Spatial Distribution of Atom Velocities in the Mercury Sodium Tail - Observation at Haleakala in June H. Fukazawa, M. Kagitani, S. Okano H. Fukazawa, M. Kagitani, S. Okano Planetary Plasma and Atmospheric Research Center Planetary Plasma and Atmospheric Research Center Tohoku University Tohoku University

Potter and Killen, 2008 Previous observations of Mercury’s sodium tail Baumgardner et al Kameda et al. 2008

Anti-sunward velocity of sodium atoms in the tail - [Potter et al., 2002] ： ～ 20R M - [Potter et al., 2007] ： ～ 20R M - [This study] ： ～ 120R M => Sodium atom velocities were derived for the region not studied before. Potter and Killen, 2008 June 6, 2007 This study 20R M

Observation - Date (UT) ： June 6; 8; 10; 11, :00 ～ 06:30LT (evening twilight) - Site ： Haleakala observatory, Maui island (20.7 ﾟ N 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 ～ 589nm - Exposure time ： 300 – 840 sec Slit position on June 6. N S 2.5Rm （ 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 θ PA VtVt V LO S 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/sec 2) 6/6/ /8/ /10/ /11/ Planetary configuration on June 6–11, 2007 Derivation of sodium anti-sunward velocity V LOS : LOS velocity V t : tailward velocity c : light velocity  : Doppler shift : D 2 line wavelength θ PA : phase angle

Anti-sunward velocities of sodium atoms on June 6, 2007 Anti-sunward velocities of sodium atoms on June 8, 2007 Anti-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). Brightness [kR] Distance from Mercury center [R M ]

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 20R M to 50 R M from Mercury center on June 8, Time scale of velocity change ～ 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/sec 2) 06/27/ /03/ /10/ /14/ 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 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