Observations (new) and Comparison with one (ours) Exospheric Model F. Leblanc

1. Observations (new) and Comparison with one (ours) Exospheric Model F. Leblanc Service d'Aéronomie du CNRS/IPSL

2. What is new since Boston’s meeting? • Mercury’s Sodium Tail • Potter et al. (2008) Observations at different TAA •  Source rate : 1 to 10% of ejected Na from Mercury surface •  Only particles with more than 3 eV when ejected are populating the tail (+radiation pressure = escape energy) • Baumgardner et al. (2008) Observation up to 1400 RM •  Past evolution of Mercury’s sodium ejection rate •  Ionization lifetime • Modeling of formation of Sodium tail should provide: • Peak of neutral sodium loss rate (max of radiation pressure) • Dynamic evolution of the ejection rate • Measurement of ionization lifetime

3. What is new since Boston’s meeting? • Mercury’s sodium exosphere: Statistical Sample • Potter et al. (2006)6 years of exospheric images • Dawn/Dusk asymmetry • Dawn brighter than the limb in relation with solar pressure • Dusk less bright than the limb without relation with solar pressure High latitude peaks • North/South asymmetry: 1/3 of the time with random TAA and solar longitude distributions  % of the time with open magnetosphere (IMF) • Simultaneous peaks in North and South hemispheres: in relation with high solar pressure (but only 7 cases!)

4. Potter et al. (2007)6 years of integrated exospheric brightness • - Effect of the solar pressure • on the measured intensity: • Relation between column density and brightness depends on it • Comparison with Smyth & Marconi (1995) : Energy accommodation coefficient β > 0.5 • Na atoms interact weakly with the surface

5. Comparison with Exospheric Model • An exospheric model should be able to describe: • - Tail formation • - Dawn / Dusk asymmetry • - Coupling with magnetosphere • - Role of solar pressure on exospheric 3D distribution • - Role of solar pressure on measured brightness • - Variation along Mercury’s year of integrated brightness • Leblanc and Johnson (2003) 3D EM, New version: • Ambient / source populations : • The surface population is now described in term of binding energy • Potassium species is described •  Analysis of the dependency of the simulated exosphere with respect to ejection mechanisms (in progress)

6. Ionization loss ionization lifetime Neutral loss Ejection mechanisms Reabsorption Ejection mechanisms Meteoroid gardening Supply rate Meteoroid Supply Supply rate Solar wind implantation: negligible for sodium atoms Magnetospheric recycling: negligible for sodium atoms (Leblanc et al. 2003) • Ejection mechanisms: • Thermal desorption (Vs temperature and binding energy) • Solar wind sputtering (Yield and magnetospheric penetration) • Micro-meteoritic vaporization (Flux and vapor temperature) • Photo-stimulated desorption (Cross section) • Total supply rate : variation with heliocentric distance

7. First example: Infinite reservoir in surface Peak of emissivity Retrograde Sun motion Average D2 emission brightness (kR) % of exosphere produced by… Thermal desorption Solar Wind sputtering PSD Meteoritic vaporization Magnetospheric sputtering

8. Second Example Role of Thermal desorption Important Negligible

9. Best Comparison with observations (Work in Progress) % of exosphere produced by… Respective role of ejection Mechanisms related to surface density variation along Mercury’s year (not shown)

10. CONCLUSIONS • The 6 years data base of Potter et al. is a very rich source of information • The dawn/dusk variation is correctly reproduced • The upleg and downleg of Mercury’s orbit are not symmetric and dependent on ejection mechanisms •  Work in progress to understand the global structure of the measured • Emission vs different parameters •  May be a way to constrain the ejection mechanisms.

11.  Comparison with THEMIS