Doctor of Science, The University of Tokyo, Mar. 2014

Planetary Atmosphere

telescope

planetary exploration

planetary science

Natural Science, Space and planetary sciences

Bachelor's degree program, School of Science

Master's degree program, Graduate School of Science

Doctoral (PhD) degree program, Graduate School of Science

Polarimetric observations of the near-Earth asteroid 2024 MK during its close approach within lunar distance
Kazuya Doi; Seiko Takagi; Tomohiko Sekiguchi
Publications of the Astronomical Society of Japan, Oxford University Press (OUP), 15 Apr. 2026, [Peer-reviewed]
Scientific journal, Abstract

The near-Earth asteroid (NEA) 2024 MK was discovered by the Asteroid Terrestrial-Impact Last Alert System on 2024 June 16, less than two weeks before its close approach to Earth at approximately 0.76 lunar distance on 2024 June 29. NEAs that would cause significant regional damage upon impact are classified as potentially hazardous asteroids, and 2024 MK belongs to this category. In this study, the linear polarization degree of 2024 MK was measured using the 1.6 m Pirka telescope at Hokkaido University’s Astronomical Observatory in Hokkaido, Japan, on 2024 June 29 and July 2, at solar phase angles ($\alpha$, Sun–target–observer) of approximately $26^{\circ }$ and $101^{\circ }$. We found that the high linear polarization degree of 2024 MK reached $\sim$35% at a solar phase angle of $101^{\circ }$, which is a characteristic typical of C-type asteroids. In contrast, spectroscopic observations conducted by McGraw et al. (2024, ApJ, 977, L25), before and after its close approach to Earth, revealed that 2024 MK exhibits a reflectance spectrum typical of S-type asteroids. We hypothesized that this discrepancy is due to the grain size and/or porosity of the regolith on 2024 MK’s surface. Although 2024 MK is an S-type asteroid, its surface appears to be composed of fine grains, a characteristic likely resulting from space weathering.

Dust Properties of the Interstellar Object 3I/ATLAS Revealed by Optical and Near-Infrared Polarimetry
Seungwon Choi; Masateru Ishiguro; Jun Takahashi; Tomoki Saito; Yoonsoo P. Bach; Bumhoo Lim; Hiroyuki Naito; Jooyeon Geem; Sunho Jin; Jinguk Seo; Hyeonwoo Ju; Hiroshi Akitaya; Koji S. Kawabata; Mahito Sasada; Kazuya Doi; Hisayuki Kubota; Seiko Takagi; Makoto Watanabe; Tomohiko Sekiguchi; Myungshin Im
The Astrophysical Journal Letters, 1000, 1, L29, L29, American Astronomical Society, 18 Mar. 2026, [Peer-reviewed]
Scientific journal, Abstract

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible R _C band (0.64  μ m) to the near-infrared K _s band (2.25  μ m), and investigated its dependence on solar phase angle (polarization phase curve, PPC) and wavelength (polarization color curve, PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical solar system comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the R _C band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6–1.2  μ m and peaks at 1.5–2.0  μ m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among solar system comets, despite sharing a similar size scale of the aggregate building blocks.

Multi-epoch spectrophotometric characterization of the mini-moon 2024 PT₅ in the visible and near-infrared
Jin Beniyama; Bryce T. Bolin; Alexey V. Sergeyev; Marco Delbo; Laura-May Abron; Matthew Belyakov; Tomohiko Sekiguchi; Seiko Takagi
Astronomy & Astrophysics, 700, A183, A183, EDP Sciences, 15 Aug. 2025, [Peer-reviewed]
Scientific journal, Context. 2024 PT₅ is a tiny (D ≤ 10 m) near-Earth asteroid (NEA) discovered in August 2024. 2024 PT₅ was gravitationally bound to the Earth-Moon system from September to November 2024 and classified as a mini-moon. Several quick response observations suggest that 2024 PT₅ has a lunar ejecta origin, while the rotation state and albedo, essential properties for investigating its origin, are not well constrained.

Aims. We aim to characterize the spectrophotometric properties of 2024 PT₅ by ground-based observations to test its taxonomic classification and origin.

Methods. We performed visible to near-infrared multicolor photometry of 2024 PT₅ from data taken using the TriColor CMOS Camera and Spectrograph (TriCCS) on the Seimei 3.8 m telescope during January 4–10, 2025. The Seimei/TriCCS observations of 2024 PT₅ cover phase angles from 14 deg to 27 deg, and were obtained in the ɡ, r, i, and ɀ bands in the Pan-STARRS system. In addition, we analyzed Y, J, H, and K photometry taken with the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) on the Keck I 10-m telescope on January 16–17, 2025.

Results. Our light curves show brightness variations over time periods of several tens of minutes. We infer that 2024 PT₅ is in a tumbling state and has a light curve amplitude of about 0.3 mag. Visible and near-infrared color indices of 2024 PT₅, ɡ – r = 0.567 ± 0.044, r – i = 0.155 ± 0.009, r – ɀ = 0.147 ± 0.066, Y – J = 0.557 ± 0.046, J – H = 0.672 ± 0.078, and H – Ks = 0.148 ± 0.098, indicate that 2024 PT₅ is an S-complex asteroid, largely consistent with previous observations. Using the H–G model, we derived an absolute magnitude, H_V,HG, of 27.72 ± 0.09 and a slope parameter, G_V, of 0.223 ± 0.073 in the V band. A geometric albedo of 2024 PT₅ was derived to be 0.26 ± 0.07 from the slope of its photometric phase curve. This albedo value is typical of the S- and Q-type NEAs.

Conclusions. Using the albedo and absolute magnitude, the equivalent diameter of 2024 PT₅ is estimated to be 7.4 ± 1.0 m. The color properties of 2024 PT₅ derived from our observations match the ones of rock samples taken from the lunar surface, which agrees with previous studies.

Visible Spectral Slope of Saturn’s Rings from Ground-based Observations and Comparison with Cassini Data
Jun Kimura; Tomoe Yamaguchi; Seiko Takagi; Sho Sasaki
Research Notes of the AAS, 9, 7, 203, 203, American Astronomical Society, 25 Jul. 2025
Scientific journal, Abstract

We present visible wavelength observations of Saturn’s rings conducted using 1.6 m ground-based telescope. The goal is to assess long-term compositional changes in the rings by comparing spectral characteristics with data acquired by the Cassini spacecraft. Reflectance spectra from 450 to 900 nm were obtained during four nights in 2021 and 2022. Spectral slopes, sensitive to water ice purity, were derived for two wavelength ranges: 450–550 and 550–900 nm. Our measured slopes agree well with values previously reported from Cassini’s VIMS data, suggesting minimal compositional change over the past decade. In addition, no clear correlation is found between spectral slope and the orbital position of Enceladus. These results support the reliability of visible-wavelength ground-based monitoring as a complementary tool to past spacecraft observations.

A Search for Calcium in the Atmosphere of Europa with Ground-based Telescope
Seiko Takagi; Tatsuhiro Tsurumi; Jun Kimura; Taro Matsuo; Kazuo Yoshioka
Research Notes of the AAS, 9, 4, 88, 88, American Astronomical Society, 17 Apr. 2025, [Lead author, Corresponding author]
Scientific journal, Abstract

Europa’s tenuous atmosphere mainly consists of molecular oxygen, with traces of sodium, potassium, and other elements, potentially reflecting the composition of its surface and interior. While sodium- and magnesium-bearing hydrated salts have been suspected to exist on the surface, other materials like calcium-bearing salts dissolved in the subsurface ocean could also be present. Atmospheric emissions could originate from sputtering or sublimation processes, and potential water plume activity, possibly influenced by tidal interactions, could significantly contribute to the atmosphere. Additionally, the composition of the atmosphere and surface could be affected by neighboring moons, particularly Io. We searched for calcium emissions using the Multi-Spectral Imager of a 1.6 m Pirka telescope at Hokkaido University’s Astronomical Observatory. Observations from 2021 September to 2023 October revealed suggestive emission lines attributed to calcium atoms. These suggestive emission lines originated from the trailing hemisphere, suggesting that sputtering is likely the dominant atmospheric source.

A search for water vapor plumes on Europa by spatially resolved spectroscopic observation using Subaru/IRCS
Jun Kimura; Taro Matsuo; Hitomi Kobayashi; Yuji Ikeda; Kazuo Yoshioka; Seiko Takagi; Shigeru Ida
Publications of the Astronomical Society of Japan, Oxford University Press (OUP), 28 Oct. 2024, [Peer-reviewed]
English, Scientific journal, Abstract

We present near-infrared high-dispersion spectroscopic observations of Europa using the Infrared Camera and Spectrograph (IRCS) onboard the Subaru Telescope, seeking direct evidence of water plumes on Europa and exploring spatial variations in plume activity. Using the high spectral/spatial resolution and sensitivity of Subaru/IRCS, our observations have enabled a spatially resolved search for water plumes on Europa. Within our detection limits and time of observation, we found no evidence for the presence of water emission. For a rotation temperature of 50 K, we derived an upper limit on the H$_{2}$O abundance of 9.46 $\times$ 10$^{19}$–5.92 $\times$ 10$^{20}$ m$^{-2}$ in each divided slit area and 4.61 $\times$ 10$^{19}$ m$^{-2}$ in the entire area covered by the slit. This upper limit lies below the inferred water abundance from previous UV observations by the Hubble Space Telescope, while being less sensitive by a factor of three compared to the Keck telescope and by one order of magnitude or more than the James Webb Space Telescope observations. Our results align with previous studies and demonstrate that using Subaru/IRCS is an effective strategy for searching for water plumes on Europa with high spatial resolution. Continued observations across different surface areas and orbital phases are essential to fully characterize Europa’s plume activity and complement upcoming space missions.

New evidence supporting past dust ejections from active asteroid (4015) Wilson–Harrington
Sunho Jin; Masateru Ishiguro; Jooyeon Geem; Hiroyuki Naito; Jun Takahashi; Hiroshi Akitaya; Daisuke Kuroda; Seitaro Urakawa; Seiko Takagi; Tatsuharu Oono; Tomohiko Sekiguchi; Davide Perna; Simone Ieva; Yoonsoo P. Bach; Ryo Imazawa; Koji S. Kawabata; Makoto Watanabe; Hangbin Jo
Astronomy & Astrophysics, 690, A193, A193, EDP Sciences, 08 Oct. 2024, [Peer-reviewed]
Scientific journal, Context. (4015) Wilson-Harrington (hereafter, WH) was discovered as a comet in 1949 but has a dynamical property consistent with that of a near-Earth asteroid. Although there is a report that the 1949 activity is associated with an ion tail, the cause of the activity has not yet been identified.

Aims. This work aims to reveal the mysterious comet-like activity of the near-Earth asteroid.

Methods. We conducted new polarimetric observations of WH from May 2022 to January 2023, reanalyses of the photographic plate images taken at the time of its discovery in 1949, and dust tail simulation modelings, where the dust terminal velocity and ejection epoch are taken into account.

Results. We found that this object shows polarization characteristics similar to those of low-albedo asteroids. We derived the geometric albedo ranging from p_V = 0.076 ± 0.010 to p_V = 0.094 ± 0.018 from our polarimetry (the values vary depending on the data used for fitting and the slope-albedo relationship coefficients). In addition, the 1949 image showed an increase in brightness around the nucleus. Furthermore, we found that the color of the tail is consistent with sunlight, suggesting that the 1949 activity is associated with dust ejection. From the dust tail analysis, ~9 × 10⁵ kg of material was ejected episodically at a low velocity equivalent to or even slower than the escape velocity.

Conclusions. We conclude that WH is most likely an active asteroid of main belt origin and that the activity in 1949 was likely triggered by mass shedding due to fast rotation.

Study of hydrated asteroids via their polarimetric properties at low phase angles
Jooyeon Geem; Masateru Ishiguro; Hiroyuki Naito; Sunao Hasegawa; Jun Takahashi; Yoonsoo P. Bach; Sunho Jin; Seiko Takagi; Tatsuharu Ono; Daisuke Kuroda; Tomohiko Sekiguchi; Kiyoshi Kuramoto; Tomoki Nakamura; Makoto Watanabe
Astronomy & Astrophysics, 688, A195, A195, EDP Sciences, 22 Aug. 2024, [Peer-reviewed]
Scientific journal, Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated.

Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids.

Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations.

Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids.

Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.

Estimation of the drift rate and intensity of Neptune's storm by Pirika telescope
Yuki Sato; Yukihiro Takahashi; Mitsuteru Sato; Seiko Takagi; Masataka Imai; Tatsuharu Ono
02 May 2024

Spectroscopic and Imaging Observation of the Venus Atmosphere by a Balloon-Borne Stratospheric Telescope FUJIN-2
Masataka IMAI; Makoto TAGUCHI; Yasuhiro SHOJI; Toshihiko NAKANO; Yukihiro TAKAHASHI; Mitsuteru SATO; Tatsuharu OHNO; Kyoko TANAKA; Shion MORI; Daisuke KOHNO; Ko HAMAMOTO; Seiko TAKAGI
Journal of Evolving Space Activities, 1, Dec. 2023, [Peer-reviewed]
English, Scientific journal

Photometry and Polarimetry of 2010 XC₁₅: Observational Confirmation of E-type Near-Earth Asteroid Pair
Jin Beniyama; Shigeyuki Sako; Katsuhito Ohtsuka; Tomohiko Sekiguchi; Masateru Ishiguro; Daisuke Kuroda; Seirato Urakawa; Fumi Yoshida; Asami Takumi; Natsuho Maeda; Jun Takahashi; Seiko Takagi; Hiroaki Saito; Tatsuya Nakaoka; Tomoki Saito; Tomohito Ohshima; Ryo Imazawa; Masato Kagitani; Satoshi Takita
The Astrophysical Journal, 955, 2, 143, 143, American Astronomical Society, 27 Sep. 2023, [Peer-reviewed]
Scientific journal, Abstract

Asteroid systems such as binaries and pairs are indicative of the physical properties and dynamical histories of small solar system bodies. Although numerous observational and theoretical studies have been carried out, the formation mechanism of asteroid pairs is still unclear, especially for near-Earth asteroid (NEA) pairs. We conducted a series of optical photometric and polarimetric observations of a small NEA 2010 XC₁₅ in 2022 December to investigate its surface properties. The rotation period of 2010 XC₁₅ is possibly a few to several dozen hours and the color indices of 2010 XC₁₅ are derived as g − r = 0.435 ± 0.008, r − i = 0.158 ± 0.017, and r − z = 0.186 ± 0.009 in the Pan-STARRS system. The linear polarization degrees of 2010 XC₁₅ are a few percent at the phase angle range of 58°–114°. We found that 2010 XC₁₅ is a rare E-type NEA on the basis of its photometric and polarimetric properties. Taking the similarity of not only physical properties but also dynamical integrals and the rarity of E-type NEAs into account, we suppose that 2010 XC₁₅ and 1998 WT₂₄ are of common origin (i.e., an asteroid pair). These two NEAs are the sixth NEA pair and first E-type NEA pair ever confirmed, possibly formed by rotational fission. We conjecture that the parent body of 2010 XC₁₅ and 1998 WT₂₄ was transported from the main belt through the ν₆ resonance or Hungaria region.

Spectral Type and Geometric Albedo of (98943) 2001 CC21, the Hayabusa2# Mission Target
Jooyeon Geem; Masateru Ishiguro; Mikael Granvik; Hiroyuki Naito; Hiroshi Akitaya; Tomohiko Sekiguchi; Sunao Hasegawa; Daisuke Kuroda; Tatsuharu Oono; Yoonsoo P Bach; Sunho Jin; Ryo Imazawa; Koji S Kawabata; Seiko Takagi; Makoto Yoshikawa; Anlaug A Djupvik; Julie Thiim Gadeberg; Tapio Pursimo; Oliver Durfeldt Pedros; Jeppe Sinkbaek Thomsen; Zuri Gray
Monthly Notices of the Royal Astronomical Society: Letters, 525, 1, L17, L21, Oxford University Press (OUP), 12 Jun. 2023, [Peer-reviewed]
Scientific journal, Abstract

We conducted optical polarimetry and near-infrared spectroscopy of JAXA’s Hayabusa2# mission target, (98943) 2001 CC21, in early 2023. Our new observations indicate that this asteroid has a polarimetric inversion angle of ∼20○, absorption bands around 0.9 and 1.9 μm, and a geometric albedo of 0.23±0.04. All these features are consistent with those of S-types but inconsistent with L-types. Based on this evidence, we conclude that JAXA’s Hayabusa2# spacecraft will explore an S-type asteroid (more specifically, an L or LL chondrite-like object) with albedo and size (0.44–0.53 km when we assume the absolute magnitude of 18.8) similar to (25143) Itokawa.

Multicolor and multi-spot observations of Starlink’s Visorsat
Takashi Horiuchi; Hidekazu Hanayama; Masatoshi Ohishi; Tatsuya Nakaoka; Ryo Imazawa; Koji S Kawabata; Jun Takahashi; Hiroki Onozato; Tomoki Saito; Masayuki Yamanaka; Daisaku Nogami; Yusuke Tampo; Naoto Kojiguchi; Jumpei Ito; Masaaki Shibata; Malte Schramm; Yumiko Oasa; Takahiro Kanai; Kohei Oide; Katsuhiro L Murata; Ryohei Hosokawa; Yutaka Takamatsu; Yuri Imai; Naohiro Ito; Masafumi Niwano; Seiko Takagi; Tatsuharu Ono; Vladimir V Kouprianov
Publications of the Astronomical Society of Japan, 75, 3, 584, 606, Oxford University Press (OUP), 08 Apr. 2023, [Peer-reviewed]
Scientific journal, Abstract

This study provides the results of simultaneous multicolor observations for the first Visorsat (STARLINK-1436) and the ordinary Starlink satellite (STARLINK-1113) in the U, B, V, g′, r, i, RC, IC, $z$, J, H, and Ks bands to quantitatively investigate the extent to which Visorsat reduces its reflected light. Our results are as follows: (1) in most cases, Visorsat is fainter than STARLINK-1113, and the sunshade on Visorsat therefore contributes to the reduction of the reflected sunlight; (2) the magnitude at 550 km altitude (normalized magnitude) of both satellites often reaches the naked-eye limiting magnitude (<6.0); (3) from a blackbody radiation model of the reflected flux, the peak of the reflected components of both satellites is around the $z$ band; and (4) the albedo of the near-infrared range is larger than that of the optical range. Under the assumption that Visorsat and STARLINK-1113 have the same reflectivity, we estimate the covering factor, Cf, of the sunshade on Visorsat, using the blackbody radiation model: the covering factor ranges from 0.18 ≤ Cf ≤ 0.92. From the multivariable analysis of the solar phase angle (Sun–target–observer), the normalized magnitude, and the covering factor, the phase angle versus covering factor distribution presents a moderate anti-correlation between them, suggesting that the magnitudes of Visorsat depend not only on the phase angle but also on the orientation of the sunshade along our line of sight. However, the impact on astronomical observations from Visorsat-designed satellites remains serious. Thus, new countermeasures are necessary for the Starlink satellites to further reduce reflected sunlight.

Morpho-kinematic Modeling of the Expanding Ejecta of the Extremely Slow Nova V1280 Scorpii
Hiroyuki Naito; Akito Tajitsu; Valério A. R. M. Ribeiro; Akira Arai; Hiroyuki Maehara; Shinjirou Kouzuma; Takashi Iijima; Atsuo T. Okazaki; Makoto Watanabe; Seiko Takagi; Fumitake Watanabe; Itsuki Sakon; Kozo Sadakane
The Astrophysical Journal, 932, 1, 39, 01 Jun. 2022, [Peer-reviewed]
English, Scientific journal

A polarimetric study of asteroids in comet-like orbits
Jooyeon Geem; Masateru Ishiguro; Yoonsoo P. Bach; Daisuke Kuroda; Hiroyuki Naito; Hidekazu Hanayama; Yoonyoung Kim; Yuna G. Kwon; Sunho Jin; Tomohiko Sekiguchi; Ryo Okazaki; Jeremie J. Vaubaillon; Masataka Imai; Tatsuharu Oono; Yuki Futamura; Seiko Takagi; Mitsuteru Sato; Kiyoshi Kuramoto; Makoto Watanabe
Astronomy and Astrophysics, 2022, 658, Feb. 2022, [Peer-reviewed]

Polarimetric properties of the near-Sun asteroid (155140) 2005 UD in comparison with other asteroids and meteoritic samples
Masateru Ishiguro; Yoonsoo P Bach; Jooyeon Geem; Hiroyuki Naito; Daisuke Kuroda; Myungshin Im; Myung Gyoon Lee; Jinguk Seo; Sunho Jin; Yuna G Kwon; Tatsuharu Oono; Seiko Takagi; Mitsuteru Sato; Kiyoshi Kuramoto; Takashi Ito; Sunao Hasegawa; Fumi Yoshida; Tomoko Arai; Hiroshi Akitaya; Tomohiko Sekiguchi; Ryo Okazaki; Masataka Imai; Katsuhito Ohtsuka; Makoto Watanabe; Jun Takahashi; Maxime Devogèle; Grigori Fedorets; Lauri Siltala; Mikael Granvik
Monthly Notices of the Royal Astronomical Society, 509, 3, 4128, 4142, Oxford University Press (OUP), 09 Nov. 2021, [Peer-reviewed]
Scientific journal

Implications of High Polarization Degree for the Surface State of Ryugu
Daisuke Kuroda; Jooyeon Geem; Hiroshi Akitaya; Sunho Jin; Jun Takahashi; Koki Takahashi; Hiroyuki Naito; Kana Makino; Tomohiko Sekiguchi; Yoonsoo P. Bach; Jinguk Seo; Shuji Sato; Hiroshi Sasago; Koji S. Kawabata; Aoi Kawakami; Miyako Tozuka; Makoto Watanabe; Seiko Takagi; Kiyoshi Kuramoto; Makoto Yoshikawa; Sunao Hasegawa; Masateru Ishiguro
The Astrophysical Journal Letters, 911, 2, L24, L24, 01 Apr. 2021, [Peer-reviewed]
English, Scientific journal

(85989) 1999 JD6 : a first Barbarian asteroid detected by polarimetry in the NEA population
Daisuke Kuroda; Masateru Ishiguro; Hiroyuki Naito; Makoto Watanabe; Sunao Hasegawa; Seiko Takagi; Kiyoshi Kuramoto
Astronomy & Astrophysics, 646, A51, A51, Feb. 2021, [Peer-reviewed]
English, Scientific journal

SN 2019ein: New Insights into the Similarities and Diversity among High-velocity Type Ia Supernovae
Miho Kawabata; Keiichi Maeda; Masayuki Yamanaka; Tatsuya Nakaoka; Koji S. Kawabata; Ryo Adachi; Hiroshi Akitaya; Umut Burgaz; Hidekazu Hanayama; Takashi Horiuchi; Ryohei Hosokawa; Kota Iida; Fumiya Imazato; Keisuke Isogai; Ji-an Jiang; Noriyuki Katoh; Hiroki Kimura; Masaru Kino; Daisuke Kuroda; Hiroyuki Maehara; Kazuya Matsubayashi; Kumiko Morihana; Katsuhiro L. Murata; Takashi Nagao; Masafumi Niwano; Daisaku Nogami; Motoki Oeda; Tatsuharu Ono; Hiroki Onozato; Masaaki Otsuka; Tomoki Saito; Mahito Sasada; Kazuki Shiraishi; Haruki Sugiyama; Kenta Taguchi; Jun Takahashi; Kengo Takagi; Seiko Takagi; Masaki Takayama; Miyako Tozuka; Kazuhiro Sekiguchi
The Astrophysical Journal, 893, 2, 143, 143, American Astronomical Society, 24 Apr. 2020, [Peer-reviewed]
Scientific journal

An uppermost haze layer above 100 km found over Venus by the SOIR instrument onboard Venus Express
Seiko Takagi; Arnaud Mahieux; Valérie Wilquet; Séverine Robert; Ann Carine Vandaele; Naomoto Iwagami
Earth, Planets and Space, 71, 1, Springer Science and Business Media LLC, 21 Nov. 2019
Scientific journal, Abstract

The Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express was designed to measure the Venusian atmospheric transmission at high altitudes (65–220 km) in the infrared range (2.2–4.3 μm) with a high spectral resolution. In this work, we investigate the optical properties of Venus’s haze layer above 90 km using SOIR solar occultation observations. Vertical and latitudinal profiles of the extinction coefficient, optical thickness, and mixing ratio of aerosols are retrieved. One of the most remarkable results is that the aerosol mixing ratio tends to increase with altitude above 90 km at both high and low latitudes. We speculate how aerosols could be produced at such high altitudes.

Diagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM Cephei
P. C. Huang; W. P. Chen; M. Mugrauer; R. Bischoff; J. Budaj; O. Burkhonov; S. Ehgamberdiev; R. Errmann; Z. Garai; H. Y. Hsiao; C. L. Hu; R. Janulis; E. L. N. Jensen; S. Kiyota; K. Kuramoto; C. S. Lin; H. C. Lin; J. Z. Liu; O. Lux; H. Naito; R. Neuhauser; J. Ohlert; E. ̇Pakstienė; T. Pribulla; J. K. T. Qvam; St. Raetz; S. Sato; M. Schwartz; E. Semkov; S. Takagi; D. Wagner; M. Watanabe; Yu Zhang
The Astrophysical Journal, 871, 183, Feb. 2019, [Peer-reviewed]
English

Morphology and Dynamics of Venus’s Middle Clouds With Akatsuki/IR1
J. Peralta; N. Iwagami; A. Sánchez‐Lavega; Y. J. Lee; R. Hueso; M. Narita; T. Imamura; P. Miles; A. Wesley; E. Kardasis; S. Takagi
Geophysical Research Letters, 46, 5, Feb. 2019, [Peer-reviewed]
English

X-Ray, Optical, and Near-infrared Monitoring of the New X-Ray Transient MAXI J1820+070 in the Low/Hard State
Megumi Shidatsu; Satoshi Nakahira; Satoshi Yamada; Taiki Kawamuro; Yoshihiro Ueda; Hitoshi Negoro; Katsuhiro L. Murata; Ryosuke Itoh; Yutaro Tachibana; Ryo Adachi; Yoichi Yatsu; Nobuyuki Kawai; Hidekazu Hanayama; Takashi Horiuchi; Hiroshi Akitaya; Tomoki Saito; Masaki Takayama; Tomohito Ohshima; Noriyuki Katoh; Jun Takahashi; Takahiro Nagayama; Masayuki Yamanaka; Miho Kawabata; Tatsuya Nakaoka; Seiko Takagi; Tomoki Morokuma; Kumiko Morihana; Hiroyuki Maehara; Kazuhiro Sekiguchi
Astrophysical Journal, 868, 1, 20 Nov. 2018, [Peer-reviewed]
English, Scientific journal

High polarization degree of the continuum of comet 2P/Encke based on spectropolarimetric signals during its 2017 apparition
Y. G. Kwon; M. Ishiguro; Y. Shinnaka; T. Nakaoka; D. Kuroda; H. Hanayama; J. Takahashi; S. Baar; T. Saito; M. Kawabata; M. Uemura; T. Morokuma; K. L. Murata; Seiko Takagi; Kumiko Morihana; Takahiro Nagayama; K. Sekiguchi; K. S. Kawabata; H. Akitaya
Astronomy & Astrophysics, Oct. 2018, [Peer-reviewed]

Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images
Sanjay S. Limaye; Shigeto Watanabe; Atsushi Yamazaki; Manabu Yamada; Takehiko Satoh; Takao M. Sato; Masato Nakamura; Makoto Taguchi; Tetsuya Fukuhara; Takeshi Imamura; Toru Kouyama; Yeon Joo Lee; Takeshi Horinouchi; Javier Peralta; Naomoto Iwagami; George L. Hashimoto; Seiko Takagi; Shoko Ohtsuki; Shin‐ya Murakami; Yukio Yamamoto; Kazunori Ogohara; Hiroki Ando; Ko‐ichiro Sugiyama; Nobuaki Ishii; Takumi Abe; Chikako Hirose; Makoto Suzuki; Naru Hirata; Eliot F. Young; Adriana C. Ocampo
Earth, Planets and Space, Feb. 2018, [Peer-reviewed]
English, Scientific journal

Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results
Atsushi Yamazaki; Manabu Yamada; Yeon Joo Lee; Shigeto Watanabe; Takeshi Horinouchi; Shin‐ya Murakami; Toru Kouyama; Kazunori Ogohara; Takeshi Imamura; Takao M. Sato; Yukio Yamamoto; Tetsuya Fukuhara; Hiroki Ando; Ko‐ichiro Sugiyama; Seiko Takagi; Hiroki Kashimura; Shoko Ohtsuki; Naru Hirata; George L. Hashimoto; Makoto Suzuki; Chikako Hirose; Munetaka Ueno; Takehiko Satoh; Takumi Abe; Nobuaki Ishii; Masato Nakamura
Earth, Planets and Space, 70, Feb. 2018, [Peer-reviewed]
English, Scientific journal

Initial products of AKATSUKI 1-um Camera
Naomoto Iwagami; Takeshi Sakanoi; George L. Hashimoto; Kenta Sawai; Shoko Ohtsuki; Seiko Takagi; Kazunori Uemizu; Munetaka Ueno; Shingo Kameda; Shin‐ya Murakami; Masato Nakamura; Nobuaki Ishii; Takumi Abe; Takehiko Satoh; Takeshi Imamura; Chikako Hirose; Makoto Suzuki; Naru Hirata; Atsushi Yamazaki; Takao M. Sato; Manabu Yamada; Yukio Yamamoto; Tetsuya Fukuhara; Kazunori Ogohara; Hiroki Ando; Ko‐ichiro Sugiyama; Hiroki Kashimura; Toru Kouyama
Earth, Planets and Space, Jan. 2018, [Peer-reviewed]
English

Topographical and Local Time Dependence of Large Stationary Gravity Waves Observed at the Cloud Top of Venus
T. Kouyama; T. Imamura; M. Taguchi; T. Fukuhara; T. M. Sato; A. Yamazaki; M. Futaguchi; S. Murakami; G. L. Hashimoto; M. Ueno; N. Iwagami; S. Takagi; M. Takagi; K. Ogohara; H. Kashimura; T. Horinouchi; N. Sato; M. Yamada; Y. Yamamoto; S. Ohtsuki; K. Sugiyama; H. Ando; M. Takamura; T. Yamada; T. Satoh; M. Nakamura
Geophysical Research Letters, 44, 24, 12, 105, Blackwell Publishing Ltd, 28 Dec. 2017, [Peer-reviewed]
English, Scientific journal

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki
Takeshi Imamura; Hiroki Ando; Silvia Tellmann; Martin Pätzold; Bernd Häusler; Atsushi Yamazaki; Takao M. Sato; Katsuyuki Noguchi; Yoshifumi Futaana; Janusz Oschlisniok; Sanjay Limaye; R. K. Choudhary; Yasuhiro Murata; Hiroshi Takeuchi; Chikako Hirose; Tsutomu Ichikawa; Tomoaki Toda; Atsushi Tomiki; Takumi Abe; Zen Ichi Yamamoto; Hirotomo Noda; Takahiro Iwata; Shin Ya Murakami; Takehiko Satoh; Tetsuya Fukuhara; Kazunori Ogohara; Ko Ichiro Sugiyama; Hiroki Kashimura; Shoko Ohtsuki; Seiko Takagi; Yukio Yamamoto; Naru Hirata; George L. Hashimoto; Manabu Yamada; Makoto Suzuki; Nobuaki Ishii; Tomoko Hayashiyama; Yeon Joo Lee; Masato Nakamura
Earth, Planets and Space, 69, 1, 01 Dec. 2017, [Peer-reviewed]
English

Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki
Tetsuya Fukuhara; Makoto Taguchi; Takeshi Imamura; Akane Hayashitani; Takeru Yamada; Masahiko Futaguchi; Toru Kouyama; Takao M. Sato; Mao Takamura; Naomoto Iwagami; Masato Nakamura; Makoto Suzuki; Munetaka Ueno; George L. Hashimoto; Mitsuteru Sato; Seiko Takagi; Atsushi Yamazaki; Manabu Yamada; Shin-ya Murakami; Yukio Yamamoto; Kazunori Ogohara; Hiroki Ando; Ko-ichiro Sugiyama; Hiroki Kashimura; Shoko Ohtsuki; Nobuaki Ishii; Takumi Abe; Takehiko Satoh; Chikako Hirose; Naru Hirata
EARTH PLANETS AND SPACE, 69, 141, Oct. 2017, [Peer-reviewed]
English, Scientific journal

Large stationary gravity wave in the atmosphere of Venus
Tetsuya Fukuhara; Masahiko Futaguchi; George L. Hashimoto; Takeshi Horinouchi; Takeshi Imamura; Naomoto Iwagaimi; Toru Kouyama; Shin-ya Murakami; Masato Nakamura; Kazunori Ogohara; Mitsuteru Sato; Takao M. Sato; Makoto Suzuki; Makoto Taguchi; Seiko Takagi; Munetaka Ueno; Shigeto Watanabe; Manabu Yamada; Atsushi Yamazaki
NATURE GEOSCIENCE, 10, 2, 85, +, Feb. 2017, [Peer-reviewed]
English, Scientific journal

AKATSUKI returns to Venus
Masato Nakamura; Takeshi Imamura; Nobuaki Ishii; Takumi Abe; Yasuhiro Kawakatsu; Chikako Hirose; Takehiko Satoh; Makoto Suzuki; Munetaka Ueno; Atsushi Yamazaki; Naomoto Iwagami; Shigeto Watanabe; Makoto Taguchi; Tetsuya Fukuhara; Yukihiro Takahashi; Manabu Yamada; Masataka Imai; Shoko Ohtsuki; Kazunori Uemizu; George L. Hashimoto; Masahiro Takagi; Yoshihisa Matsuda; Kazunori Ogohara; Naoki Sato; Yasumasa Kasaba; Toru Kouyama; Naru Hirata; Ryosuke Nakamura; Yukio Yamamoto; Takeshi Horinouchi; Masaru Yamamoto; Yoshi-Yuki Hayashi; Hiroki Kashimura; Ko-ichiro Sugiyama; Takeshi Sakanoi; Hiroki Ando; Shin-ya Murakami; Takao M. Sato; Seiko Takagi; Kensuke Nakajima; Javier Peralta; Yeon Joo Lee; Junichi Nakatsuka; Tsutomu Ichikawa; Kozaburo Inoue; Tomoaki Toda; Hiroyuki Toyota; Sumitaka Tachikawa; Shinichiro Narita; Tomoko Hayashiyama; Akiko Hasegawa; Yukio Kamata
EARTH PLANETS AND SPACE, 68, 75, May 2016, [Peer-reviewed]
English, Scientific journal

Ground-based IR observation of oxygen isotope ratios in Venus's atmosphere
N. Iwagami; G. L. Hashimoto; S. Ohtsuki; S. Takagi; S. Robert
PLANETARY AND SPACE SCIENCE, 113, 292, 297, Aug. 2015, [Peer-reviewed]
English, Scientific journal

Science requirements and description of the 1 mu m camera onboard the Akatsuki Venus Orbiter
Naomoto Iwagami; Seiko Takagi; Shoko Ohtsuki; Munetaka Ueno; Kazunori Uemizu; Takehiko Satoh; Takeshi Sakanoi; George L. Hashimoto
EARTH PLANETS AND SPACE, 63, 6, 487, 492, 2011, [Peer-reviewed]
English, Scientific journal

Contrast sources for the infrared images taken by the Venus mission AKATSUKI
Seiko Takagi; Naomoto Iwagami
EARTH PLANETS AND SPACE, 63, 5, 435, 442, 2011, [Peer-reviewed]
English, Scientific journal

Polarimetry of (155140) 2005 UD: the Dynamical Association with (3200) Phaethon
ISHIGURO Masateru; GEEM Jooyeon; NAITO Hiroyuki; BACH Yoonsoo P; KURODA Daisuke; ONO Tatsuharu; TAKAGI Seiko; SATO Mitsuteru; KURAMOTO Kiyoshi; SEKIGUCHI Tomohiko; IMAI Masataka; WATANABE Makoto; OHTSUKA Katsuhito; HASEGAWA Sunao; ITO Takashi; YOSHIDA Fumi; ARAI Tomoko, 日本地球惑星科学連合大会予稿集(Web), 2019, ROMBUNNO.PPS03‐P17 (WEB ONLY), 2019
English

Polarimetric Study of Asteroids in Comet‐Like Orbits (ACOs)
GEEM Jooyeon; ISHIGURO Masateru; BACH Yoonsoo P; KURODA Daisuke; NAITO Hiroyuki; HANAYAMA Hidekazu; KIM Yoonyoung; KWON Yuna G; JIN Sunho; SEKIGUCHI Tomohiko; OKAZAKI Ryo; VAUBAILLON Jeremie J; IMAI Masataka; OONO Tatsuharu; FUTAMUTS Yuki; TAKAGI Seiko; SATO Mitsuteru; KURAMOTO Kiyoshi; WATANABE Makoto, 日本地球惑星科学連合大会予稿集(Web), 2019, ROMBUNNO.PPS03‐P04 (WEB ONLY), 2019
English

一番星へ行こう!日本の金星探査機の挑戦その39 「第74回藤原セミナー/International Venus Conference 2019」開催報告
高木聖子, 日本惑星科学会誌「遊星人」, 28, 3, 226, 230, 2019

一番星へ行こう!日本の金星探査機の挑戦その30 〜1 umカメラで金星昼＆夜〜
岩上直幹; はしもとじょーじ; 澤井健太; 坂野井健; 高木聖子; 亀田真吾, 日本惑星科学会誌「遊星人」, 26, 1, 21, 22, 2017

「あかつき」搭載中間赤外カメラによる金星雲頂温度観測
田口 真; 福原 哲哉; 神山 徹; 今村 剛; 中村 正人; 鈴木 睦; 佐藤 隆雄; 上野 宗孝; 岩上 直幹; はしもと じょーじ; 佐藤 光輝; 高木 聖子, Mar. 2016
第29回大気圏シンポジウム（2016年3月7日-8日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県資料番号: SA6000051002レポート番号: I-2, Japanese

一番星へ行こう! 日本の金星探査機の挑戦その29 〜巨大な弓形雲頂温度構造の発見〜
田口真; 福原哲哉; 今村剛; 神山徹; 二口将彦; はしもとじょーじ; 鈴木睦; 上野宗孝; 岩上直幹; 佐藤隆雄; 佐藤光輝; 高木聖子; 中村正人, 日本惑星科学会誌「遊星人」, 25, 4, 2, 16, 2016
Japanese, Report scientific journal

A search for emission lines in the atmosphere of the Galilean moons with ground-based telescope
Japan Geoscience Union Meeting 2022, 24 May 2022

Weathering of the surface layer of ice satellites revealed by ground-based telescopes
Seiko Takagi
The International Symposium on Planetary Science 2022
[Invited]

大学院共通授業科目（一般科目）：自然科学・応用科学, 2024年, 修士課程, 大学院共通科目

物質進化学特論２, 2024年, 修士課程, 理学院

宇宙理学入門, 2024年, 修士課程, 理学院

物理学Ⅱ, 2024年, 学士課程, 全学教育

情報実習, 2024年, 学士課程, 理学部

教科教育法（理科Ⅰ）, 2024年, 学士課程, 教育学部

地球惑星科学のための物理数学Ⅱ, 2024年, 学士課程, 理学部

地球惑星科学のための物理数学Ⅱ演習, 2024年, 学士課程, 理学部

地上望遠鏡で探る木星系の形成環境と進化過程
科学研究費助成事業
01 Apr. 2026 - 31 Mar. 2029
高木 聖子
日本学術振興会, 基盤研究(C), 北海道大学, 26K07160

Establishment of a Prediction Method for Typhoon Genesis and Development Based on Lightning Observations in Asia and Vortex Gas Mathematical Models
Grants-in-Aid for Scientific Research
Apr. 2025 - Mar. 2029
佐藤 光輝; 久保田 尚之; 高木 聖子; 木村 芳文; 藤原 宏志; 芳松 克則
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 25H00776

Spatiotemporal variation of planetary lightning activities measured by ground-based telescope and spacecrafts and its relation to atmospheric circulations
Grants-in-Aid for Scientific Research
01 Apr. 2020 - 31 Mar. 2024
佐藤 光輝; 高木 聖子; 杉山 耕一朗; 今井 正尭
太陽系の金星および木星に焦点をあて、両惑星の雷放電を観測する惑星雷放電発光検出装置（PLD）を新規開発し、北海道大学附属天文台光学望遠鏡（ピリカ）に搭載する。ピリカ望遠鏡を専有することで長期連続観測を実現し、(1)両惑星での雷活動の時間・空間変動を特定する、(2)金星における雷放電発生の可能性に決着をつけ、金星探査機あかつきとの同時観測から雷放電発生点の雲対流構造を特定する、(3)木星大気雲解像モデルの数値計算と観測結果との比較から、木星雷雲の鉛直・水平対流の結合過程を検証する、これらを目的とする。
2021年度はPLDの設計を進め、2020年度で未購入であった必要部品の選定と購入を行うとともに、設計変更に応じた部品等の調達を適宜行った。本年度は筐体の最終組み上げを行い、機能・性能試験とピリカ望遠鏡に搭載しての試験観測を実施する予定であったが、新型コロナウイルスによって研究活動が大幅に制限されたため、前年度に引き続きこれらを実施することができなかった。同時に、海外の学会に参加して本研究計画の紹介や、海外研究者と連携した共同研究の計画を企画する予定であったが、参加予定にしていた全ての国際学会の開催がオンライン形式となり、有益な成果を残すことができなかった。また、調達物品に関しても、世界的な半導体不足の影響を受けて納期が間に合わない等の事態が度々発生した。これらの点から、全体的に進捗は計画よりもやや遅れ気味である。
2022年度は、遅れを取りもどずべく、PLDの機能・性能試験とピリカ望遠鏡に搭載しての試験観測を早期に実施することを目標とする。
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), Hokkaido University, 20H01954

金星雲に働く物理・化学機構と大気ダイナミクスの理解
稲盛研究助成
Apr. 2020 - Mar. 2021
稲盛財団, Principal investigator

「小惑星「2001 CC21」は「イトカワ」に似ていることが判明 JAXA探査機「はやぶさ2」の接近探査予定天体
Aug. 2023
sorae
[Internet]

「はやぶさ2」が次に目指す小惑星、イトカワと類似
Jul. 2023
アストロアーツ
[Internet]

「はやぶさ2」の次の訪問天体は小惑星イトカワに似ている!?
Jul. 2023
アストロピクス
[Internet]

北大など、はやぶさ2がフライバイ観測予定の小惑星を地上から詳細に観測
Jul. 2023
マイナビニュース
[Internet]

「はやぶさ2」探査の小惑星、主成分は石質 北大など組成解明
Jul. 2023
日刊工業新聞
[Paper]