MESSENGER First Flyby Magnetospheric Results J. A. Slavin and the MESSENGER Team

1. MESSENGER First Flyby Magnetospheric Results J. A. Slavin and the MESSENGER Team BepiColombo SERENA Team Meeting Santa Fe, New Mexico 11 May 2008

2. Mariner 10 View of Magnetosphere Miniature Terrestrial-type Magnetosphere according to Mariner 10 measurements (mid-1970’s) with some Exospheric Neutrals from ground-based telescopic observations (mid-1980’s)

3. Mariner 10 Energetic Particle Events Christon (1989) Mariner 10 Measurements Strongly Suggest Earth-like Substorms, but with durations of only a few minutes – consistent with the Dungey Cycle DT ~ 1 min for Mercury due to Siscoe et al. (1975)

4. Other Mariner 10 Substorm Phenomena Slavin et al. (1997) Similarity to Terrestrial-type Substorms includes Near-Tail Dipolarization and SCW Field-Aligned Currents

5. MESSENGER First Fly-by Trajectory Low Latitude Near-Tail Encounter

6. First Flyby Overview IMF Bz > 0, so No Substorms, but Planetary Ions Everywhere!

7. First Ion Composition Measurements at Mercury! Na+ Not resolved? Zurbuchen et al. (2008) Mercury’s Surface and Atmosphere provide a Compositionally Rich Source of Photo-Ions throughout the Magnetosphere and the Near-by Solar Wind.

8. Can Mercury’s Magnetosphere Thermalize Heavy Ions? Potter et al. (2002) Boardsen and Slavin (2007) Small Magnetospheric Dimensions may prevent significant Photo-Ion Thermalization – e.g., No ICW’s in Mariner 10 data

9. BS & MP Boundary Models Bow Shock indicates M ~ 3; Magnetopause is Terrestrial in Shape with Factor of ~ 8 Spatial Scaling

10. Large Flux Transfer Event Russell and Elphic (1978) Magnetosheath FTE following Bz < 0; Diameter is ~ 1200 km or about 30% of Rmp as compared with ~ 10% at Earth (supporting predictions by Kuznetsova and Zeleny,1986)

11. Flank Region Vortex Signatures Fujimoto et al. (1998) Spatial scale lengths for individual rotation signatures are ~ 1 RM or ~ 0.7 – 1 Rmp as compared to ~ 2 – 3 Rmp at the Earth.

12. “Double Magnetopause” and ULF Waves Na+ ionized in the flank magnetosheath will have an energy of ~ 6 keV and a depth of penetration into the magnetosphere of ~ 1 gyro-radius or ~ 1000 km – which is comparable to the scale of the “double” magnetopause. ULF waves with frequencies below H+ gyro-frequency are common and appear to map out closed field line region.

13. What is our estimate of the dipole? Anderson et al. (2008)

14. Summary of Initial Results • Has Mercury’s magnetic field changed since Mariner 10? • Less than ~10% moment and 20º in direction. • Smaller than variations between different analysis techniques. • What is our current best estimate for the main field? • Moment: -290 nT-RM3, 5º tilt, 51º lon. • Is there evidence for higher order structure? • Best fit to data requires a g20 term with g20/g10 in the range 0.2 to 0.4. • Driven by equatorial field that is lower than a dipole would give. • Could be due to plasma pressure. • 2nd and 3rd encounters near equator at ~180º opposite longitude: • will help resolve longitude asymmetry; • will determine prevalence of plasma pressures. Anderson et al. (2008)

15. Post - MESSENGER First Flyby View Magnetosphere Immersed in a Comet-Like Planetary Ion Coma.

16. MESSENGER (2011) & BepiColombo (2019) MESSENGER Orbit BepiColombo MPO & MMO Orbits

17. Second Mercury Encounter will be October 6th, 2008!