スケジュール 2020

The Earth has sustained its intrinsic magnetic field for at least 3.5 billion years as revealed by paleomagnetic studies [e.g., Biggin et al., 2015]. Studies of thermochemical evolution of the Earth’s core suggest that the solid inner core has been growing up to the present size, whose ratio of the inner to outer core radii (r_i/r_o) is 0.35, for approximately one billion years [e.g., O’Rourke and Stevenson, 2016]. The characteristic of the current geomagnetic field is dipolar dominant, so revealing mechanism and investigating conditions of a dipolar dynamo in a rotating spherical shell with various inner core radii are important in the perspective of understanding the Earth’s history. Some numerical dynamo simulations are focusing on the relation between dynamo regime and inner core size. Hori et al. (2010) found that sustained magnetic field is likely to be more dipolar on the fixed heat flux (FF) condition than on the fixed temperature (FT) condition with r_i/r_o = 0.1 and 0.35. Driscoll (2016) indicated dynamos are strong dipolar after the inner core nucleation. Lhuillier et al. (2019) pointed out that sustained magnetic field is less dipolar when 0.2 < r_i/r_o < 0.22. However, it is still unclear how dipolar or non-dipolar regime is controlled with different inner core radii. In the present study, we investigate dipolar component dominancy on thermal conditions and inner core radii. Using a numerical dynamo code Calypso [Matsui et al, 2014], we perform thermal and MHD dynamo simulations changing the Rayleigh number (Ra) with r_i/r_o = 0.15, 0.25, and 0.35 on FT and FF boundary conditions. We examine the dominancy of the dipole component in two approaches; one is calculating the dipolarity, f_dip, which is the ratio of the amplitude of the axial dipole magnetic field to the total amplitude, and the other is comparing magnetic energy for the dipole component with extrapolated magnetic energy for l = 1 using an exponential function for odd degree components from l = 3 to l = 19. We calculate the ratio of magnetic energy for the dipole component to that extrapolated from the fitting curve. f_dip is an ordinary index and the fitting curve ratio is our new index for representing the dipolar dominacy. f_dip and the fitting curve ratio become smaller with the larger Ra because the intense convection cannot sustain the strong dipolar field. We find dipolar dominancy becomes weakened with the smaller inner core from calculating the ratio. Axial structure is needed to be sustained dipolar field. Our result that dynamos are less dipolar in FF than in FT is reverse to that of Hori et al. (2010). Dynamo simulations with cooling from the core-mantle boundary are next step to understand the radius ratio dependency in the more realistic condition.

abstract

Measurements of trace-gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbor, Venus, has cloud decks that are temperate but hyper-acidic. We report the apparent presence of phosphine (PH3) gas in Venus’ atmosphere, where any phosphorus should be in oxidized forms. Single-line millimeter-waveband spectral detections (quality up to ~15) from the JCMT and ALMAtelescopes have no other plausible identification. Atmospheric PH3 at ~20 parts-per-billion abundance is inferred. The presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently-known abiotic production routes in Venus’ atmosphere, clouds, surface and subsurface, or from lightning,volcanic or meteoritic delivery. Phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life. Other PH3 spectral features should be sought, while in-situ cloud/surface sampling could examine sources of this gas.

We report on the first direct measurements of zonal winds at 80 km during the 2018 planet-encircling dust event (PEDE) by ground-based infrared heterodyne spectroscopy. The observed line-of-sight Doppler shift implies an intense retrograde zonal wind in the equatorial mesosphere reaching -221 m s^-1 on average with the standard deviation of ~103 m s^-1 during the 2018 PEDE, which is much larger than that during clear sky conditions (-141 m s^-1). This implies a substantial acceleration of the retrograde zonal wind during the PEDE. The observed intensification of the retrograde zonal wind at 0.1 Pa pressure level is consistent with the predictions made by Mars general circulation models (MGCMs), however of a much larger magnitude. We evaluated the mechanism of acceleration of the retrograde zonal wind using the output of a high-resolution MGCM to find out that the stronger retrograde zonal wind are related to strengthening the meridional circulation across the equator and forcing by gravity waves. 

TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our solar system and possibly habitability. Both transit timing variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derived from N-body simulations possible planetary evolution sce- narios, and show that all the planets are likely in synchronous rotation. We then used a versatile 3D global climate model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We investigated the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explored the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N2, CO, or O2 are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/EUV radiation and stellar wind atmo- spheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO2, CH4, or NH3. CO2 can easily condense on the permanent nightside, forming CO2 ice glaciers that would flow toward the substellar region. A complete CO2 ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO2 ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least ∼103 × Titan’s flux), CH4 and NH3 are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO2 would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should very likely sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

Solar-locked and geographical atmospheric structures of daily averaged wind and temperature on Venus were investigated using an atmospheric general circulation model with Venusian topography and a two-stream radiative code and were compared with wind fields obtained from the Akatsuki ultraviolet imager. The horizontal wind fields simulated around the subsolar region are similar to the observed ones at the cloud top. Mid-latitude jets of ∼120 m s–1 and an equatorial fast flow of ∼90 m s–1 are formed around the cloud top. A poleward flow of >8 m s–1 is formed above the cloud layer, where solar heating is strong. Around the cloud top, a poleward flow of ∼1 m s–1 is confined within the equatorward flank of the jet core, whereas an indirect circulation is formed in the jet core by the eddy heat fluxes owing to the thermal tide and baroclinic waves. In solar-fixed coordinates, the subsolar-to-antisolar circulation is predominant around the cloud top. Thus, differences are significant between the zonal and dayside averages of the meridional wind and its related fluxes within the cloud layer. This suggests the zonal mean meridional wind of the Hadley circulation, eddy momentum, and heat fluxes from the one-side hemisphere must be estimated carefully. In the experiment including topography, a near-surface subrotation is formed in latitudinal zones over high land and mountains, a weakly stable layer is formed at 10–20 km at low latitudes, and the zonal wind is weakened at the cloud top over the Aphrodite Terra. Regional stationary modification of the atmospheric structure due to topographical waves appears in the cloud layer.

The Martian thermosphere-ionosphere is a source for atmospheric escape, which is affected by both the lower-middle atmosphere and external forces. Recent studies suggest a significant impact of the lower-middle atmosphere on the upper atmosphere. However, its effect on ionospheric compositions and compositions of escape ions is not well understood.

Seasonal variations of atmospheric compositions in the dayside thermosphere and ionosphere have been investigated using data from December 2014 to March 2018 obtained by Neutral Gas and Ion Mass Spectrometer (NGIMS) on Mars Atmosphere and Volatile EvolutioN (MAVEN). We have investigated the effect of variations of homopause altitude (Jakosky et al., 2017; Slipski et al., 2018; Yoshida et al., under review) on atmospheric composition in the upper atmosphere using CO₂ and N₂ densities. Additionally, the coupling between neutral and ion species in the upper atmosphere has observationally confirmed among CO₂, O, CO₂⁺, O₂⁺, and O⁺ densities, as well as N₂ and N⁺ densities below the photochemical equilibrium region below ~200 km. We find that seasonal variations of CO₂⁺, O₂⁺, and O⁺density, during solar minimum activity, are by ~6, ~3, and ~1.5 at 180 km, respectively, which are mainly controlled by the seasonal variation of CO₂ density in the upper atmosphere. It suggests that inflation and contraction of the atmosphere have more impacts on atmospheric compositions in the upper atmosphere (cf. Fox, 2009; Krasnopolsky, 2002).

A depletion of O, O⁺, and O₂⁺ densities between 150 and 250 km altitude are simultaneously found during L_s = 342-346 in MY 33. A regional dust storm event was found during the same period (Liu et al., 2018). A decrease of O density in the thermosphere can qualitatively explain the depletion of O⁺ and O₂⁺ density in the ionosphere. Our result suggests that there would be a different behavior of ionospheric species during the dust storm event.

Solar Energetic Particles (SEP) consist of protons, electrons and
heavy ions in the energy range between a few tens of keV and GeV,
which are originated from solar flares in the low corona, shock waves
driven by coronal mass ejections (CME), planetary magnetospheres, and
bow shock. SEPs are well known to damage not only spacecraft and
detectors but also human body. For future international space
explorations in the 2020s, the human activity is going to expand to
the Moon and Mars. Thus, the impact of SEP on future explorations on
Mars needs to be addressed. However, the effect of SEP on the Martian
atmosphere and surface environment is not well understood. In
addition, SEP penetrates into Earth’s atmosphere down to tens of
kilometers at high geomagnetic latitudes, which affects the
composition in the middle atmosphere. During the large solar flare in
October 2003, SEP caused NO2 enhancement of several hundred percent
and tens of percent ozone depletion between 36 and 60 km altitudes
(e.g., Seppälä et al., 2004; Rohen et al., 2005). In contrast, the
vertical distribution of ozone on Mars was extensively observed by
SPICAM onboard Mars Express from 2004 to 2014. Results of previous
studies show that the ozone layer was located around 50 km altitude in
the southern polar winter, and around 30-40 km altitude in low to
middle latitudes in both hemispheres at aphelion season (Montmessin et
al., 2013, Maattanen et al., 2019). Mars has a thin atmosphere, less than 1% of that on Earth. The surface and atmosphere are heavily affected by energetic photons and particles that easily penetrate it owing to insufficient magnetospheric and atmospheric shielding. The Imaging UltraViolet Spectrograph (IUVS) spectroscopy onboard Mars Atmosphere and Volatile EvolutioN(MAVEN) detected diffuse aurora, suggesting widespread occurrences with increased SEPs (Schneider et al., 2015; Schneider et al., 2018). Schneider et al. (2018) reported the low-altitude, “diffuse” aurora spanning across Mars’ nightside hemisphere, coincident with a SEP outburst. The emission extended down to an altitude of ~60 km. Deep precipitation in the middle atmosphere requires extremely energetic
electron fluxes up to 100 keV. This fact suggests that SEP may have
additional effects on global atmospheric structures in the Martian
atmosphere. Incident particles ionize and dissociate atmospheric
species deeply, as well as heat the target atmosphere.
The purpose of this study is to investigate the effects of SEP on the
Martian atmosphere, especially on ozone, which will help us to
understand the study of astrobiology and the greenhouse effect of the
early Martian atmosphere. In this study, we use vertical profiles of ozone, carbon dioxide, and oxygen number densities observed by stellar occultation measurements by Imaging UltraViolet Spectrograph (IUVS) on board Mars Atmosphere and Volatile EvolutioN (MAVEN), and the energy fluxes of electrons and ions by Solar Energetic Particles onboard MAVEN (MAVEN/SEP). Since March 2015, MAVEN/IUVS has generally conducted stellar occultation campaign for 1-2 days on average for every 2-3 months. MAVEN/IUVS observations cover whole longitudinal area, and wide range of latitudes from 80°S to 75°N. MAVEN/SEP detects an energy spectrum of
electrons from 30 keV to 1 MeV and ions from 30 keV to 12 MeV up to
10-10^6 eV/[cm^2 s sr eV]. The distribution of ozone observed by MAVEN/IUVS confirms the seasonal, latitudinal and altitudinal characteristics of the ozone distribution observed by MEX/SPICAM. MAVEN/IUVS observations show the peak number density of ozone to be the order of 10^8 /cc at the southern polar winter and the order of 10^9 /cc for both hemispheres at aphelion season. These peak number densities are generally well in agreement with the SPICAM observation, but some data are not quantitatively consistent in detail. Using the MAVEN/SEP, we analyzed the energy fluxes of solar energetic particles observed between March 2014 and January 2020 in order to identify the periods of strong solar energetic particle arrival events, related to the IUVS observations. Here, we focus on the events; Occurred in 3-4th November 2015 for 27 hours after the arrival of electrons to the arrival of ions, with 34 profiles. The effect of SEP on the atmosphere is addressed by comparing with 26 profiles in 8-9 October 2017 during the quiet period.
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Mars is expected to be a target of resource exploration and
immigration for business in a decade or two, next to Moon. In the
human activities on Mars, water and oxygen would be main resources as
they may be the materials of hydrogen energy and life-support, so the
exploration and securement of them would be important. In this
context, making the frequent opportunities to send spacecrafts to Mars
to get the information of their distributions is indispensable, and
the use of low-cost micro-satellites would extremely help that.
We are developing micro-satellite Mars spacecrafts with a terahertz
spectrometer onboard, named TEREX (TERahertz EXplorer). A terahertz
spectrometer can be made very small and lightweight (several
kilograms) with a high-frequency heterodyne spectroscopic system,
covering the wavelength range in which the simultaneous detection of
H2O, O2, H2O2 and O3 are possible with high sensitivities. TEREX-1 is
planned to be launched as a piggyback payload in 2022 at the earliest,
costing 10-20M US-dollars which is less than 1/10 of previous Mars
missions. We are also planning the following TEREX-2 as a Mars
orbiter, intended TEREX to be a micro-satellite mission series.
In addition, O2 on Mars is a hot target also in the astrobiological
aspect, as new findings about the mixing ratios and their variability
near the surface have been reported (Hartogh et al., 2010, A&A;
Trainer et al., 2019, JGR Planets). We discuss the mission and
observational plans of TEREX for both the economic and scientific
benefits.

Vertical profiles of ozone were retrieved from ground-based infrared
spectra observed with FTIR at Tsukuba. The profiles were retrieved from
the spectra between 1000.0 and 1005.0 cm-1 with SFIT4 spectral fitting
program using the recommended parameters by NDACC/IRWG. There are many absorption lines of ozone in this spectral window and we can achieve a high degrees of freedom for signal (DOFS) of 5. This means that we can get information for 5 independent layers. The instrumental line shape (ILS) is important for profile retrieval. As for the Bruker IFS125HR
which is used at Tsukuba, the alignment of the optics is very good and
the ILS derived from HBr cell measurements are nearly perfect.
Therefore, we don’t need to correct the spectra.
The derived ozone profiles were validated using ozonesonde and Brewer
data observed at Tateno. Tateno is located very close to our site and we
can consider them as the same location. Total columns derived from FTIR
are 5 % higher than the Brewer in average for 2019. This difference
agree with the results of Vigouroux, et al.[2008] and may be due to the
bias of the line intensity. The profiles between 17 and 35 km agree with
ozonsonde within 15 % for two case. More comparison results will be
presented in the seminar.

Exoplanets with substantial Hydrogen/Helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars. The extreme irradiation levels these atmospheres experience causes them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and lower-mass super-Earths/mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, and multi-dimensional calculations that incorporate magnetic-fields and interactions with the interstellar environment are cutting edge. However, there remains very limited comparison between simulations and observations. While hot Jupiters experience atmospheric escape, the mass-loss rates are not high enough to affect their evolution. However, for lower mass planets atmospheric escape drives and controls their evolution, sculpting the exoplanet population we observe today. 

There are two moons on Mars, Phobos and Deimos. They have common characteristics of small inclination and eccentricity. The main scenarios of origin are the capture scenario and the giant impact scenario. The reason why the capture scenario is supported is that the reflectance spectra of the two moons match with the D-type asteroids, but it is difficult to explain the current characteristics of the inclination and eccentricity. On the other hand, the giant impact scenario can explain the characteristics of the current orbit. In this study, we evaluate the validity of the capture scenario by calculating the orbital evolution of Phobos, taking into account the deviation from the co-rotation of the early Mars expanded atmosphere.
 Previous studies have calculated orbit that take into account the co-rotation of the Martian atmosphere and have shown that the lifetime of the captured asteroid is relatively long when the atmosphere rotates at the angular velocity of Mars near geostationary orbit. However, in order to follow the orbital evolution of captured asteroid after its capture, we need to consider the evolution of the Martian atmosphere. When the Martian atmosphere was expanded significantly beyond the geostationary orbit, it is possible that the atmosphere rotated at an angular velocity relatively close to the rotation of Mars. However, considering the process of shrinkage of the expanded atmosphere, it deviates from co-rotation for various reasons. In this study, we calculate the orbit of Phobos, taking into account the deviation from the co-rotation of the atmosphere, and estimate the change in the lifetime of the captured asteroid.

In recent years, the coupling of atmospheric regions on Mars has been actively studied. Due to its thin atmosphere and lack of strong magnetic field, Mars has a much more intense solar radiation and intense convection activity than Earth, atmospheric waves excited by the intense solar radiation, the dynamic circulation and mass transport associated with the Hadley cells that extend to the middle atmosphere, and the disappearance of the cold trap (hygropause) due to the dust heating. Characteristic features, such as vertical water vapor transport, have been reported by the latest observations. In this presentation, we will review our recent activities and try to look forward to the future..

The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. Overall, the electron temperature is lower in the crustal-field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet (EUV) conditions, which is associated with the local EUV energy deposition. The current models can explain the electron temperature in the draped-field region, while underestimating the electron temperature above 250 km altitude in the crustal-field region. Electron heat conduction associated with a photoelectron transport in the crustal-field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron’s upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal-field region should be considered for future numerical simulations. In this presentation, I will summarize the previous studies on the electron temperature in the Martian ionosphere, and discuss our plans for the upcoming year, including a coupling of the electron temperature model with the Monte Carlo method, which enable us to deal with kinetic effects.

The mid-IR laser heterodyne spectroscopy provides high spectral
resolution > 106 by combining an IR source from the observing target
and an IR laser, such as a quantum cascade laser (QCL) and/or a CO2
gas laser, as the local oscillator (LO). We have developed the
mid-infrared laser heterodyne spectrometer MILAHI (Mid Infrared LAser
Heterodyne Instrument) mounted on our dedicated Tohoku 60 cm telescope
(T60) at the summit of Mt. Haleakala, Hawaii, which has successfully
operated for measurements of Venusian and Martian atmosphere (Nakagawa
et al., 2016, Takami et al., 2020).

The two beams are combined at the ZnSe beam splitter and then focused
onto a HgCdTe photomixer. A precise optical alignment is required to
combine two beams. In this study, we aim to simplify the optics by
applying mid-IR transmissive hollow fibers.

There is few optical fiber which has a high transmittance at the
wavelengths longer than 2 μm. Recently mid-IR (5-20 μm) transmissive
hollow fibers has been developed by Tohoku University (e.g. Matsuura
et al., 2002). The fibers are made of glass tubing whose inner
diameter are 1 mm and have a metallic layer of Ag on the inside of
glass tubing and then a dielectric layer of AgI over the metallic
layer. Transmittance of 95 %/m at 10.6 μm was reported in previous
studies (e.g. George and Harrington, 2004), meanwhile we have achieved
about 70% transmittance with a 600 mm hollow fiber at 10.4 μm from our
laboratory measurements. Transmittance of hollow fibers strongly
depends on its incident angle of the light. Better transmittance might
be possible by improving the alignment.
 In this study, we made a fiber system which guides two beams by 300mm
hollow fibers and combines them by a conventional beam splitter to
evaluate the efficiency of the heterodyne signal using hollow fibers.
A CO2 gas laser which emits IR at 10.3 μm was used as a LO and a small
blackbody furnace and a QCL were used as IR sources.  I will report
the result of this experiment.

In addition, I will introduce the technology related to the fiber
coupler and divider for the hollow fibers. (Tamura et al., 2017)  The
fiber coupler can provide downsizing, weight saving, high
stabilization of the instrument, which are essential to develop the
instrument for space-born missions.