na+ precipitations! $Rescaled! analytical! model! $Case 1:! MHD_lc, IMF Bz=-30 nt! $Case 2:! MHD_lc, IMF Bz=-5 nt! $Case 3:! MHD_hc, IMF Bz=-5 nt!

14 Effects of Surface Conductivity 14! [Seki et al., JGR, 2013] Summary and Discussions (1)! Systematic trajectory tracings of Na + ions in the 3 cases of MHD fields (sbz30_lc, sbz5_lc, sbz5_hc) are carried out. Initial conditions of each particle is taken from an exospheric Na model [Leblanc and Johnson, 2002]. Comparison of the results with Na + dynamics in a rescaled analytical model shows:! 1.# The density profile obtained for MHD with the low conductivity (lc) boundary at planetary surface is similar to that obtained with analytical fields.! 2.# Na + precipitation band around 30 latitude in analytical model disappeared in the lc MHD case due to formation of the near- Mercury neutral line (NMNL) in the magnetotail, while the NMNL formation causes high-energy Na+ precipitation into equatorial region.! 3.# The change in the strength of the southward IMF (sbz) changes the location of NMNL and Na+ precipitation pattern. In sbz 5 case, both the equatorial precipitation and Na+ band ~30 Na+ band are formed.!

15 Effects of Surface Conductivity 15! [Seki et al., JGR, 2013] Summary and Discussions (2)! Systematic trajectory tracings of Na + ions in the 3 cases of MHD fields (sbz30_lc, sbz5_lc, sbz5_hc) are carried out. Initial conditions of each particle is taken from an exospheric Na model [Leblanc and Johnson, 2002]. Comparison of the results with Na + dynamics in a rescaled analytical model shows:! 4. In the high conductivity case, magnetospheric convection through polar regions are suppressed and it results in a region of dense Na+ near the planet.! These results suggest that the Na+ precipitation pattern onto the Mercury s surface are highly variable depending on the solar wind conditions.! It is also suggested that we may be able to infer the surface conductivity from the Na+ distribution.!

16 16! Effects of Off-set dipole" (Discussion)

17 Effects of offset dipole Introduction: Mercury s intrinsic magnetic field! Offset dipole by MESSENGER observations! [Anderson et al., Science, 2011]! Best-fit model:! Magnetic moment: 195±10 nt R M 3! Offset to north: 484±11 km! Tilt angle: less than 3!

18 Effects of offset dipole MHD model with an off-set dipole! pressure & magnetic field (meridional plane)! %=35cm -3, V=400km/s, Bz=-10nT, moderate surface conductivity! MP:1.1R M Dipolarization: 3.0R M # Location of the neutral line in the tail does not change significantly.! # Dayside magnetosphere is eroded to ~1.1R M by magnetic reconnection.!

19 Effects of offset dipole MHD model with an off-set dipole! pressure & magnetic field (meridional plane)! %=35cm -3, V=400km/s, Bz=10nT, moderate surface conductivity! P dyn : Low (%=35 cm -3 )! meridional (X-Z)! P dyn : High (%=140 cm -3 )! meridional (X-Z)! MP:1.5R M Tail MP:5.5R M MP:1.2R M Tail MP:7R M # South pole magnetic field is connected to the lobe field line in the northern hemisphere in the distant magnetotail.! # From southern to front magnetosphere is collapsed in high P dyn case, while cusp structure is clear in low P dyn case.!

20 Effects of offset dipole Discussions! Preliminary results of MHD simulations with the off-set dipole intrinsic magnetic field show:! 1.# In the southward IMF case with typical solar wind condition at Mercury, dayside magnetosphere is eroded significantly to ~1.1R M by magnetic reconnection under a moderate surface conductivity condition.! 2.# In southward IMF cases, average location of the neutral line in the magnetotail depends on the surface conductivity, and the tendency is the same as the no off-set dipole cases.! 3.# In northward IMF cases, the south pole magnetic field is connected to the lobe field line in the northern hemisphere in the distant magnetotail.! 4.# In northward IMF case, the location of open-closed field line is sensitive to the solar wind dynamic pressure (P dyn ), and the dayside magnetosphere is collapsed in high P dyn case.! These results suggest that the southward IMF condition is more suitable to assessthe surface conductivity from the Na + distribution.!