Transcript Document

Dynamical Evolution
of Sodium Anysotropies
in the Exosphere of Mercury
Mangano V.,
Massetti S., Milillo A.,
Mura A., Orsini S., Leblanc F.
INAF-IAPS Roma, Italy, CNRS Paris, France
HEWG-SERENA meeting Key Largo, May 15th 2013
Why Mercury Exosphere?
• Exosphere is almost ‘nothing’…it is so tenuous! (density < 10−14 bar)
• Peculiar because it is directly in contact with the surface
• It is the result of many interactions and equilibrium of sources and sinks
...hence, a very active dynamics !
(Milillo et al, 2005)
Why Mercury Exosphere?
• It is almost ‘nothing’…so tenuous! (density less than 10-14 bar)
• Peculiar because it is directly in contact with the surface
• It is the result of many interactions and equilibrium of sources and sinks
• For this reason it has a very active dynamics
Why Sodium?
• Na is a minor species BUT...
• Thanks to resonant scattering, it is a very good ‘tracer’!
Mercury Variability/1
...both in time and in space !
Na D2 emission in MR (Potter et al, 1999)
Na D2 tail (Potter et al, 2002)
Mercury Variability/2
...on time-scales of hours and days, with peculiar and recurring morphologies.
D2 Na intensity variability in kR
(Leblanc et al., 2009)
D1+D2 tail variability with TAA
in R (Schmidt et al, 2012)
THEMIS 0.90 m Solar Telescope
F/16 Ritchey-Chretien telescope in alt-az mounting
Helium filled telescope tube
N
MTR mode for multiline spectropolarimetry
SUN
Spectral range 400 to 1000 nm at :
R ~ 220,000 Slit: 0.5" & 120 " long
SSP
SEP
R ~ 400,000 Slit: 0.25" & 70 " long
E
(low and high resolution)
W
Spectral resolution 0.027 Å to 0.016 Å
Spectral dispersion 10.2 to 6 mÅ
Two individual cameras: S
D1 Na at 5896 Å
&
D2 Na at 5889 Å
THEMIS – Observatorio del Teide, Tenerife
Lat.: N 28° 18' 12.42" Long.: W 16° 30’ 32.04"
Elevation: 2429 m
Six years of observations (2007-2012): ~ 150 days!
Observations on July 13th, 2008
06:52 UT
08:16 UT
09:33 UT
10:50 UT
13:38 UT
• The two-peaks feature is visible for the whole
day  related to the magnetic cusps
• Southern peak intensity is higher
• The intensity trend is decreasing
16:55 UT
Dependance on seeing
Scan
sequence
Time (UT, LT+2h)
Resolution
Seeing (”)
1
2
3
4
5
6
06:52-08:05
08:16-09:28
09:33-10:45
10:50-12:02
13:38-15:03
16:55-17:38
high
high
high
high
high
low
1.61±0.69
1.33±0.61
1.45±0.60
1.77±0.59
1.70±0.59
1.55±1.15
• Comparison is misleading without ‘averaging’ the different
seeing values to a single one
• Convolution of the observations with a proper gaussian
profile degrades all the images to the worse seeing value
(1.77’’)
Dependance on TAA
Average Intensity
Emission vs Time
(for the whole 7
days) is also
decreasing  this
is because of TAA
dependance
Normalization is
needed
Leblanc et al., 2010
Final images
• All degraded to 1.77’’ seeing value (the one of 4th scan)
• Normalized to the average intensity trend of the period (TAA)
Exospheric Model
(Mura et al., 2009)
• Mid-latitude peaks are the results of a two step process:
1) IS (ion-sputtering)
2) PSD (photon-stimulated desorption) + TD (thermal desorption)
• Simulations agree in magnitude
•
Peaks (as the effect of a single ion precipitation event) decrease,
causing a migration of Na towards the equatorial region
•
Unfortunately the model fails in the decreasing time-scale (~2-5 hours)
Magnetospheric Model
(Massetti et al., 2007)
• Earth-like magnetosphere generated
by an intrinsic magnetic dipole
• MESSENGER found that it is shifted
northward by 0.2 RM
• This may cause broadening of the
southern cusp footprint
• Simulations with IMF=[-10 0 -30] nT
fits remarkably well with the first
observation
• High pressure SW can cause a shift
toward the poles of the cusp
footprints
Analysis/1: emission regions
N
E
S
• Even if error bars are quite big, S region is clearly higher than the rest
• A second ‘event’ seem to occur at the fourth scan
• A decreasing trend with time can also be hypothesised
Analysis/2: peaks evolution
• Going deeper in the analysis 21 thin ‘slices’ along Z axis
• Some interesting features are revealed…
Analysis/2: peaks evolution
• Going deeper in the analysis 21 thin ‘slices’ along Z axis
• Some interesting features are revealed…
Analysis/2: peaks evolution
• Intensity in latitude vs time (time increasing bottom to top)
• Intensity variations in latitude are evidenced  4th scan
Our interpretation
• During a quasi-steady reconnection regime solar wind precipitates
toward the cusp footprints
• Additional pulsed reconnections are superposed causing more
intense localized plasma precipitation
• Hence, the plasma impacting onto the surface would produce the
localized peaks of the Na exosphere
• In addition, observations show the effects of a discrete sequence of
precipitation events due to pulsed magnetic reconnection,
superposed to the precipitation due to quasi-steady reconnection, as
a global modulation of Na release average intensity
Summary
• Daily and ~1-hour time-scale observations with THEMIS solar telescope
allow a detailed monitoring of the highly dynamic exosphere of Mercury
• Cusp related peaks in the Na exosphere are observed on July 13th
2008 in high resolution
• Analysis of peaks intensity variations with time along latitude is
performed
• Comparison with both the Exospheric Model by A. Mura (2009) and the
Magnetospheric Model by S. Massetti (2007) may explain the main
features:
1. peaks broadening & equatorial enhancement
2. cusp footprint
• A reasonable scenario with IMF conditions and interactions with the
Mercury magnetic field is given. Unfortunately local data of IMF on July
2008 are not available to confirm it.
For details:
Mangano V., Massetti S., Milillo A., Mura A., Orsini S., Leblanc F.
Dynamical evolution of sodium anysotropies in the exosphere of
Mercury
Planet. Space Sci. 2013, in press
http://www.sciencedirect.com/science/article/pii/S0032063313000597