Mercury Disk Observations by Japanese team 1. Observation of Mercury transit on the solar disk on November 9, 2007 [Dawn-Dusk Asymmetry] by Junya Ono and Ichiro Yoshikawa, University of Tokyo 2. Sodium abundance vs. Mercury’s distance from equatorial plane [Micro meteoroid and dust distribution vs. Mercury sodium] by Shingo Kameda, ISAS/JAXA 3. Observation of Mercury disk at the time of Messenger flyby in January 2008 by Masato Kagitani and Shoichi Okano, Tohoku University

Hunten and Sprague, 1997 Schleicher et al., 2004 It is impossible to observe both dawn and dusk sides at a time by ground-based observation. However, based on statistics, sodium density on the dawn side is ～ 3 times higher than that on the dusk side It is thought that sodium atoms are adsorbed in the night side (low temp), while they are released from the dayside. Dawn-Dusk Asymmetry observed at Mercury transit on November 9, 2006 by Junya Ono and Ichiro Yoshikawa (University of Tokyo) Dawn-Dusk Asymmetry was observed at a time of Mercury transit on the solar disk.

Observation was made at Hida Observatory of Kyoto University on November 9, 2006 using a 60-cm vacuum solar telescope and a 10-m spectrograph (R ～ 210,000). Conditions at a time of observation Start time (UT) End time (UT) Mercury diameter Mercury-Sun distance True Anomaly Angle Mercury-Sun velocity Rotational velocity of the Sun Wavelength Doppler shift g-factor 22:06 00: arcsec AU 329° 5.3 → 5.2 km/s 0.9 → 1.4 km/s nm (Na D 1 ) 8.5 → 7.5 pm 0.18 → 0.12

Example of observed Na absorption in a single frame data far from limb (>2.5”) close to limb co-added 6 data at the North polar region (improved S/N)

Column density of Na atoms along a line of sight vs. distance from the limb Na atoms column densities at limb locations [Na atoms/cm 2 ] 6.1 ± 1.1×10 10 Morning Evening North South 4.1 ± 1.8× ± 1.2× ± 1.3×10 10 Morning-Evening asymmetry 1.5 ±0.71

Na temperatures derived from observed line width

Na temperature were also derived from scale height Atmospheric seeing was determined from shadow region (red lines) Morning1.59 arcsec Evening North South 1.76 arcsec 1.73 arcsec 1.72 arcsec Determined seeing Temp. from line widthTemp. from scale heightscale height Morning Evening North South 1750 ±500 K 2350 ±900 K 2700 ±950 K 3200 ±1150 K 152 ±30 km 124 ±40 km 128 km 134 km 1560 ±260 K 1300 ±410 K 1320 K 1380 K

Summary of Mercury transit observation on Nov. 9, Morning–Evening asymmetry was ～ No high densities at polar regions 3. Temperatures derived from line width are different from those derived from scale height. → meaning that the Mercury atmosphere is not in the hydrostatic equilibrium.

・ source process Relation between dust distribution and atmospheric density Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #1 Tilt angle of Mercury’s orbit is 7 degrees. Assuming that dust and micro-meteoroids are concentrated near ecliptic plane, source rate for meteoroid vaporization will be higher (possibly).

Potter et al., 2007 TAA vs Sodium density Radiation pressure (Potter et al., 2007) Radiation pressure is Minimum at the TAA of 0 and 180. Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #2

Sprague et al., 1997 Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #3 TAA vs Sodium density Radiation pressure is Minimum at the TAA of 0 and 180. However, From other results, It is not definite..

Sprague et al., 1997 As a trend, Mercury is away from ecliptic plane  small density Potter et al., 2007 Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #4

Potter et al., 2007 In Northern side, Heliospheric distance is small  large dust density (?) Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #5 As a trend, Mercury is away from ecliptic plane  small density

Problem: 1. Accuracy of absolute value for each observation result 2. The cause of significant increase is still unknown. Potter et al., 2007; Sprague et al., 1997 □ : Observation at Haleakala in 2006 Micro meteoroid and dust distribution vs. Mercury sodium by Shingo Kameda, ISAS/JAXA #6

D=60cm λ/ ∆ λ~59,000 Platescale:0.92 ”/pix Japan Iitate observatory Observation of Mercury disk at the time of Messenger flyby in January 2008 by Masato Kagitani and Shoichi Okano, Tohoku University

Observation D=60cm λ/ ∆ λ~59,000 Platescale:0.92 ”/pix Slit: 2.1”x180” Fig: Slit configuration ・ Long-slit spectroscopy ・ High-dispersion Echelle spectrograph

Observation DateTime (UT) Seeing (FWHM ) QualityDay or Night Jan. 15 8:136.9MidNight Jan. 17 8:095.0MidDay 8:176.5HighDay 8:246.5LowNight 8:306.9LowNight Jan. 19 8:047.0MidDay dateTAAPhase angle Ang.- Diam. Jan ” ” ”

Data Reduction Earth’s sodium emission Mercury sodium tail Mercury continuum Spatial axis Spectral axis Sky background subtraction Sky background

Calibration To calibrate absolute intensity, Hapke’s reflection model was used. Hapke’s reflection model Observed continuum Seeing convolved Hapke’s reflection model NaD2 NaD1 MR/nm (continuum) MR (Sodium emission)

Result DateTime (UT) Seeing (FWH M) QualityDisk NaD2 [MR] Column density x10 10 [cm -2 ] Total sodium atoms within 3R M [x10 28 ] Jan. 15 8:136.9Mid Jan. 17 8:095.0Mid :176.5High :246.5Low :306.9Low Jan. 19 8:047.0Mid