Spatial Distribution of Atom Velocities in the Mercury Sodium Tail - Observation at Haleakala in June 2007 -

H. Fukazawa, M. Kagitani, S. OkanoH. Fukazawa, M. Kagitani, S. Okano Planetary Plasma and Atmospheric Research CenterPlanetary Plasma and Atmospheric Research Center Tohoku UniversityTohoku University

Potter and Killen, 2008

Previous observations of Mercury’s sodium tail

Baumgardner et al. 2008

Kameda et al. 2008

Anti-sunward velocity of sodium atoms in the tail

　 - [Potter et al., 2002] ： ～ 20RM

　 - [Potter et al., 2007] ： ～ 20RM 　　 - [This study] ： ～ 120RM => Sodium atom velocities were derived for the region not studied before.

Potter and Killen, 2008

June 6, 2007

This study

20RM

Observation- Date (UT) ： June 6; 8; 10; 11, 2007 05:00 ～ 06:30LT (evening twilight)

- Site ： Haleakala observatory, Maui island (20.7 ﾟ N 156.3 ﾟW 3040m ）- Telescope ： 40cm Schmidt Cassegrain （ F/10.9 ）- Spectrograph ： Echell High Resolution Spectrograph

- Data ： 1dataset => equator = 1 frame, North-South = 3 frames

- Slit Width ： 50 – 90 μm

- Plate Scale ： 0.85”/pixel, 2.1 pm/pixel

- Spectral Resolution ： λ/ λ⊿ ～ 70,000 @ 589nm

- Exposure time ： 300 – 840 sec

Slit position on June 6.

N

S2.5Rm（ 5,580km）

8Rm（ 19,500km）

17Rm（ 41,900km）

Spectral image with slit aligned anti-sunward direction

Terrestrial Na airglow

Na emission on Mercury

Example of spectral image (June 6, 2007)

Sun

Earth

Mercury θPA

Vt

VLOS

cVLOS

90sin PA

LOSt

VV

Geometry of the Sun, Mercury, and the Earth

Date

(UT)

TAA

(Degrees)

PA

(Degrees)

Heliocentric Distance

(AU)

Radial Velocity,

(km/sec)

Solar Radiation Acceleration,

(cm/sec2)

6/6/07 134.9 114.5 0.433649 7.05 87.5

6/8/07 141.2 119.4 0.441392 6.24 73.1

6/10/07 147.3 124.5 0.448213 5.37 55.8

6/11/07 150.3 127.0 0.451231 4.93 46.6

Planetary configuration on June 6–11, 2007

Derivation of sodium anti-sunward velocity

VLOS: LOS velocityVt : tailward velocityc : light velocity : Doppler shift : D2 line wavelengthθPA : phase angle

Anti-sunward velocities of sodium atoms on June 6, 2007Anti-sunward velocities of sodium atoms on June 8, 2007Anti-sunward velocities of sodium atoms on June 10, 2007Anti-sunward velocities of sodium atoms on June 11, 2007

Non-uniform accelerations, exceeding error bars, are seen in the data.

Sodium brightness in the tail on June 6, 2007Sodium brightness in the tail on June 8, 2007Sodium brightness in the tail on June 10, 2007Sodium brightness in the tail on June 11, 2007

Non-uniformities are also seen in sodium brightness (noticeable on June 8).

Bri

ghtn

ess

[kR

]

1

0.1

10

100

1000

Distance from Mercury center [RM]

0

40 80 120

Comparison between sodium velocity and brightness

■　 At regions where velocity non-uniformity is seen, brightness is also increased.

Velocity and brightness of sodium atoms in the tail on June 8 and 10, 2007.

Velocity of sodium atoms at a distance of 20RM to 50 RM from Mercury center on June 8, 2007.

Time scale of velocity change

～ 20min～ 20min

Conclusions

- Non-uniform acceleration of sodium atoms was identified in the tail. - Time scale between successive non-uniform acceleration is about 20 minutes.

Future Strategy

- Simulation of Mercury magnetosphere

- 3-D simulation of the sodium tail

Combining above two simulations, we may reproduce what is happening on Mercury.

- Another observation

Date

(UT)

TAA

(degrees)

PA

(degrees)

Heliocentric Distance

( AU)

RadialVelocity

(km/sec)

Solar Radiation Acceleration

(cm/sec2)

06/27/08 238.3 118.5 0.415636 -8.65 115

07/03/08 260.9 101.1 0.383247 -10.3 151

07/10/08 292.9 78.4 0.343348 -9.38 180

07/14/08 314.4 63.0 0.324070 -7.30 164

Table.2. Planetary configuration in June-July, 2008

Mercury orbit.

outleg

inleg

Energy needed for escape[Potter et al, 2008]

TAA

Sun

Mercury

apherion periherion

Next Observation - Purpose : to get more positive results on the non-uniform acceleration - Necessity : sufficient S/N and spatial resolution - Time window : end of June – early July this year

Data processing and Error estimation

ガウシアンフィッティングNO

binnig

10000 times loop

wavelength error in fitting

calculate 1

Estimation of sodium velocity error.

Tail Spectroscopic Data

ガウシアンフィッティングGaussian fitting

OK

Smoothed Tail Data

Random readout noise is added

Noise added tail data

Gaussian fitting

Determine center pixel

histogram of center pixel