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5 – 6 May 2008 IMW Meeting PARIS

Observations (new)and

Comparison with one (ours) Exospheric Model

F. LeblancService d'Aéronomie du CNRS/IPSL

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5 – 6 May 2008 IMW Meeting PARIS

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

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5 – 6 May 2008 IMW Meeting PARIS

What is new since Boston’s meeting?Mercury’s sodium exosphere: Statistical Sample

Potter et al. (2006) 6 years of exospheric images

Dawn/Dusk asymmetryDawn brighter than the limb in relation with solar pressureDusk 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!)

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5 – 6 May 2008 IMW Meeting PARIS

Potter et al. (2007) 6 years of integrated exospheric brightness

- Effect of the solar pressureon 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

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5 – 6 May 2008 IMW Meeting PARIS

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

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5 – 6 May 2008 IMW Meeting PARIS

Ionization loss ionization lifetimeNeutral loss Ejection mechanismsReabsorption Ejection mechanismsMeteoroid gardening Supply rateMeteoroid Supply Supply rateSolar wind implantation: negligible for sodium atomsMagnetospheric 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

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5 – 6 May 2008 IMW Meeting PARIS

First example: Infinite reservoir in surface

Thermal desorptionSolar Wind sputtering

PSDMeteoritic vaporization

Magnetospheric sputtering

% of exosphere produced by…

Average D2 emission brightness

(kR)

Peak of emissivityRetrograde Sun motion

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5 – 6 May 2008 IMW Meeting PARIS

Second ExampleRole of Thermal

desorption

Important

Negligible

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5 – 6 May 2008 IMW Meeting PARIS

BestComparison

with observations

(Work inProgress)

Respective role of ejectionMechanisms related to surface

density variation along Mercury’s year (not shown)

% of exosphere produced by…

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5 – 6 May 2008 IMW Meeting PARIS

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 measuredEmission vs different parameters

May be a way to constrain the ejection mechanisms.

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5 – 6 May 2008 IMW Meeting PARIS

Comparison with THEMIS