THE SODIUM EXOSPHERE OF MERCURY: COMPARISON BETWEEN OBSERVATIONS AND MODEL

A.Mura, P. Wurz, H. Lichtenegger, H. Lammer, A. Milillo, S. Orsini,S. Massetti, H. Schleicher, M. Kodachenko

Abstract. In this study we compare the sodium observations made by Schleicher et al. (2004) with the result of a numerical simulation. The observations, made during the transit of Mercury across the solar disk on May 7, 2003, shown a maximum of sodium emission near the polar regions, with north prevalence, and the presence of a dawn-dusk asymmetry. We interpret this distribution as the resulting effect of two combined process: the s/w proton precipitation causing chemical alteration of the surface, freeing the Na atoms from bounds in the crystalline structure on the surface, and the subsequent photon stimulated desorption of the Na particles. The observed and simulated distributions agree very well, indicating that the proposed process is able to explain the observed features.

Observations

2

Mangano et al., 2007Potter et al., 2002

2

More observations…

33

Transit observations (1)

44

From Schleicher, 2004

Transit observations (2)

5

Observed Na column density. Data from Schleicher et al., (2004), obtained during the Mercury transit of May. 7, 2003. Y and Z axis are orientated according to the MSE frame i.e. Z is positive towards north; Y is positive towards dusk.

Max density 2500 cm-3

Max Col. den.

71010 cm-

2

Total amount

31027

Scale height 200500 km

parallel doppler width

1.6 km/s

5

Transit observations (2)

66

Anomaly 150°

Distance 0.45 AU

S/W velocity 700 km/s

IMF -20, 10, -10 nT

Radiation pressure -60 cm/s2

Measured by ACE, during May 7-9, 2003, distance from spacecraft to Earth, components of magnetic field, solar wind proton speed and density

(http://www.srl.caltech.edu/ACE/ASC/level2/index.html).

Influence of IMF

7

Negative Bx component of IMF causes reconnection in the North Emisphere (Sarantos et al., 2003, Kallio et al, 2003, Massetti et al., 2005)

7

Reconnection in the North Emisphere causes higher S/W proton precipitation fluxes

North

South

Simulated H+ flux

8

Day NightNight

North-South Asymmetry

Simulated precipitation flux using Montecarlo single-particle model, 106 test-particles/run

8

Using only Ion sputtering…

99

1st Run: S/W precipitation causes Na ion sputtering

Results: scale height too high, density too low (factor 100), no dawn dusk asymmetry

Y(RM)

col d

ens.

(cm

-2)

Other model

10

Surface element

Na

TD

PSD

S/W

1) Thermal desorption and PSD depleted Na contents

2) S/W causes chemical alteration of the surface,

freeing the Na atoms from bounds in the crystalline structure on the surface

2H + Na2SiO3 → 2Na + SiO2 + H2O

•Production of sodium and water by proton sputtering of sodium- bearing silicates was considered by the following mechanism (Potter, 1995)

Proton precipitation Montecarlo model

Surface evolution model

Sodium exospheric Montecarlo model

Sodium variability

11

Dawn Dusk

Rotation

11

Ions

PS

D

Ions TD

PS

D

Ions TD

Some equations…

1212

• 24 x 48 elements surface grid,

• time step = 10 m

• total simulation time: 2 Mercury years

• S/W flux: from numerical model

• PSD flux:

,,cos,,,, tCAtCNtpsd

•TD flux:

Tk

U

tdB

td

etCNt

,,,,

4/1cos ndn TTTT

21

UE

EUEf

…more equations.

1313

•The surface elements is “moved” and the following equation is solved numerically:

,...tCkdt

tdCN lossspu

loss = k spu

If we assume that particles released by TD always fall back onto the surface, this process does not contribute to the net flux from the surface; however, the Na atoms fall back within an area of radius <300 km. Thus, thermal desorption will also cause a smearing of the places of Na release on the day side. This effect has been simulated using 10000 test-particle for each time-step

and k takes into account the overall process yield and the probability that the proton found a Na atoms in the surface, considering the fraction of Na bearing minerals in the regolith (Wurz et al., 2008).

•For the elements in the dayside:

Time evolution of Na: first hour

1414

TSC = N / loss

Time evolution of Na: a year

1515

H+ and Na fluxes: comparison

1616

Na flux from the surface. The blurring is due to Thermal desorption

H+ flux onto the surface

Finding optimal parameters

1717

• The simulation parameters can be tuned to better match the observed quantities.

• Since the PSD flux is directly proportional to the H+ one, the main parameter is the energy distribution (controlled by the parameter U)

• The energy distribution of the source controls also the scale height and the density of Na

• Here we have chosen to use a value for U in order to match the wavelength dependence of the excess adsorption, which is related to the Doppler velocity shift along the line of sight

Finding optimal parameters

1818

Parallel (x) velocity distribution of the simulated particles (blue line). The simulated data can be fitted by using a gaussian function: exp (-v2/vth

2), with vth = 1.4 km/s. The observed velocity distribution can be reproduced by a gaussian function with with vth = 1.4 km/s (in red).

From Schleicher et al. 2004

Results

2020

Parameter Data SimulationDensity (max) 2500 cm-3 1000 cm-3

Column dens. (max) 71010 cm-2 91010 cm-2

Total amount 31027 91027 (*)

Scale height 200500 km ~1000 kmparallel doppler width 1.6 km/s 1.4 km/s

OBSERVATIONSSIMULATION

2 106 test particle run

Model limits

2121

•A critical hypothesis is to assume that the proton precipitation flux (on the night side) is constant over a very long time-scale (~ weeks): not realistic;

•the time-scale for the equilibrium between H+ and PSD fluxes, on the dayside, is very short (~ one hour);

•the north-south asymmetry is due to H+ precipitation on the dayside, which rapidly results in an enhancement of Na density in the high latitude regions;

•the dawn-dusk asymmetry is caused by the planetary rotation and by the H+ precipitation on the night side; such a precipitation is predicted for most of the IMF conditions (for example, Kallio et al., 2003)

Results (2)

2222

From Kallio and Janhunen, GRL, 2003

From Delcourt et al, AG, 2003

Results (2)

2323

Including a uniform precipitation of 107 cm-2 s-1 on the night side (corresponding to a Na flux of 106 cm-2 s-1 ), we obtain a better fit of the dawn-dusk asymmetry.

OBSERVATIONSSIMULATION

105 test particle run

Conclusions

2424

1)Neither PSD, nor IS alone are able to explain the observed features

2)There is a very good agreement between:

Column densities

Scale heights

Doppler widths

3) Effect of other ion precipitation, and more refilling mechanism are going to be added

4) A paper has be submitted to Icarus; poster to EGU: EGU2008-A-09131