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

9 May 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)

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

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

9 May MESSENGER First Fly-by Trajectory Low Latitude Near-Tail Encounter

9 May First Flyby Overview IMF B z > 0, so No Substorms, but Planetary Ions Everywhere!

9 May Not resolved? First Ion Composition Measurements at Mercury! 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. Na +

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

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

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

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

9 May “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.

9 May What is our estimate of the dipole? External fieldMoment (nT-R M 3 ) Pole tilt (North H) Pole lon (North H) RMSCond No 1. TS º19º28 nT1 nT 2. SHA: 1 fit-24712º42º26 nT10 nT 3. SHA: 3 fits-2905º51º14 nT17 nT  13 24%Dir: 5.4º  23 16%Dir: 7.1º Anderson et al. (2008)

9 May 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-R M 3, 5º tilt, 51º lon.  Is there evidence for higher order structure? -Best fit to data requires a g 20 term with g 20 /g 10 in the range 0.2 to Driven by equatorial field that is lower than a dipole would give. -Could be due to plasma pressure.  2 nd and 3 rd encounters near equator at ~180º opposite longitude: -will help resolve longitude asymmetry; -will determine prevalence of plasma pressures. Anderson et al. (2008)

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

9 May MESSENGER (2011) & BepiColombo (2019) MESSENGER OrbitBepiColombo MPO & MMO Orbits

9 May Second Mercury Encounter will be October 6th, 2008!