PCG21-01

Japan Geoscience Union Meeting 2016

Akatsuki returns to Venus*Masato Nakamura1
1.Institute of Space and Astronautical Science of Japan Aerospace Exploration Agency
ISAS successfully launched Akatsuki at 06:58:22JST on May 21st 2010, by H-IIA F17. After a halfyear successful cruise from the earth to Venus, the malfunction happened on the propulsion system
during the Venus orbit insertion (VOI) on December 7th, 2010. The engine shut down at 158 sec
during the VOI, while we planned 12 min operation. The spacecraft did not enter the Venus orbit but
entered an orbit around the Sun with a period of 203 days. The orbital maneuvering engine (OME) was
found to be broken and unusable, but most of the fuel still remained. ISAS’s engineers decide touse the reaction control system (RSC) for orbital maneuver and three minor maneuvers in November
2011 were successfully done so that Akatsuki would meet Venus in 2015.
The Akatsuki spacecraft was rotating about the sun with a period of 199 days and was on the
trajectory to meet Venus on 22nd of November, 2015 after the orbital maneuvers in November 2011.
The date, November 22nd, 2015, was chosen as the shortest encounter timing with consideration of
spacecraft's lifetime. Trajectory analysis done later revealed that the orbit around Venus after
insertion on 22nd of November, 2015 is unstable. We decided to perform another orbital maneuver inJuly 2015 to let the spacecraft to meet Venus on 7th of December, 2015 with this date the orbit
around Venus would be more stable.
On 7th of December, 2015, the spacecraft approached from outside of Venus orbit and captured by
Venus. For the Venus orbit insertion in 2015, termed VOI-R1, four 23 Newton-class thrusters wereused as opposed to 500 Newton-class OME used at the 1st VOI in 2010. VOI-R1 burn (1228 seconds) was
successfully achieved from 23:51:29 on 6th of December through 00:11:57 on 7th of December (UTC,on-board time).
Akatsuki became the first satellite of a planet in Japan. After VOI-R1, the apoapsis altitude is
0.44 million km with the inclination of 3 degrees. The orbital period is 13 days and 14 hours. The
figure shows the VOI-R1 geometry depicted with the Venus center coordinate. For two purposes, todecrease the apoapsis altitude and to avoid long eclipse during the orbiter, we performed a trim
maneuver at the first periapsis. The apoapsis altitude is now 0.36 million km with periapsis
altitude of 1,000 km - 8,000 km (varying) and the period is 10 days and 12 hours.
Akatsuki will send data over two years to us, and it means that our exploration enters the new era
when Japan deliver the continuously changing planet’s data to the whole world.
Keywords: Akatsuki, Venus, Exploration©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

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Japan Geoscience Union Meeting 2016

Initial Results and Radiometric Properties of Ultraviolet Imager on AKATSUKI*Manabu Yamada1, Atsushi Yamazaki2, Takeshi Imamura2, Shigeto Watanabe3
1.Planetary Exploration Research Center, Chiba Institute of Technology, 2.Institute of Space andAstronautical Science / Japan Aerospace Exploration Agency, 3.Hokkaido Information University
Introduction:
The beautiful UV images of the Venusian cloud top were previously performed by several spacecraft
such as Mariner 10 [Bruce et al., 1974], Pioneer Venus [Travis et al., 1979; Rossow et al., 1980],
Galileo [Belton et al., 1991], Venus Express [Markiewicz et al., 2007a, 2007b; Titov et al., 2008].
These previous instruments have taken images at the wavelength around 365-nm, but what material
distribution reflects the contrasting density has been unknown yet. Under wavelength of 320nm, SO2absorption consistent with Pioneer Venus measurements [Pollack et al., 1979], and images in thiswave length can clarify the distribution of SO2.
The ultraviolet imager (UVI) on the AKATUSKI satellite takes ultraviolet images of the solar
radiation scattered at the Venusian cloud top level at the both 283- and 365-nm wavelengths. There
are absorption bands of SO2 and unknown absorber in these wave-length regions. UVI result intomeasurements of the SO2 and the unknown absorber distributions, and the sequential images lead tounderstand the velocity vector of the wind at the cloud top altitude.
First Image of Venus:
UVI has taken the two UV wavelength images of Venus immediately after the operation of the Venus
orbit insertion last year. The first memorial images by the AKATSUKI “satellite” of Venus weretaken at the positions of ~72,000 km far from the Venus center. The solar phase angle at the
sub-observer point was ~45 degrees with the evening terminator. The UVI image at a wavelength of
283 nm presents solar radiation attenuated by SO2 absorption near the cloud top altitudes. This is
the first time to capture the snapshot of Venus with this wavelength. Together with the 365-nm
images, the continuous UVI images will be used to derive horizontal cloud-tracked velocities near
the cloud top altitudes (62—70 km) [e.g., Ogohara et al., 2012; Kouyama et al., 2013].
Although UVI had experienced under interplanetary radiation environment for an unexpected longtime, the performance of UVI is fortunately very high because CCD is strongly shielded. Estimatedradiance from the first image is from 50 to 200 W/m2/sr/μm. It is very reasonable for brightness of
the Venusian cloud top. The image quality is very comfortable suite to study scientific objectives
before launch. We expect the interesting results from the UVI images of Venus.
Keywords: Venus, AKATSUKI, Ultraviolet Imager©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-02 -PCG21-02©2016. Japan Geoscience Union. All Right Reserved.

Japan Geoscience Union Meeting 2016

- PCG21-02 -PCG21-03

Japan Geoscience Union Meeting 2016

INITIAL RESULTS OF THE VENUSIAN CLOUD-TOP TEMPERATUREOBSERVATION BY AKATSUKI/LIR*Makoto Taguchi1, Tetsuya Fukuhara2, Takeshi Imamura3, Masato Nakamura3, Makoto Suzuki3, Takao M.
Sato3, Toru Kouyama4, Naomoto Iwagami5, George HASHIMOTO6, Mitsuteru Sato7, Seiko Takagi8, Munetaka
UENO9
1.Rikkyo University, 2.NICT, 3.ISAS, 4.AIST, 5.University of Tokyo, 6.Okayama University,
7.Hokkaido University, 8.Tokai University, 9.Kobe University
The Longwave Infrared Camera (LIR) onboard Akatsuki took Venus images just after the firstchallenge in Venus orbit insertion (VOI) in December 20101). They showed several interesting
features in the brightness temperature distribution at the cloud top, however, quality and quantityof the data were far insufficient for studies in details. Akatsuki was finally thrown into a Venus
orbit at the second attempt at VOI (VOI-R1) in December 2015 after the unwilling five year cruise
around the Sun. It had been confirmed before VOI-R1 that LIR as well as the other cameras onboard
Akatsuki was still very good in health. Observations of Venus were started immediately after the
VOI-R1 operation. More than 20 Venus images in thermal infrared have been acquired by LIR so far,and observations are continuing to accumulate Venus images day by day.
This presentation will introduce initial results of observation of Venus by LIR, and also show aperspective in future studies in the atmospheric dynamics using brightness temperature and wind
distributions derived from the LIR data.
LIR is a small light-weighted thermal infrared camera using an uncooled micro-bolometer array with
320 x 240 effective pixels as an image sensor, and acquires a snapshot of thermal radiation emitted
from the cloud top of Venus in the wavelength region of 8 to 12 µm2). The FOV of LIR is designed to
fit the full Venus disk to it from the distance of 4.8 RV from the center of Venus. Since Akatsuki
is orbiting in a far elongated elliptical orbit compared to the originally planned orbit, LIR cancapture the full Venus disk in most of an orbiting period. The pixel field-of-view is 0.05°, which
is four times larger than those of UVI, IR1 and IR2.
LIR has an internal image accumulation function to improve noise-equivalent temperature difference(NETD). This function is called as primary accumulation, which is performed during each exposure.
Image data are sent to DE, and up to 32 images can be accumulated. This is called as secondary
accumulation. In the nominal observation sequences both primary and secondary accumulation numbers
are set to be 32 which gives the best NETD according to the pre-launch test results, and an image
acquisition sequence takes about two minutes. An image acquisition sequence without accumulation is
also equipped to take an image with a very short exposure time of 1/30 sec, and used when the
ground speed of spacecraft is large.
In the first orbiting period LIR took 19 images from Dec. 7 to Dec. 11. The shortest time
separation between successive images was two hours. This is chosen so that a wind vector field canbe properly derived by a cloud-tracking method. From Dec. 12 to Jan. 14 observation was suspended
due to important operations on spacecraft which did not allow the observations in parallel.
Observation restarted on Jan. 15.
As a matter of course data amount that can be transferred from spacecraft to ground is limited bybit rate of telecommunication. In the nominal observation plan time interval of image acquisition
by LIR is two hours, and it can be shortened to be one hour in a special observation period.
In the first shot by LIR after VOI-R1 several amazing features which have never been seen before
are identified at a glance. A huge bow-shape high temperature region extending from the northernhigh-latitudes across the equator to the southern high-latitudes exists around the evening©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

terminator. The temperature in the southern polar region seems to be the highest in the snapshot.
Dark filament-like streaks aligned north-south direction are found in the low latitudes. They arealso identified in the UV image. Interpretation of these interesting features will be discussed in
the presentation.
Keywords: cloud morphology©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-03 -PCG21-04

Japan Geoscience Union Meeting 2016

Initial results of IR2 camera on board Akatsuki*Takehiko Satoh1, Makoto Suzuki1, Yasumasa Kasaba3, Munetaka UENO2, George HASHIMOTO4, Takao M. Sato1, Takayuki Enomoto5
1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Kobe
University, 3.Tohoku University, 4.Okayama University, 5.SOKENDAI
Akatsuki, after its successful orbit insertion on 7 Dec 2015, is under the initial check-out phase.
IR2, a 2-um camera on board Akatsuki, is to investigate the Venusian atmospheric dynamics,
distribution of the trace gas, aerosol properties and mechanisms of their production andmaintenance.
The detector of IR2 is a 1024x1024 pixels PtSi (17-um pixel pitch), combined with the f=84.2 mm
lens (F/4, Nikon). The field of view is a square of 12 degrees on a side. To suppress the darkcurrent in PtSi, it is cooled to ~65 K, while the optics including filters are < 190 K to reducethe infrared radiation. Such cooling is achieved by a single-stage Stirling cooler from Sumitomo
Heavy Industries, the company responsible to the entire IR2 system.
IR2 has 4 Venus filters: 1.735, 2.26, and 2.32 um are for the night-side of Venus. These "windows"
are wavelength regions where CO2 absorption is relatively weak and the thermal emissions
originating the lower atmosphere can leak to the space. Therefore, these windows allow imaging oflower clouds as silhouette and enable measurements of atmospheric dynamics in deeper levels. The
2.32-um filter, in CO absorption band, allows to map the distribution of CO in the lower
atmosphere. We de-cloud by differentiating 2.32-um image from nearly-simultaneous 2.26-um image.
The 2.02-um filter for the day-side allows the cloud top altimetry by utilizing the stron CO2
absorption.
In this paper, we present initial results of data analysis acquired during the check-out phase, andwill present the latest data from the orbit.
Keywords: Venus atmosphere, near-infrared, cloud property, CO©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-04 -PCG21-05

Japan Geoscience Union Meeting 2016

Radio occultation observation of Venus atmosphere in Akatsuki mission*Takeshi Imamura1, Hiroki Ando1
1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
The Radio Science experiment (RS) in the Akatsuki mission of JAXA aims to determine the verticalstructure of the Venus atmosphere, thereby complementing the imaging observations by onboard
instruments. The physical quantities to be retrieved are the vertical distributions of theatmospheric temperature, the electron density, the H2SO4 vapor density, and small-scale density
fluctuations. The uniqueness of Akatsuki RS is quasi-simultaneous observations with multi-band
cameras dedicated to meteorological study; the locations probed by RS are observed by the cameras ashort time before or after the occultations. An ultra-stable oscillator (USO) provides a stablereference frequency, which is needed to generate the X-band downlink signal used for RS. The USO
was switched on and tested successfully in February 2016.
Keywords: Venus, Akatsuki, radio occultation©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Cloud top structure of Venus retrieved from Subaru/COMICS mid-infrared spectra*Takao M. Sato1, Takehiko Satoh1, Hideo Sagawa2, Atsushi Yamazaki1, Toru Kouyama3, Takeshi Imamura1
1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Kyoto Sangyo
University, 3.National Institute of Advanced Industrial Science and Technology
Venus is completely shrouded by a curtain of dense clouds (50-70 km) with total optical thicknessof 20-40 at visible wavelengths. The upper sulfuric acid (H2SO4) clouds reflect ~76% of theincident solar radiation back to space [Crisp and Titov, 1997]. Approximately 50% of the solar
energy absorbed by Venus is deposited at altitudes higher than 64 km mainly due to unknown UV
absorber mixed in the upper clouds [Tomasko et al., 1980]. In addition, infrared radiation from thelower atmosphere is absorbed by the clouds. To elucidate the cloud structure which controls thermalbalance of the planet, we analyzed the mid-infrared images obtained by the Cooled Mid-Infrared
Camera and Spectrometer (COMICS) mounted on Subaru Telescope [Sato et al., 2014]. We found several
important findings near the cloud top altitudes (~70 km), such as the possibility that the westward
rotation of the polar features is synchronized between the northern and southern hemispheres, andtemporally variable small-amplitude patterns distributed in the entire disk. In order toinvestigate what atmospheric parameters are responsible for these features seen in the images, wehave also analyzed mid-infrared spectra taken on the same date by the same instrument.
A ground-based spectroscopy of Venus was carried out at the solar phase angle of ~90 deg, with themorning terminator in view, using Subaru/COMICS on October 29, 2007 (UT). The entire N-band (8-13 μm) spectra were obtained with a spectral resolving power of R ~ 250, which is equivalent to that of
the Fourier Spectrometer onboard Venera 15 [Moroz, 1986]. The slit, which was sufficient to capture
the northern and southern limbs of Venus (angular diameter ~25 arcsec), was set to be parallel tothe central meridian of Venus just off the nightside limb and Venus was scanned toward the daysidelimb. The observed thermal radiation in this wavelength range is emitted mainly from altitudes~65-70 km.
From slit-scan images composed of a total of 78 spectra, polar hot spots and cold collars in bothhemispheres are clearly seen and day-night asymmetry is also found, which are consistent with thecharacteristics of snapshots at 8.66 μm and 11.34 μm taken by imaging observations on the same date
[Sato et al., 2014]. The spectra near 9.6 μm are unavailable due to the contamination of O3 in the
Earth’s atmosphere even after the careful data reduction. There are two identifiable CO2 bands(12.1 μm and 12.7 μm). For both bands, the spectral features appear in absorption for theequatorial region and in emission for the southern cold collar. This qualitative characteristic is
consistent with our knowledge obtained from Venera 15 [Moroz, 1986]. Such information as well as
overall spectral shape is useful to retrieve atmospheric parameters, for example, cloud top
temperature, cloud top altitude, and cloud scale height.
To estimate how accurately atmospheric temperature can be retrieved from 8-13 μm spectra, as afirst step, we performed a sensitivity test with VIRA-2 temperatures [Zasova et al., 2006]. The
pseudo observed spectra were calculated from several combinations of VIRA-2 temperatures and a
cloud model [Zasova et al., 2007; Eymet et al., 2009], and an inversion technique [Smith, 1970] wasapplied to these spectra while changing the initial guess of the temperature profile. As a result,
we found that the temperature in altitudes ~65-70 km can be retrieved with the uncertainty of ~2 K.
In this presentation, we will show the mid-infrared spectra of Venus obtained by Subaru/COMICS andprimitive results of atmospheric parameters retrieved from the observed spectra.

©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Keywords: Venus, cloud structure, ground-based infrared spectroscopy, radiative transfer©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Ground-based observations of the formation and periodical rotation of the global scale
UV-feature on Venus cloud top*Masataka Imai1, Yukihiro Takahashi1, Makoto Watanabe1, Toru Kouyama2
1.Department of Cosmosciences, Graduate School of Science, Hokkaido University, 2.Artificial
Intelligence Research Center National Institute of Advanced Industrial Science and Technolog
On Venus, the atmosphere moves rapidly in the westward direction, reaching velocities 60 times therotation velocity of the solid globe. This atmospheric ”super-rotation,” was first detected in the
1960s, however, the mechanism of super-rotation remains mysterious. A planetary-scale bright anddark UV feature, known as the “Y-feature,” rotates around Venus with a period of 4-5 days and has
been long-time interpreted as a planetary wave. When assuming this, its rotation period and spatial
structure might help to understand the propagation of the planetary-scale waves and find out theirrole in the acceleration-deceleration of the zonal wind speed, which is essential for understanding
the super-rotation of the planet. The rotation period of the UV feature varied over the course ofobservation by the Pioneer Venus orbiter (PVO). However, last work issuing this crucial topic wasmade more than 15 years ago, and, since PVO was operated in nearly fixed inertial space, theperiodicity variations on sub-yearly timescales (one Venusian year is ~224 Earth days) were
obscured by the limitation of continuous dayside observations.
We newly conducted ground-based Venus imaging observations at 365 nm, which consists of six
periods covering over half or one month from mid-August 2013 to the end of June 2014 and one
continuous periods from mid-April to end of July 2015. Distributions of the relative brightness
were obtained from the equatorial to mid-latitudinal regions in both hemispheres, and from thecyclical variations of these distributions we deduced the rotation periods of the UV features of
the cloud tops albedo. The relative brightness exhibited periods of 5.2 and 3.5 days above 90% of
significance. The relative intensities of these two significant components also seemed subject totemporal variations.
In 2013 and 2014, although the 3.5-day component persisted throughout the observation periods, its
dominance over the longer period varied in a cyclic fashion. The prevailing period seems to change
from 5.2 to 3.5 days in about nine months, what is clearly not-coincident with the Venusian year
(224 days). The amplitude of relative brightness variation is weak during the transition periods ofdominant-wave changing. It was indicated that the stability of the planetary scale UV-feature wereobserved only in the presence of single longer or shorter periodic waves. In 2015, 3.5-days and5.2-days wave periods could be observed. We success to obtain the change of the first significant
mode from 3.5-day wave to 5.2-day wave continuously. As the former observation results, Venus
experienced the absence of dominant-wave mode during the transition periods, and the time scale ofthe transition is estimated about one month in that period.
Keywords: Venus, Y-feature, Ground-based telescope©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Analysis of fine structures of Venusian clouds using VMC on Venus Express*Yusuke Nara1, Takeshi Imamura2, Shin-ya Murakami2
1.Department of Earth and planetary Science, University of Tokyo, 2.Japan Aerospace Exploration
Agency, Institute of Space and Astronautical Science
At low latitudes of Venus observed by ultraviolet wavelength range, turbulent-like patterns areobserved at the cloud top (Markiewicz et al., 2007). Although it is usually thought that the cloudin the visible wavelength range has little contrasts, structures of clouds are discernible by theobservation of Venus Express (Titov et al., 2007). The structures observed by visible wavelength
range reflect the horizontal distribution of clouds’ thickness because the absorption of sunlightby the “unidentified absorber” which is important in ultraviolet wavelengths is not significant.
Since visible light is strongly related to the energy budget of Venus, to study features of cloudsin visible wavelength range is important.
In this study, we analyzed the structures of Venusian cloud at low latitude using the dataobtained by VMC on Venus Express spacecraft. Using a method of enhancing contrast of the cloudfeatures, which simultaneously eliminates one-dimensional streaky noises fixed to the detector, thecontrasts of the cloud features become clear even in the visible wavelength range. We discuss thespatial structures of the Venusian clouds in the visible wavelength range.
Keywords: Venus, visible image, cloud morphology©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Detailed temperature structure of the Venusian atmosphere revealed by radio holographic
analysis of radio occultation data
mayu miyamoto2, *Takeshi Imamura1
1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Department ofEarth and Planetary Science, The University of Tokyo
Radio occultation (RO) is one of the most important measurement techniques for studying planetaryatmospheres. Radio occultation method relies on the measurement of the frequency shift of thereceived signal caused by the bending of radio waves in the radial gradient of the refractive index
in the atmosphere. The relationship between the impact parameter and the bending angle is retrieved
from the observed frequency shift, and this relationship yields the vertical profiles of therefractive index, from which the temperature profile is obtained. The geometrical optics (GO)method has long been used for the analysis of RO data. However, this method cannot disentangle
multipath rays, which often occur in the temperature inversion region at high latitudes in the
Venusian middle atmosphere. Furthermore, vertical resolution is limited by the size of the Fresnel
zone (∼1 km for a typical spacecraft-tangential point distance of 2 Venus radii and the wavelengthof the radio wave of 3.6 cm).
Radio holographic methods have been proposed for processing of RO signals in multipath regions andobtaining atmospheric profiles with high resolution. One of them is the Full Spectrum Inversion(FSI), which was recently applied to GPS occultation data of the Earth’s atmosphere. By applying
the FSI technique to ESA’s Venus Express RO data, we demonstrated the applicability of thistechnique to Venus’ atmosphere.
The new vertical temperature profiles obtained by FSI technique seem to well repro- duce thetemperature structure in the multipath region and achieve high vertical resolution (∼150 m). Thin,near-neutral layers were found in the high-resolution static stability pro- files in the highlatitude. This feature can be attributed to generation of turbulent layers by breaking gravity
waves. The static stability profile in the middle and high latitude tends to show sharp transition
from neutral stratification below ∼60 km to stable stratification above, suggesting that theconvective layer and the overlying stable layer are adjacent to each other with a very thintransition region. Such a condition is favorable to convective generation of gravity waves.
Keywords: Venus, radio occultation, FSI©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Two dimensional numerical experiment of the Venusian gravity waves by using a cloud
resolving model*Hiroki Ando1, Ko-ichiro SUGIYAMA1, Masatsugu Odaka2, Kensuke Nakajima3, Takeshi Imamura1,
Yoshi-Yuki Hayashi4
1.ISAS/JAXA, 2.Hokkaido University, 3.Kyushu University, 4.Kobe University
Atmospheric gravity waves influences the atmospheric circulation due to the heat and momentum
transports associated with their propagation and breaking. Recently gravity waves are often
observed in the Venusian atmosphere by optical and radio occultation measurements (e.g. Peralta etal. 2008; Ando et al. 2015). Convection in the Venusian cloud layer, which is located at 50 to 70
km altitudes, is thought to be one of the main sources of the gravity waves. We have investigatedthe convective motion and wave generation and propagation due to it by using two dimensional
numerical model (Ando et al. JPGU 2014) and found that the reproduced gravity waves satisfy thedispersion relationship, but wave amplitude and energy depend on the resolution and numericalviscosity set in our model. In this presentation, we change the resolution and numerical viscosityand examine the shape of the energy spectrum obtained in each case to determine the appropriate
resolution and numerical viscosity to reproduce the gravity waves. Furthermore, we show the energy
spectrum distribution and calculate the acceleration rate associated with the momentum convergencein the vertical direction in the case where we use these appropriate values.
We use a cloud resolving numerical model “deepconv” (Sugiyama et al. 2009). The horizontal domain
is 500 km, and the vertical domain corresponds to 35 to 135 km altitudes in the Venus atmosphere.
There are no stress, vertical flow and potential temperature flux at the upper and lower
boundaries. Side boundary is a periodic one. To prevent the wave reflection at the upper and lower
boundaries, we introduce Rayleigh friction within 35 km down from top boundary and 5 km up frombottom boundary. To prevent generation of the mean zonal wind we also add 
the
 artificial
friction to zonal wavenumber 0 component. Newtonian cooling is also introduced in our model on thebasis of Crisp (1989). Initial vertical temperature profile is based on Ikeda et al. (2010), who
derived the temperature distribution under the radiative-convective equilibrium. This profile has aneutral stable layer within the altitude rage of 48-54 km and a stable layer above and below it. We
use net solar heating and infrared radiative cooling profiles based on Ikeda et al. (2010),
assuming that they are horizontally uniform and do not change temporally. We assume that theatmosphere is stationary at the initial stage. To generate convective motion potential temperature
perturbation with the maximum amplitude of 1 K at 50 km altitude, and we perform the numericalcalculation for 15 Earth days. Horizontal resolution is constant of 200 m, and we use threevertical resolution cases (16, 32, 64 m) and four numerical viscosity cases (1x10-4, 3x10-4, 1x10-3,
3x10-3, 1x10-2 m s-2).
As a result, in the case where vertical resolution is less than 32 m and numerical viscosity isless than 3x10-3 m s-2, all the calculated spectral amplitude are almost equal. Within the altitude
region of 66 to 98 km, where gravity waves propagate upward, the spectral amplitude decreases withthe altitude. The spectral slope is about -2 within the horizontal wavenumber range of 10-4 < k <10-3 (1/m) and -3 of 10-3 < k (1/m). In particular, the spectral slope in the former case isconsistent with that empirically suggested on the basis of the observations of gravity waves in the
Earth’s atmosphere. Furthermore, the acceleration rate in the horizontal direction associated withthe wave dissipation increases with altitude, and it is ~1 m s-1 day-1 at 90 km altitude.

©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Keywords: Venus, Atmospheric gravity waves, Numerical calculation©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Study of the Venusian cloud formation and distribution in low- and mid-latitudes using a
GCM: Effects of atmospheric chemistry and circulation
Kazunari Ito1, *Takeshi Kuroda1, Yasumasa Kasaba1, Naoki Terada1, Kohei Ikeda2, Masaaki Takahashi3
1.Department of Geophysics, Tohoku University, 2.National Institute for Environmental Studies,
3.Atmosphere and Ocean Research Institute, The University of Tokyo
Sulfuric acid clouds cover the Venusian atmosphere in 48-70 km, and are important in the Venusianclimate through their radiative processes. The clouds are the main objective of the JAXA Akatsuki
orbiter for the investigations of atmospheric dynamics, so the modeling study of the formation andadvection of them using a Venusian General Circulation Model (VGCM) is of significance also for thehelp of the mission. We have implemented the sulfuric acid cloud formations and related chemical
processes into a VGCM, and investigated their distribution and the formation systems.
We used the VGCM modified from the CCSR/NIES/FRCGC AGCM [Ikeda, 2011], with the horizontal
resolution of T21 (longitude and latitude grids of about 5.6 degrees) and 52 levels in verticalwith the sigma (equivalent to the pressure) coordinate (the top altitude of ~95 km). For the cloud
condensation and evaporation processes, the effects of supersaturation are not considered and theradius of cloud aerosol is artificially distributed into 4 modes by ratios based on the verticalprofiles shown by Haus and Arnold [2010]. Note that our model at the moment does not include thegrowth or reduce of the particle size, and only traces the advection of produced clouds. Also notethat the cloud distributions should modulate the thermal distributions through radiative effects,
but the current model still assumes constant heat input profile (as well as the former code by
Ikeda [2011]).
Our model includes the chemical processes related to the production and loss of H2SO4 vapor (SO3, SO
2

and H2O) in order to reproduce the realistic maintenance processes of cloud distributions. In themodel with those chemical processes (hereafter WC model), the simulated latitudinal distributionsof the optical thickness of clouds agree with the observational results by the Visible and InfraRed
Thermal Imaging Spectrometer (VIRTIS) onboard Venus Express (VEX), and also the simulated verticalprofiles of H2SO4 vapor agree with the Magellan radio occultation data, in low- and mid-latitudes(0-70o N). On the other hand, in the model without the chemical processes (hereafter NC model), thecloud opacity distributions become less than half of the observed ones, and also the abundance of H
2SO4vapor around 48 km becomes less than half in comparison with the WC model. Clouds are formed

above ~50 km in both models, but the abundances in the WC model are larger than those in the NC
model. The difference of the WC model is caused by the production of H2SO4 vapor due to thechemical processes and the condensation of it into the clouds in the upper cloud region (60-80 km).
We conclude that our VGCM reproduces the observed features with a good reliability in low- andmid-latitudes with chemical processes.
With this VGCM, we investigated the maintenance and circulation processes of the sulfuric acidclouds and vapor in low- and mid-latitudes. Our model indicated that, in the upper cloud region,
the clouds are produced centered at ~65 km altitude, and flow upward and poleward by the meridional
circulation and vertical diffusion. Meanwhile, in the lower cloud region (50-60 km), H2SO4 vapor
transported by the upward advection and vertical diffusion condenses into the cloud in theequatorial region at the altitude of 50-54 km, and the formed clouds are transported poleward along
the meridional circulation. These cycles are consistent with those simulated in a 2-D
latitude-altitude cloud formation model by Imamura and Hashimoto [1998] with a given meridional
circulation, and we first reproduced the cycle in a VGCM. Using this new VGCM, we showed that thediurnal tide mainly contributed to drive the meridional circulation through the wave-zonal mean

©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

flow interaction in low- and mid- latitudes.
Keywords: Venus, Sulfuric acid clouds, Atmospheric chemistry, General circulation model, Akatsuki©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Small-scale disturbances reproduced by AFES for Venus(Atmospheric general circulation model For the Earth Simulator)*Norihiko Sugimoto1, Masahiro Takagi2, Hiroki Ando3, Hiroki Kashimura4, Takeshi Imamura3, Yoshihisa
Matsuda5, Wataru Ohfuchi4, Takeshi Enomoto6, Yoshiyuki O. Takahashi7, Yoshi-Yuki Hayashi7
1.Department of Physics, Research and Education Center for Natural Sciences, Keio University,
2.Kyoto Sangyo University, 3.JAXA/ISAS, 4.JAMSTEC, 5.Tokyo Gakugei University, 6.Disaster
Prevention Research Institute, Kyoto University, 7.Graduate School of Science, Kobe University
An atmospheric general circulation model (AGCM) for Venus on the basis of AFES (AGCM For the Earth
Simulator) have been developed to perform a very high-resolution simulation (e.g., Sugimoto et al.,2014a). The highest resolution is T319L120, namely, there are 960 times 480 horizontal grids (gridintervals are about 40 km) with 120 vertical layers (layer intervals are about 1 km). In the model,
the atmosphere is dry and forced by the solar heating with the diurnal and semi-diurnal change. The
infrared radiative process is simplified by the Newtonian cooling. Then the temperature is relaxed
to a prescribed horizontally uniform temperature distribution which has a virtual static stabilityof Venus with almost neutral layers. We set a fast zonal wind in a solid-body rotation as theinitial state.
Starting from this idealized superrotation, the model atmosphere reaches a quasi-equilibrium state
within 1 Earth year. This state is stably maintained for more than 10 Earth years. The zonal-mean
zonal flow with weak midlatitude jets has almost constant velocity of 120 m/s in latitudes between
45°S and 45°N at the cloud top levels, which agrees very well with observations. We haveinvestigated small-scale disturbances reproduced in the model. In the cloud layer, baroclinic waves
develop continuously at midlatitudes and generate Rossby-type waves at the cloud top (Sugimoto etal., 2014b). At the polar region, warm polar vortex zonally surrounded by a cold latitude band(cold collar) appears successfully (Ando et al., 2016). As for horizontal kinetic energy spectra,
divergent component is broadly (k > 10) larger than rotational component compared with that on
Earth (Kashimura et al., in preparation). In the presentation, the relation between small-scale
gravity waves and large-scale thermal tides will be also shown.
Keywords: Venus, GCM, Waves©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-12 -PCG21-12©2016. Japan Geoscience Union. All Right Reserved.

Japan Geoscience Union Meeting 2016

- PCG21-12 -PCG21-13

Japan Geoscience Union Meeting 2016

Status of Millimeter-wave Band Ground-based 10m-SPART Telescope for Monitoring
Observations of the Middle Atmospheres of Terrestrial Planets in the Solar System.
*Hiroyuki Maezawa1
1.Department of Physical Science Osaka Prefecture University
To understand the influence of activities of central stars on the atmospheres of terrestrial
planets in the solar system and of exoplanets, we are performing monitoring observations of
millimeter-waveband spectral lines of carbon monoxide,12CO( J = 1-0, J=2-1) and

13

CO( J = 2-1) of

the Martian and Venusian atmosphere with a 10-m telescope, Solar Planetary Atmosphere Research
Telescope (SPART). Heterodyne spectroscopy with high frequency resolution is a powerful tool to
trace the weak and narrow spectral lines of minor constituents in the middle atmosphere of planets.
The results obtained with SPART suggest that the disk-averaged mixing ratio of carbon monoxide
derived at an altitude of 80 km in Venus was about 60 ppmv during 2012 and 2015. This observing
period corresponds to the solar cycle 24 maximum phase. The mixing ratios of CO seem to be lower
than those observed during the solar cycle 22 maximum phase. These may suggest that the abundance
variations of CO are related to solar activities. For understanding of the CO production induced bysolar energetic particles (SEP) events, we performed a numerical model on the basis of theBethe–Bloch formula under conditions of typically great solar-proton events with incident-protonenergies of less than 1 GeV. The obtained results suggest that the ionization rate reaches its
maximum at an altitude of 80–90 km in the Venusian atmosphere. However it was found that the
production rate of CO is smaller than that induced by incident ultraviolet radiation at thealtitude. Although currently the operation environment of the SPART telescope is renewed for
opening to the general public at Nobeyama Radio Observatory, we will restart the monitoring
observations soon after the construction within this year in anticipation of integrated researcheswith the AKATSUKI.
In this conference, we will present the current status of the SPART project briefly.
Keywords: Venusian atmosphere, Radio Telescope, Solar Activity©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-13 -PCG21-14

Japan Geoscience Union Meeting 2016

Radiative Transfer Simulations for 1.9 THz Band Remote Sensing Observations of the MartianAtmosphere with SMILES-2*Yuji Nishida1, Kosuke Saito1, Kazunori Morimae1, Daiki Haraguchi1, Hiroyuki Maezawa1, Hideo Sagawa2, Makoto Suzuki3,4, Masato Shiotani5
1.Osaka Prefecture University, 2.Kyoto Sangyo University, 3.JAXA, 4.ISAS, 5.Kyoto University
For better understanding of global warming and space weather predictions, recovery of Ozone layer,
physics and dynamics such as planetary and gravity waves, and chemical reaction networks of the
Earth’s middle atmosphere, the Submillimeter Wave Limb Emission Sounder 2 (SMILES-2) mission iscurrently being studied. The target species of the SMILES-2 are H2O, N2O, NO2, NO, CO, H2CO, OH andO-atom, and so on, which are also key species to study the CO2 stability problem on the terrestrial
planets of the solar system and the formations/sources and destruction processes of CH4 on Mars.
The SMILES-2 will employ 4 K-cooled superconductor/insulator/superconductor mixer detectors andsuperconducting hot-electron bolometer mixer (HEBM) detectors for the frequency bands below 600 GHz
and above 1.8 THz, respectively.
Currently we are developing a superconducting NbTiN HEBM detector integrated with a waveguide slot
and a horn antenna for 1.8-2.0 THz band remote sensing observations. We also carried out thefeasibility studies of the spectral lines of HOx, CO, NOx, and their isotopes in the Martian
atmosphere observed with SMILES-2 equipped with the HEBM by performing the radiative transfer
simulations. In this conference, we will present the results of these simulations.
Keywords: SMILES-2, THz, Remote Sensing, Mars

©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-14 -PCG21-15

Japan Geoscience Union Meeting 2016

Effects of ion-ion collisions on vertical CO2+ profiles in Martian ionosphere undermagnetic field penetration: Multi-fluid MHD*Kyohei Koyama1, Kanako Seki2, Naoki Terada3, Kaori Terada3
1.Graduate School of Science, Nagoya University, 2.Graduate School of Science, University of Tokyo, 3.Graduate School of Science, Tohoku University
The escape of the planetary atmosphere is an important phenomenon related to evolution of theatmosphere, and numerical simulations are an effective method to understand the global atmosphericescape processes. The escape of CO2+ from Mars observed by Mars Express is presumed that the origin
of CO2+ escape flux is result of the ionospheric outflow. In this process, the escape of massive
amounts of CO2+ requires relatively high density of CO2+ at high altitude ionosphere. Ionosphericmodel developed by Fox and Hac [2010] presumed chemical reaction and velocity difference for each
ion fluxes in ionosphere to reproduce the density disturbance of CO2+ in high altitude ionosphere.
This result suggests velocity difference is important to reproduce the high CO2+ density in highaltitude ionosphere. Multi-fluid MHD, it is the model allows ion fluxes to take individual
velocities, has developed Najib et al,. [JGR, 2011], but the model has not include the effects ofthe collisions between ions. In our previous research, we developed Multi-fluid MHD and reproduced
Martian ionosphere. To investigate effects of collisions, we conducted 5 cases of the simulation.
Case1: Multi-fluid MHD includes electron-neutral and ion-ion collisions, Case2: Multi-fluid MHD
without electron-neutral collision, Case3: Multi-fluid MHD without ion-ion collisions. Case4:
Multi-fluid MHD without electron-neutral and ion-ion collisions. Case5: Multi-fluid MHD. From thisprevious study, we obtained the effects of velocity differences between ion fluxes and collision
for CO2+ vertical density distribution. Our recent study is the effect of solar wind magnetic field
on Martian ionosphere using under developing Multi-fluid model. Mars has no intrinsic magnetic
field. So that, solar wind magnetic field penetrates into Martian ionosphere when the solar windmagnetic field is in active state. When solar wind magnetic field penetrates, Martian ionosphere iscontracted, and ion-fluids are accelerated by magnetic field. Our Multi-fluid MHD code can describe
individual velocity of ion fluxes, ion-ion collisions, electron-neutral collision, and effects ofmagnetic field for ion fluxes with different speeds. In this presentation, we report the dependenceof ionospheric condition on magnetic field strength and ion velocity in the upper boundary.
Keywords: Mars, Ionosphere, Multi-Fluid MHD©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-15 -PCG21-16

Japan Geoscience Union Meeting 2016

An MHD simulation study of the Kelvin-Helmholtz instability at the Martian ionopause witha day-to-night density gradient*Sae Aizawa1, Naoki Terada1, Yasumasa Kasaba1, Manabu Yagi1, Yosuke Matsumoto2
1.Graduate School of Science, Tohoku University, 2.Graduate School of Science, Chiba University
The effect of a day-to-night density gradient on the evolution of the Kelvin-Helmholtz instability(KHI) at the Martian ionopause is investigated using 2-D extended-local MHD simulations. The KHI is
expected to play a major role in transporting mass, momentum and energy across the ionopause
between the sheath flow and the Martian ionospheric plasmas, and is thus regarded as one of thecandidate processes that have removed a huge amount of ions from Mars through its long history.
Recent local MHD simulation studies have pointed out that a density gradient in the verticaldirection significantly reduces its linear growth rate and its maximum growing wavelength mode
becomes longer. A Longer wavelength mode makes KH vortices more inhomogeneous during the nonlinear
phase [Amerstorfer et al., 2010]. However, the actual ionopause has a density gradient not only invertical but also in the horizontal (day-to-night) direction. In order to investigate the effects
of a day-to-night density gradient, we have developed two extended-local MHD models by
incorporating two elements of a global model, i.e., an aperiodic boundary condition and theday-to-night density gradient, into a local model.
Comparing the results of the aperiodic case (extended-local model without a day-to-night density
gradient) with those of the periodic case (local model), we find two notable differences in theevolution of the KHI. Firstly, while the evolution of the main vortices group is mostly the same inboth cases, that of the leading vortex is quite different[YK1] .[AS2] The main vortices group rises
and its intrusion into two regions is symmetrical with the ionopause but the leading vortex does
not enter the sheath region in the linear growth phase. On the other hand, the leading vortex seems
to be squeezed by the sheath region while the main vortices group does not show such a squeezing
like structure. Secondly, the ionospheric plasma in the aperiodic case is excavated about 1.5 times
deeper. We find that these two differences are caused by the asymmetry in the structure of bothsides of each vortex. When there is another vortex with a larger amplitude just downstream of avortex, this structure behaves like a wall, the sheath flow will be stagnated by this wall-likestructure. This stagnated sheath flow induces an enhanced vortex return flow, resulting in a deeper
excavation of the ionospheric plasma. Previously, it has been thought that the mixing area will
spread widely over time. The Deeper excavation of the leading vortex enhances mixing of ions. In
addition, we also find the elongated filament structure is caused by the asymmetry in the structureof both sides of the vortex. A wall-like structure downstream side which mentioned above and an
insufficient vortex motion on the leading (upstream) side of a KH vortex leads to vortex return
flow and an imbalance between the pressure gradient force and the centrifugal force associated withthe vortex motion. The vortex cannot keep its structure and will be an elongated filament. These
asymmetries in the vortex structure are responsible for making two differences between theaperiodic and the periodic cases.
We also add the day-to-night density gradient to the aperiodic case by reference to MEX observation
results [Duru et al., 2008]. We find that the KHI is quickly excited in the downstream (lowdensity) region. It has been thought that the KH wave propagates from upstream to downstream, i.e.,
one-way propagation. This excitation in the downstream indicates that the perturbations associatedwith the KH wave propagate not only to the downstream but also toward the opposite direction, withhighly elongated filamentary structures in downstream.
In those simulations, we evaluate the effect of the day-to-night density gradient on the loss rate

©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-16 -PCG21-16

Japan Geoscience Union Meeting 2016of the ionospheric ions. We find that the day-to-night density gradient reduces the ions loss
efficiency with 30-40%.
Keywords: Mars, Kelvin-Helmholtz instability, MHD simulation©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-16 -PCG21-17

Japan Geoscience Union Meeting 2016

Comparison of Martian Magnetic Pileup Boundary with Ion Composition Boundary Observed by
MAVEN*Kazunari Matsunaga1,2, Kanako Seki3, David A. Brain4,5, Takuya Hara6,7, Kei Masunaga3, James P.
McFadden6,7, Jasper S. Halekas8, David L. Mitchell6,7, Christian Mazelle9,10, Jack E. P. Connerney11,
Bruce M. Jakosky4,5
1.Graduate School of Science, Nagoya University, 2.Institute for Space-Earth Environmental Research(ISEE), 3.Graduate School of Science, University of Tokyo, 4.Laboratory for Atmospheric and Space
Physics (LASP), 5.University of Colorado at Boulder, 6.Space Sciences Laboratory (SSL),
7.University of California, Berkeley, 8.University of Iowa, 9.IRAP CNRS, 10.University Paul
Sabatier, Toulouse, 11.NASA Goddard Space Flight Center
The Martian upper atmosphere directly interacts with the solar wind, since Mars does not possess
the intrinsic global magnetic field. This interaction forms a transition region between the shocked
solar wind (magnetosheath) and the ionosphere, in which characteristic boundary structures areembedded. In this transition region, the neutral atmospheric heavy atoms can be ionized andinvolved into the solar wind flow. This is called the mass loading process [e.g., Dubinin andLundin, 1995]. The loaded heavy ions form a dense layer which called “ion composition boundary”(ICB). ICB separates the solar wind protons dominant region from the planetary heavy ions dominant
one [e.g., Erkaev et al., 2007]. Since the interplanetary magnetic field (IMF) frozen-in the solar
wind plasma, IMF also drape around the transition region. Due to the draping IMF piles up in thefront of the Martian upper atmosphere, the magnetic pileup boundary (MPB) is formed [e.g., Luhmann
et al., 2004].
Previous studies have shown existence of the magnetic pileup region or the induced magnetosphere in
the transition region. Mars Global Surveyor (MGS) observed MPB, a boundary between themagnetosheath and the Martian magnetic pileup region by its magnetometer and electron reflectometer[e.g., Vignes et al., 2000, Trotignon et al., 2006]. ICB was also observed by the ion mass analyzer
of Phobos 2 and Mars Express (MEX) [e.g., Breus et al., 1991, Dubinin et al., 2006]. Due to thelack of continuous simultaneous observations of the magnetic field and ion composition, however,relations between MPB and ICB are far from understood. In this study, we investigate relative
locations and characteristics of MPB and ICB, and their dependence on solar wind parameters,
utilizing a full package of plasma instruments onboard Mars Atmosphere and Volatile EvolutioN(MAVEN).
We conducted a statistical analysis of the ion, electron, and magnetic field data obtained by MAVEN
from November 2014 to March 2015 in order to investigate relations between MPB and ICB. We
identified MPB from the electron and magnetic field data by inspection based on criteria ofTrotignon et al. [2006]. We calculated the density ratio between the planetary heavy ions and thesolar wind protons to investigate the ion composition around MPB. Results show that there is anorth-south asymmetry in locations of MPB and ICB. Observations also indicate that the relative
location of MPB and ICB has deference between dayside and nightside. Moreover, the southern crustal
magnetic fields seem to play a role of the north-south asymmetry in locations of MPB and ICB.
However, dependences of MPB and ICB on the solar wind dynamic pressure, density, and velocity are
not clear. The solar wind induced magnetic field direction also has no clear effects on ICB and MPB
locations.
Keywords: Mars, Induced magnetosphere, Magnetic Pileup Boundary, Ion Composition Boundary,Unmagnetized planet, MAVEN©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-17 -PCG21-17©2016. Japan Geoscience Union. All Right Reserved.

Japan Geoscience Union Meeting 2016

- PCG21-17 -PCG21-18

Japan Geoscience Union Meeting 2016

O+ ion beams reflected below the Martian bow shock*Kei Masunaga1, Kanako Seki1, David Brain2, Xiaohua Fang2, Yaxue Dong2, Bruce Jakosky2, James
McFadden3, Jasper Halekas4, Jack Connerney5
1.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo,
Japan, 2.Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, USA, 3.Space
Science Laboratory, University of California Berkeley, USA, 4.Department of Physics and Astronomy,
University of Iowa, USA, 5.NASA Goddard Space Flight Center, USA
We investigate a generation mechanism of O+ ion beams observed above the Martian bow shock byanalyzing ion velocity distribution functions (VDFs) measured by the Superthermal and Thermal Ion
Composition (STATIC) instrument on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft.
In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (~10 keV or higher).
Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in themagnetosheath: a partial ring distribution or a hook-like distribution. The partial ring
distribution corresponds to pickup ions with a finite initial velocity (i.e. not newborn pickupions). Thus the partial ring distribution is most likely to be produced by the reflection of the
precipitating O+ ions below the bow shock. After being injected into the magnetosheath from thesolar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field inthis region, and consequently, a part of precipitating O+ ions are reflected back to the solar
wind, generating O+ beams in the solar wind. The hook-like distribution contains two ion
populations: a mixture of local O+ pickup ions and O+ pickup ions precipitating from the solar windright above the bow shock, and O+ pickup ions precipitating from the upstream solar wind and beingreflected below the bow shock. The latter population also generates the O+ ion beams in the solar
wind. The reflected O+ beams are reaccelerated by the convection electric field in the solar windand may escape Mars.
Keywords: Mars, ion reflection, MAVEN©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-18 -PCG21-19

Japan Geoscience Union Meeting 2016

Global structure of Mercury's magnetosphere : Dependence on solar wind parameters*Manabu Yagi1, Kanako Seki2, Yosuke Matsumoto3, Dominique Delcourt4, Francois Leblanc4
1.Graduate School of Science, Tohoku University, 2.Graduate School of Science, Tokyo University,
3.Graduate School of Science, Chiba University, 4.CNRS
Based on observations by MESSENGER, Mercury's magnetosphere is thought to be a miniature of the
Earth's magnetosphere. These two magnetospheres have several characteristics in common, however,some critical differences are also evident. First, there is no atmospheric layer, but only tenuous
exosphere. Second, the kinetic effects of heavy ions might not be negligible because Mercury
magnetosphere is relatively small compared to the large Larmor radii. Recent observation by
MESSENGER also found that the center of dipole is shifted to northward about 485km from the centerof Mercury. Trajectory tracings is one of the dominant methods to estimate the kinetic effect ofheavy ions which originate the exosphere, though the results of the simulation are quite sensitive
to the electric and magnetic field. Therefore, it is important to provide a realistic field modelin the trajectory tracings. In order to construct a large scale structure, we developed a MHD
simulation code, and adopted to the global simulation of Mercury magnetosphere. In this study,
first we performed two cases of simulation, low and high solar wind density cases(35cm^{-3},70cm^{-3}, and 140cm^{-3}) with velocity for 400km/s and northward IMF condition. When solar winddensity is low, magnetopause is formed at 1.4R_{M}, and the global structure has weak north-south
asymmetry in the MHD simulation. One of the important characteristics is open field line from southpole even in the northward IMF condition without Bx and By components. When solar wind dynamic
pressure is high, Mercury's magnetosphere is compressed to the scale of Mercury itself. In thiscase, planetary surface disturbs the magnetospheric convection, and the north-south symmetry as
well as similarity to Earth's magnetosphere are strongly violated. Trajectory trancings in the MHD
fields show that there are enough space for energetic (~ few keV) sodium ions which are the main
component of 'sodium ring' at the vicinity of the planet to go through the dayside magnetosphere in
the low density case. In the high density case, dayside is too compressed and there are no spacefor sodium ions to go through. As a result, 'sodium ring' became not isotropic ring but formed onlyat nightside. In the next step, we performed higher dynamic pressure of the solar wind condition,
it is, density for 140cm^{-3} and velocity for 800km/s. This parameter is rarely occurred except
for the extreme case such as CME events. The result of MHD simulation shows that most of magnetic
filed lines are opened, and continuous tail reconnection occurred by extremely high dynamic
pressure. These structure and phenomenon partly correspond to that of magnetosphere with southward
IMF, while magnetospheric convections are largely different because no magnetic reconnection occurs
at the dayside magnetosphere. Another characteristics is secondary compression region in themagnetosheath at flank side of the planet. First compression is occurred by planetary surface at
the front side and formed what we call bow shock. Second compression is caused by magnetopause at
the flank side which lies at the direction of sheath flow. In the presentation, we will also report
the ongoing simulation result of trajectory tracings in this extreme case.
Keywords: Mercury's magnetosphere, MHD©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-19 -PCG21-20

Japan Geoscience Union Meeting 2016

Ionic composition in the Io plasma torus measured using Hisaki/EXCEED and gourd-based
telescope*Masato Kagitani1
1.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University
Volcanic gases (mainly composed of SO2, SO and S) originated from jovian satellite Io are ionized
by interaction with magnetosphere plasma and then form a donut-shaped region called the plasma
torus. Ion composition in the plasma torus is a key issue to investigate a source region andproduction mechanisms of magnetospheric plasma. A coordinated observation of EUV spectroscopy by
Hisaki/EXCEED with a ground-based spectroscopy by Kitt Peak 4-m telescope enables to measurecomposition of most of ions (S+, S++, S+++, O+ and O++) in the plasma torus.
At the begging of January 2014, intense campaign observations of Jovian aurora and Io plasma torus
were made using Hisaki/EXCEED, Hubble Space Telescope and other ground-based telescopes covering
wavelength range from EUV through IR. The 4-meter R.C. Spectrograph was set up covering 550nm
through 800nm which could successfully detect NaD (589nm), [S III] 631.2nm, [S II] 671.6/673.1nm,
and [O II] 731.9/733.0nm as well. A field-of-view was 98 arc-seconds along the slit and the slit
center was pointed at the dawn or dusk edge of the centrifugal equator. We could get 54 spectrafrom the observation during January 4th through 10th, 2014.
Based on analysis of visible spectrum from Kitt-Peak 4-meter, typical emission intensity of [S II]
671.6+673.1nm, [S III] 631.2nm and [O II] 731.9+733.0nm were 700R, 100R and 60R, respectively.
Combining the visible spectrum with EUV spectrum measured by EXCEED/Hisaki, plasma diagnostics canbe made on the plasma torus. According to the atomic database, CHIANTI version 8.0, the best fit
ion composition was S+:S++:S+++:O+:O++ = 4:27:11:13:40. The result shows that the average ionization
state was higher than that at Cassini era in 2004. More accurate analysis including errors anduncertainty is ongoing, the recent result will be presented at the meeting.
Keywords: Io plasma torus©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-20 -PCG21-21

Japan Geoscience Union Meeting 2016

Variations of Io's volcanism seen in Jupiter's sodium nebula*Mizuki Yoneda1,2, Masato Kagitani2, Takeshi Sakanoi2, Fuminori Tsuchiya2, Shoichi Okano2
1.Kiepenheuer Institute for Solar Physics, 2.Planetary Plasma and Atmospheric Research Center,
Tohoku University
Io, which is one of Jupiter's remarkable moons, is the most volcanically active body in the solar
system. Io's volcanic gas forms its atmosphere, but it cannot stay on Io stable. It is ionized dueto the solar radiation or impacts by Jupiter's magnetospheric energetic particles, then escapes andforms the Io plasma torus. Sodium chloride molecules included in Io's volcanic gas are also
ionized, then picked up by Jupiter's co-rotating magnetic fields and going to the Io plasma torus.
In the torus, these molecular ions collide with the torus electron, then they distract and neutral
fast sodium atoms are produced. These sodium atoms have velocity of Jupiter's co-rotation at anorbit of Io that corresponds to 74 km/s. This is enough to escape from Jupiter's
gravitational-sphere. The sodium atoms whose origin is Io's volcano form a extend cloud around
Jupiter, and this is called Jupiter's sodium nebula. This nebula can be seen at sodium D-line
wavelengths from the ground. The brightness in the sodium nebula is changing with respect tovolcanic activity on Io. We have been making the observations of this sodium nebula from an
observatory on Mt. Haleakala in a conjunction with the HISAKI spacecraft whose one of main targets
is Jupiter's magnetosphere. The sodium nebula shows a distinct enhancement in its brightness inJanuary-April 2015. In this presentation, volcanic activity on Io in a period of the HISAKI
observations will be shown.
Keywords: Io, Jupiter, Volcanism, magnetosphere©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-21 -PCG21-22

Japan Geoscience Union Meeting 2016

The wave structure in Jupiter's polar region and the asymmetric distribution of the polar
cup haze*Yuya Gouda1, Yukihiro Takahashi1, Makoto Watanabe1
1.Department of Cosmosciences, Graduate School of Science, Hokkaido University
In the Jupiter stratosphere, the polar cup hazes exist in the both hemispheres whose edges show the
wave structure. Previous observations, such as by Cassini ISS in 2000 or the Hubble Space Telescope(HST) from 1994 to 1999 [Barrado-Izagirre at al., 2008], show that the polar region is covered bybright diffuse haze and its edge has a wavy structure spreading in longitudinal direction withwavenumber of 12 –14 at 67° S, which travels westward with a phase velocity of 0- 10 m/s. Theseobservations suggested that this wave structure is caused by a planetary Rossby wave. However,these observations had been carried out only every other year and the variance of short time scale(about month) is not clear. The spatial range of this wave structure have not been investigated.
The aim of this study is to clear dynamics of the wave structure which vary within few days or few
weeks. This is achieved by the continual observation using a methane absorption band filter at 889
nm installed at Multi-Spectral Imager (MSI) of the 1.6 m Pirka telescope. To investigate spatial
structure of this wave, we measure the boundary of the low-latitude side of the wave. We also clear
the vertical structure in polar region using observation at wavelengths that absorption by methan
are different. In addition, identify of distribution of the haze structure is also purpose of thisstudy.
In this presentation, we introduce the observational results of time variation of the wave
structure in Jupiter’s south polar region in 2011 to 2015 by the ground-based telescope. Each
result is separated by few days to few weeks. Our results show the variation of the wave structure
within few weeks for the first time. It is found that disappearance of wavy positive peak atlongitude 100° in 18 days by the time variation of the wave structure. The wave structure change
locally in a short period. The longitudinal difference of the vertical wave propagation is found byobservation at some wavelength that observational altitude are different. It is possible that which
is different from Rossby wave. It was suggested that not only Rossby wave but also locally andshort-period wave structure exist in the polar region. It is cleared that the wave structure at 67°
S is only seen at 56° S by the latitudinal variation of the wave structure. Thus, the spatial
distribution of the wave structure is constrained.
In addition, the spatial distribution of polar cup haze in the north hemisphere is asymmetry forthe north pole. This corresponds with a sharp of auroral oval at some longitude. It is suggested
the relationship haze structure and auroral oval.
Keywords: haze, wave structure©2016. Japan Geoscience Union. All Right Reserved.

- PCG21-22 -PCG21-23

Japan Geoscience Union Meeting 2016

Measurements of Jupiter's decametric source locations by LWA1 modulation lane data
*Kazumasa Imai1, Masafumi Imai2, Charles A. Higgins3, Tracy Clarke4
1.Kochi National College of Technology, 2.Kyoto University, 3.Middle Tennessee State University,
4.Naval Research Laboratory
Jupiter's decametric emissions originate along magnetic field lines within auroral zones as well as
field lines that pass through Io and the Io plasma torus. The radio waves at Jupiter are amplified
by particle-field interactions and are generated in both the X-mode and O-mode. Due to the emission
source parameters, right-hand (RH) polarized waves are generated from northern hemisphere sources(Io-A and Io-B sources) while left-hand (LH) polarized waves come from the southern hemisphere(Io-C and Io-D sources).
The modulation lanes in Jupiter’s decametric radiation, which were discovered by Riihimaa [1968],
are groups of sloping parallel strips of alternately increased and decreased intensity in the
dynamic spectra. Extensive systematic observations of modulation lanes have been made in thefrequency range 21 to 23 MHz by Riihimaa [1970, 1974, 1978]. The frequency-time slopes of the lanes
can be either positive or negative, depending on which of the Jovian sources is being observed.
In the Imai et al. model for the production of modulation lanes, the lanes are assumed to be amanifestation of interference fringes from the line source consisting of the points along the axisof the Io-activated flux tube that are emitting at the different local values of fc. The fringes
are produced as a result of the passage of the multi-frequency radiation through an interference
grating. This grating is a planar grid of almost equally spaced field-aligned columns of enhanced
plasma density, perpendicular to the ray-paths toward Earth, located near the sub-Earth point on
Io’s orbit. Radiation from each of the frequencies emitted by the line source produces a set ofinterference fringes when it is scattered by the plasma-enhanced columns. These sets of fringes are
inclined with respect to the Jovian equator. The rotation of Jupiter sweeps the inclined
interference patterns for the different frequencies across Earth, producing the modulation lanes in
the observed dynamic spectra. We developed a model to explain the production of the modulation
lanes [Imai et al., 1992a, 1992b, 1997, 2001, 2002]. By using our model the precise Jupiter's radiosource locations and beam parameters can be measured. This new remote sensing tool is called as themodulation lane method.
The Long Wavelength Array (LWA) is a low-frequency radio telescope designed to produce
high-sensitivity, high-resolution images in the frequency range of 10-88 MHz. The Long Wavelength
Array Station 1 (LWA1) is the first LWA station completed in April 2011, and is located near the
VLA site in New Mexico, USA. LWA1 consists of a 256 element array, operating as a single-station
telescope. The sensitivity of the LWA1 combined with the low radio frequency interference
environment allow us to observe the fine structure of Jupiter's decametric modulation lanes. Using
newly available wide band modulation lane data observed by LWA1, we measured source locations andbeam parameters. The results of LWA1 data analysis indicate that the radio emitting sources arelocated along the restricted range of longitude. We only receive one of the individual sources
which has a very thin beam thickness (probably less than few degrees) at a given time. We show themeasured locations of Io-related sources based on the modulation lanes observed by LWA1. The newcomponents of Io-C and Io-B sources are discussed.
Keywords: Jupiter radio, modulation lane, radio source locations©2016. Japan Geoscience Union. All Right Reserved.

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