2016年度 惑星大気・電離圏セミナー



■場所　：棟A6F会議室 (変更の場合は赤字)
■時間　：火曜日9:00-　(変更の場合は赤字)
■連絡先：韓　秀萬 (hsooman1011[at]stp.isas.jaxa.jp)


開催日時・場所 発表者 (所属・身分)

7/27 (Wed.) 16:00
5F conference room 		Takayuki Enomoto
Venusian Cloud and Haze inferred from Groud-based Imaging Polarimetry with HOPS  
Abstract:
Venus is entirely covered with thick sulfuric acid cloud. There are some studies reporting that optical thickness of the polar haze layers have temporal variations (e.g.: 0.3 to 0.05 @935nm, Sato et al., 1996). Since such variations can affect energy deposition and its time constant gives restrictions of the kinds of chemical reactions in Venusian atmosphere, it is important to perform continuous monitoring. In order to track the temporal variation of Venusian polar hazes, we have carried out observations with an imaging-polarimetry system “HOPS” (Hida Optical Polarimetry System) attached to 65cm refractor telescope at Hida Observatory form 2012. In previous seminar, we showed the quick-look results that optical thickness of polar hazes were 0.16@North, 0.09@South in 2012 August and decreased to 0.01@Both Polar in 2015 June. But this quick-look analysis did not consider the influence of atmospheric turbulence during observations, and model used for radiative transfer calculations was little simple for accurate estimations. In order to avoid this problem, we used disk-integrated polarization degrees and morphology of polarization maps. In this seminar, we will show the preliminary results.


7/22 (Fri.) 16:00
5F conference room 		Yumika Sakamoto
A numerical study on decrease of electron temperature inside the sporadic E layer  
Abstract:
The sounding rocket ”S-520-29” was launched from Uchinoura Space Center at 19:10 JST on August 17, 2014. The purpose of this experiment is to elucidate spatial structure of the Es layer in the lower ionosphere. Then, the detailed trend of the electron temperature from its boundary toward the center of Es layer was revealed due to the interpolation. In this study, I will discuss a physical implication of the observed high electron temperature for energy budget inside the Es layer by conducting numerical calculation in which I consider electrons, ions, and neutrals in the vertical 1-D direction. In this presentation, I will show a result from this numerical consideration.


7/15 (Fri.) 16:00
6F conference room 		Masahiro Shimokawa
Microphysical calculation of Venusian cloud formation  
Abstract:
Sulfuric acid is the main component of Venusian cloud, which is produced at upper atmosphere by photolysis and vanished at lower atmosphere by thermal decomposition. These chemical differences from terrestrial cloud makes unique characteristics in respect of cloud formation and transportation. It is helpful to clarify the propaties of Venusian cloud for understanding not only Venus itself but also the Earth and exoplanets, but estimation by calculation is necessary because of difficulties of observing and investigating Venus directly. In this study I calculate vertical number density of sulfuric acid and water, considering effects of advection, diffusion, sedimentation, chemical production and chemical decomposition, to elucidate the mechanism of microphysical cloud formation. This time I will show the results of simplified calculation excluding some factor and discuss the appropriateness of setting conditions.
Masayuki Omino
Cloud-tracked winds on Venus night-side obtained from images taken by VIRTIS on board Venus Express  
Abstract:
Venus is covered by a thick layer of clouds, but the mechanism that produces those clouds and the atmospheric circulation that maintains those clouds are unexplained yet. Investigating the relationship between the cloud structure and the wind velocity field around them is important to deepen our understanding for the mechanism of the cloud generation and of the atmospheric circulation, but enough studies are not done yet. In this study, we are investigating the relationship between the cloud morphology and the distribution of the wind velocity around them by tracking the clouds using the images of VIRTIS on board Venus Express. Specifically, we are investigating the atmospheric motion of the lower cloud layer of Venus night-side by using 1.74μm images which is a wavelength known as the “atmospheric window”.


7/13 (Wed.) 10:00
6F conference room 		Keishiro Muto
Analysis of the polar oval of Venus using VIRTIS images  
Abstract:
Polar oval is a circular feature observed near the South Pole in visible and ultraviolet wavelengths (Observation is absent for the North Pole). The mechanism producing the oval is not understood. In this study, we compare IR and visible images taken by VIRTIS onboard Venus Express to better characterize this feature. In images taken at 5 micrometers, which is the maximum wavelength that can be observed by VIRTIS, we can observe black-body radiation from the cloud top. Contamination of sunlight scattered by dayside clouds is removed by subtracting a cubic function fitted to the brightness temperature variation along the solar zenith angle; this procedure enables observation of black-body radiation even on the dayside. In visible range, local time dependence of the radiance was observed both in VIRTIS and VMC images. Calculation of the heat balance at the dark edge of the polar oval shows that the temperature variation across the polar oval is explained by the albedo variation.


6/30(Thu)16:00
5F conference room 		Takumi Abe
Observations of thermal electron and ions by FBP during ICI-4 campaign  
Abstract:
A series of the ICI (Investigation of Cusp Irregularities) sounding rocket campaign has been conducted to reveal the controlling mechanism(s) of plasma instabilities that generates HF radar backscatter targets and GPS scintillations in the polar ionospheres. The main objective of this campaign is to carry out in-situ measurements of plasma and fields to provide information about the underlying physics of such space weather phenomena. The 4th experiment of ICI campaign was conducted in February 2015 to investigate the micro-physics of plasma instabilities and turbulence phenomena associated with polar cap patches being pulled into the night-time aurora, referred to as auroral blobs. The auroral blob phenomenon is created when polar cap patches, islands of high electron density plasma, exit the polar cap at night, i.e., when they are entering the night-time auroral oval. A total of seven instruments were installed on ICI-4 rocket. Among them, Fixed Bias Probe (FBP) was provided by ISAS to measure small scale density perturbation of electrons and ions. In FBP instrument, a spherical probe with a diameter of 2 cm was adopted as a detector for the electron density measurement. In addition, two circular probes with a diameter of 2 cm were adopted to monitor the local ion density. The electron and ion probes are biased with a positive voltage of +4 V and a negative voltage of -3 V, respectively, with respect to the rocket potential. The electron and ion currents incident to the probe were used to investigate characteristic feature of electron/ion density irregularity. We are also trying to estimate a direction of ion drift from a spin variation of ion current incident to the planar probe. In this talk, data from FBP and other instruments obtained during ICI-4 sounding rocket experiment will be discussed.


6/23(Thu)16:00
5F conference room 		Takao Sato
Characteristics of Venus cloud particles inferred from near-infrared atmospheric windows.  
Abstract:
Optically thick sulfuric acid clouds in the altitudes of 50-70 km are considered to have impacted on the atmospheric dynamics and the climate system of Venus. Discovery of near-infrared atmospheric windows allow us to investigate the middle and lower clouds by remote sensing techniques. Akatsuki IR2 carries three channels for studying atmospheric dynamics and clouds in the lower atmosphere and has tested its performance for three months. In this presentation, I will review two important works (Carlson et al., 1993; Wilson et al., 2008) which analyzed spaceborne 1.74 and 2.3 micron images to obtain the spatial variations in properties of cloud particles. And I will explain the updated IR2 observation plan to be executed in several weeks.


6/09(Thu)16:00
5F conference room 		Takehiko Satoh
The new views of Venus as revealed with Akatsuki/IR2  
Abstract:
Since the successful orbit insertion on 7 Dec 2015, scientific instruments onboard Akatsuki started acquisition of exciting Venus data. IR2, although the first in-orbit Venus image was acquired on 11 Dec 2015, delayed regular observations due to difficulty in optimizing the operation of cryo-cooler, a key component of IR2 as the system requires appropriate cooling. The first usable 2.26-um image of Venus night side was obtained on 13 March 2016, more than 3 months after the VOI-R. However, IR2 is now almost at its full swing now and images of both day-side and night-side have been regularly acquired, surprising us every week. In this seminar, scientific objectives of IR2, design of the system will be explained briefly. Large portion of time may be used for *picture viewing* AND discussion about the phenomena that are seen in these new data.


5/16(Mon)10:00
5F conference room 		Yusuke Nara
Venus Express VMCの雲画像解析による金星雲頂の微細構造の研究  
Abstract:
金星の雲の紫外線観測では低緯度に、数十から数百kmの空間スケールの不規則 な乱流のような模様が観察される(Markiewicz et al., 2007)。従来、輝度の変 化が小さく雲の模様の変化が見にくいとされてきた可視波長でも、Venus Expressの観測により低コントラストながら模様が存在することが明らかとなっ た(Titov et al., 2007)。可視波長領域は雲の吸収物質による影響が少ないた め、雲の厚さの水平分布上層雲の水平分布をよく表していると考えられ、さらに 太陽放射の大半は可視波長領域にある。そのため雲の反射率の変化が金星大気に およぼす影響を考える上で可視波長を用いた研究は重要である。可視波長での観 測により得られる、金星の雲そのものの形態を手がかりに、どのような物理が金 星大気に働いているか理解することに意義がある。 　本研究では、欧州の金星探査機Venus Expressに搭載されていた撮像装置 VMC（Venus Monitoring Camera）により得られた可視画像を用いて金星の雲の低 緯度にある微細な構造の抽出を試みた。輝度の微細な空間変化を強調するために 空間差分を行う際に、同時に検出器の読み出しに伴う筋状の固定ノイズを取り除 くよう工夫することにより、低コントラストの模様までとりだすことができ、雲 が背景風に流される様が観察できた。本発表では、可視波長でみた金星の雲の空 間構造について議論する。


5/12(Thu)16:00
5F conference room 		Atsushi Yamazaki
Hisaki observations of interplanetary neutral helium emission and Local Interstellar Medium  
Abstract:
The Hisaki (SPRINT-A) satellite has a main scientific topic of the planetary continous observation for a long term, but carried out the non-planet observation at the time when no planet during a good observation opportunity phase exists. One case of those is observation of helium atom resonance scattering from the interplanetary space. 　A material in the local interstellar medium (LISM) travels into the heliosphere over the heliopause by the relative velocity of the heliosphere and the interstellar medium. The helium atom moves into about 0.5Au from the neighboring of the sun without ionizing because of its high ionization energy. The travelling orbit is bent by sun gravity and forms a high density region on the down wind side. It is called helium cone. The distribution of helium atoms in the helium cone can estimate the speed and direction of the interstellar wind, and the density and the temperature of the helium atom in interstellar space. Such a study was carried out from the 1970s, but the recent IBEX satellite observation results into gradual changes of the interstellar wind direction for several decades (Frisch+13). 　The Hisaki satellite carried out the observaiont of the resonance scattering from helium corn. In this season, Hisaki observed the helium cone for two months including a ecliptic longitude with the maximum density of the helium corn. In this presentation, the helium cone observation result and the change of the wind direction are reported.


4/28(Thu)16:00
5F conference room 		Yeon Joo Lee
Temperature retrieval from an observed thermal spectrum using the 4.3 um CO2 absorption band  
Abstract:
The strong absorption of CO2 at 4.3 um dominates atmospheric opacity in the mesosphere of Venus (60-100 km). For example, the mesosphere is total opaque at a band center, and then becomes rather transparent as it moves away from the band center, reaching to the cloud top level, ~65 km. So altitudes that are responsible for a thermal emission spectrum would vary across the band center. This property allows us to estimate atmospheric information, such as temperature profile in the mesosphere. There are many previous studies developed this reverse method to retrieve atmospheric temperature. In this talk, I will introduce an easiest approach for the temperature retrieval, a relaxation methods, introduced by Shahine et al (1972) and developed by Twomey et al (1977). This method has been used by many other studies (Zasova et al. 1999, Grassi et al. 2008, Haus et al. 2013, and Garate-Lopez et al. 2015). Here I will present the similar approach using 4.3 um CO2 band observed from a spectrometer onboard Venus Express during the Messenger flyby. A comparison with a temperature profile obtained by radio occultation measurement (VeRa) will be presented for a discussion.


4/22(Fri)17:00
5F conference room 		Takeshi Imamura
A review of cloud-radiatiation feedback models of Venus  
Abstract:
Venus’s atmosphere is considered a system maintained by strong coupling among cloud formation, radiative transfer and dynamics. Since modeling of such interaction including cloud physics in a realistic three-dimensional atmosphere is too complicated, simplified one-dimensional models have been employed so far. Here several previous studies on this topic are reviewed.


4/14(Wed)16:00
5F conference room
Self-Introducetion & New graduate students shortly present their previous studies.  
Abstract: