1 5 – 6 May 2008 IMW MeetingPARIS Observations (new) and Comparison with one (ours) Exospheric Model F. Leblanc Service d'Aéronomie du CNRS/IPSL

2 5 – 6 May 2008 IMW MeetingPARIS 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 R M  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 5 – 6 May 2008 IMW MeetingPARIS 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 5 – 6 May 2008 IMW MeetingPARIS 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 5 – 6 May 2008 IMW MeetingPARIS 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) Comparison with Exospheric Model

6 5 – 6 May 2008 IMW MeetingPARIS 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 5 – 6 May 2008 IMW MeetingPARIS First example: Infinite reservoir in surface Thermal desorption Solar Wind sputtering PSD Meteoritic vaporization Magnetospheric sputtering % of exosphere produced by… Average D2 emission brightness (kR) Peak of emissivity Retrograde Sun motion

8 5 – 6 May 2008 IMW MeetingPARIS Second Example Role of Thermal desorption Important Negligible

9 5 – 6 May 2008 IMW MeetingPARIS Best Comparison with observations (Work in Progress) Respective role of ejection Mechanisms related to surface density variation along Mercury’s year (not shown) % of exosphere produced by…

10 5 – 6 May 2008 IMW MeetingPARIS 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 5 – 6 May 2008 IMW MeetingPARIS  Comparison with THEMIS