Stefano Orsini1, Valeria Mangano1,
Alessandro Mura1, Francois Leblanc2, Anna Milillo1
1IFSI-INAF
Roma, Italy
2CNRS/IPSL Paris, France
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ORSINI et al.
Outline
•It is well known that solar wind particles and photons,
meteoritic impacts and interplanetary magnetic field
have an influence on the morphology, content and time
evolution of the Hermean exosphere.
•By using a wide set of data taken during three years of
Mercury sodium observation from the THEMIS solar
telescope in the Canary Islands, we intend to analyze
the variability of the Hermean exosphere sodium
emission.
•By referring to existing models, recently proposed, we
try to interpret our images respect to the model
expectations
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Exosphere
Upper part of the atmosphere where the column
density is so low (less than 1014 cm-2) that the
collision frequency between particles becomes
negligible. Exobase is the boundary between
collisional and not collisional regime (Earth case about
500 km altitude).
In the case of absence of an atmosphere (Mercury
case), we refer to surface-bounded exospheres. It
is the result of a complex dynamic equilibrium with the
surrounding systems, mainly: surface, magnetosphere
and outer space. In this case the exobase is considered
the surface itself.
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THEMIS
0.90 m Solar Telescope
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THEMIS
0.90 m Solar Telescope
F/16 Ritchey-Chretien telescope in alt-az
mounting
Helium filled telescope tube
MTR mode for multiline spectropolarimetry
Spectral range 400 to 1000 nm at :
R ~ 220,000 Slit: 0.5" & 120 " long
R ~ 400,000 Slit: 0.25" & 70 " long
Four years of campaign (2007-2010)
 96 days of observation
Spectral resolution 0.027 Å to 0.016 Å
Spectral dispersion 10.2 to 6 mÅ
Four individual cameras, 2 for low and 2 for
high resolution scanning of sodium D lines:
D1 Na at 5896 Å
D2 Na at 5889 Å
THEMIS – Observatorio del Teide,
Tenerife
Latitude:
N 28° 18' 12.42"
Longitude: W 16° 30’ 32.04"
Elevation: 2429 m
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N
SSP
SUN
Raw data are scans of a slit (0.5 x 120
low resolution or 0.25 x 70 high
resolution) normal to the solar equator,
in the E-W direction.
The scans result in a datacube λ x X x Y
giving both 2D spatial dimensions and
wavelength. A text header with
additional information about the
exposure is associated to the scan.
Raw data
SEP
E
W
Spatial direction
S
Exospheric sodium D emission
Disk of Mercury (reflected solar continuum)
Solar Fraunhofer D line
Wavelenght
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Calibrated 2D maps
Sodium 2D maps, are the result of the
analysis (code by Francois Leblanc) of
the raw data, that pass through
several steps:
Wavelenght calibration . Cleaning
from cosmic rays and instrumental
anysotropies along the CCD . Sky
substraction . Na D identification and
extraction (pixel by pixel, step by
step).
The resulting 2D maps are spatially
expressed in Rm (both x and y), and
show the sodium emission
superposed to the disk of Mercury.
white cross is the SEP
Sun is at the bottom
dashed zone is the not-illuminated
side of Mercury
kR (kRayleigh) is the unit used to measure
the emission intensity, and is defined as
the isotropic emission of 1010 photons
from a sphere of 1 m2 radius.
dashed horizontal curve is the
subsolar meridian
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Analyzed data samples/1
July 13, 2008
Same, interpolated
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Analyzed data samples/2
April 28, 2008
June 30, 2009
November 7, 2008
April 22, 2009
October 22, 2009
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Sodium variability
Na ground based
observations reveal high to
mid-latitude
enhancements.
Recently, Leblanc et al.
2008 registered intensity
and width variation of the
D1 and D2 spectral-line in
time scale of 1 hour.
K ground based
observations reveal similar
mid-latitude enhancements
(Killen et al., 2007).
(Leblanc et al., 2008)
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Sodium asymmetries
dawn-dusk
and low-high
latitudes
asymmetries
(Schleicher et al., 2004)
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Kameda et al., 2009
The distance from IPD symmetry plane is
correlated with Na density.
Similar tendency in Feb 2009.
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・Short-term spatial variability?
Concentration at high latitudes
and small variability of
average density
still cannot be explained.
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13
(Fm Mura et al., 2006)
North
South
Negative Bx component of
IMF causes reconnection in
the North Emisphere
(Sarantos et al., 2003, Kallio et
al, 2003, Massetti et al., 2005)
1
Reconnection in the North
Emisphere causes higher
S/W proton precipitation
fluxes
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(Fm Mura et al., 2008)
Night
Day
Night
North-South
Asymmetry
Simulated precipitation flux using Montecarlo singleparticle model, 106 test-particles/run
1
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From Mura et al. 2008
Dawn
Sodium Variability
Dusk
SERENA-HEWG meeting, Visby, Sweden,
Aug
2010,
BepiColombo
SWT 24-27,
meeting, Blois
TD
Ions
TD
Rotation
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Sodium distribution
SIMULATION
OBSERVATIONS
Is this complex process able to explain the short term
variability?
Parameter
Data
Model
Density (max)
2500 cm–3
1000 cm–3
Column dens. (max)
71010 cm–2
51010 cm–2
Total amount
41027
51027 (*)
Scale height
200 ... 500 km
~1000 km
parallel doppler width
1.6 km/s
1.4 km/s
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(Mura et al., 2008)
ORSINI et al.
..by applying the Mura et al., 2008 model
Impulsive ion precipitation event
See the next simulation movie
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Data analysis
Selected periods
July 2008, 7 days;
April 2009, 8 days;
June 2009,6 days;
October 2009,9 days.
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Data analysis /2
We rotate the disk of Mercury into a new reference frame,
with SubSolar point in the middle of the image, and we
consider the following parameters:
Mean Intensity = average value (/pixel) of total emission
North = sum of the emission (kR) coming from all the
region upper than 30 degrees latitude
South =sum of the emission (kR) coming from all the
region lower than -30 degrees latitude
Equator = sum of the emission (kR) coming from all the
region between +30 and -30 latitude
High / Low Lat Asymmetry= (North + South)/Equator
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Data analysis /3
Relevant intensity
profile decrease,
two evident peaks
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Data analysis /4
Subsolar point
in the middle of the image
(Density column integral
assumed as invariant)
2008, July 19
13
14
15
16
17
18
Earth view
(sun to the left)
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Data analysis/5
The asymmetry and the Mean intensity
show good correlation
2
3
4 1
Asimmetry profile verus time
7
6 5
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Conclusions
1. We have collected several images of the exospheric
Na intensity at Mercury, by means of ground
observations from the Themis solar telescope.
2. Strong signal variability has been observed
3. By selecting a specific period, when the mean
intensity was strongly decreasing versus time, we
have seen that this profile could be due to PSD
emission, induced by impulsive ion precipitations at
high latitudes, followed by a progressive decrease
and diffusion towards lower latitudes.
4. Further studies for other similar cases will be
performed in the next.
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