PPARCセミナー (2026/2/13)

PPARCセミナー (2026/2/13)

(1)

[Name]

Naoko Takatori
 

[Title]

ハレアカラT60望遠鏡のファイバー面分光器を用いた水星Na外圏大気の時空間変動の観測

Observation of spatio-temporal variations in Mercury’s sodium exosphere using the Haleakala T60 telescope with the fiber-fed spectrograph

[Abstract]

　水星はアルカリ金属を含む非常に希薄な外圏大気を有している。特に中性NaのD線（589.0nm、589.6nm）による共鳴散乱発光は明るく、地上の中小口径望遠鏡でも観測可能である。水星Na外圏大気は輝線の強さや分布が1時間未満の短時間変動から季節変動に至るまで、様々な時間スケールで変化する事が知られている。変動成分のうち短時間変動（10-20%）は外部環境（太陽風・磁気圏）に、季節変動（80%）は内部・表面プロセス（放出・移動・消失）に起因すると考えられている。
　従来の地上観測で使用されているスリット分光で水星全球分布を捉えるには、スリット移動により約1時間を要する。従来観測された短時間変動とされる数分オーダーの変動は狭視野観測で得られたものであり、全球分布の短時間変動観測には新たな手段が必要となる。
　私たちは、東北大学60cm望遠鏡（T60）にファイバー面分光器と補償光学（Adaptive Optics）システムを結合させ、水星Na外圏大気の全球空間分布の変動を数分スケールで捉えることを目指した開発を進めてきた。2026年秋のBepiColombo探査機の水星周回軌道投入後には、Mio探査機に搭載されたNa発光観測カメラMSASIによる外圏大気リモート観測を支え、同探査機による太陽風・磁気圏のその場観測と協調した観測を行う予定である。
　T60はハワイ・マウイ島ハレアカラ山頂（海抜3,040m）に位置し、太陽離角>17°の場合に遠隔操作で継続的観測が可能である。この望遠鏡に装着したファイバー面分光器は、ファイバー視野集積器と結合した可視高分散分光器（波長分解能~67,000）からなる。ファイバー視野集積器は1ファイバーあたり1.58”×1.58”の視野を持ち、120本のファイバーを2次元（10×12）に配列して15.8”×19.0”の視野を面分光する。観測中は、本装置に結合させたAOシステムで、昼間の荒れたシーイングを補正している。このAOは、2025年春には朝方の観測でFWHM~1”程度の空間分解能を達成しており、午前中や夕方といった大気が比較的安定する条件下ではファイバー視野を分解するに十分である。
　現在は、2025/8/15~8/24のシーイングが比較的良い朝方の観測に対し、短時間変動・長期変動のそれぞれを導出するための解析を行っている。短時間変動では水星diskを北、赤道、南の緯度領域に分離し、それぞれの変動の割合を導出しているところである。また、長期変動では過去の先行研究（Leblunc+2022）に倣い、LT、TAA、水星東経におけるNa発光の分布を導出しているところである。
　本発表では、水星Na発光の変動の中でも、水星Na外圏大気の軌道上における変動（長期・季節変動）に関する先行研究（Milillo+2021）に関する論文紹介を行うとともに、自身の研究の進捗と今後の展望を示す。

Mercury possesses an extremely tenuous exosphere containing alkali metals. In particular, the resonance scattering emission of neutral sodium (Na D lines: 589.0 nm, 589.6 nm) is bright enough to be observable even with small-to-medium ground-based telescopes. It is known that the intensity and distribution of Mercury’s Na exosphere vary across diverse timescales, ranging from short-term fluctuations of less than an hour to seasonal variations. Among these components, short-term variations (10–20%) are thought to be related to the response of the solar wind and the Interplanetary Magnetic Field (IMF), while seasonal variations (80%) are attributed to complex mechanisms involving surface release processes, loss, sources, and the migration of sodium atoms.
Conventional ground-based observations using slit spectroscopy require approximately one hour to capture the global distribution of Mercury due to the necessity of slit scanning. Previously observed short-term variations on the order of minutes were obtained through narrow-field observations; therefore, a new method is required to observe short-term variations in the global distribution.
We have been developing a system for the Tohoku University 60cm telescope (T60) that combines a fiber-fed Integral Field Unit (IFU) spectrograph with an Adaptive Optics (AO) system, aiming to capture global spatial distribution variations of Mercury’s Na exosphere on a minute-scale. Following the orbit insertion of the BepiColombo spacecraft in the autumn of 2026, we plan to support remote exospheric observations by the Na emission atmospheric camera (MSASI) onboard the Mio orbiter and perform coordinated observations with the orbiter’s in-situ measurements of the solar wind and magnetosphere.
The T60 is located at the summit of Mt. Haleakala, Hawaii (3,040 m above sea level), and is capable of continuous remote observation when the solar elongation is greater than 17°. The instrument mounted on this telescope consists of a visible high-dispersion spectrograph (spectral resolution ~67,000) coupled with an IFU. The IFU has a field of view of 1.58”×1.58” per fiber, with 120 fibers arranged in a 2D array (10×12) to perform integral field spectroscopy over a 15.8”×19.0” field. During observations, the integrated AO system corrects for the turbulent daytime seeing. As of Spring 2025, this AO system has achieved a spatial resolution of FWHM ~1” during morning observations, which is sufficient to resolve the fiber fields under relatively stable atmospheric conditions, such as during the morning or evening.
Currently, we are analyzing morning observations from August 15–24, 2025, characterized by relatively good seeing, to derive both short-term and long-term variations. For the short-term variations, we are dividing the Mercurian disk into northern, equatorial, and southern latitudinal regions to derive their respective fluctuation ratios. For the long-term variations, following previous research (Leblanc et al., 2022), we are deriving the distribution of Na emissions as a function of Local Time (LT), True Anomaly Angle (TAA), and Mercurian East Longitude.
In this presentation, I will introduce previous research regarding orbital (long-term/seasonal) variations in Mercury’s Na exosphere (Milillo et al., 2021) and present the current progress of my research and future prospects.

(2)

[Name]

Masamichi Waga

[Title]

Review: Search for secular changes in the 3D profile of the synchrotron radiation around Jupiter, Dun et al. 2003他

[Abstract]

We present a summary of Jupiter data taken over an eighteen year span (1981–1998) by the Very Large Array at ∼21.0 cm. At this wavelength the emission is dominated by synchrotron radiation, which is roughly proportional to the product of the electron number density and magnetic field strength (NeB). At each epoch 8–12 hours of data were taken, which allowed us to examine Jupiter during an entire rotation period. We mapped the longitudinal structure of the synchrotron radiation by using a 3D reconstruction technique developed by Sault et al. [Astron. Astrophys. 324 (1997) 1190] which enabled us to produce plots of the latitude, radial distance, and peak intensity vs. jovian longitude (System III). The results show the shape of the synchrotron radiation has remained stable (except, of course, during the period of comet Shoemaker–Levy 9 impacts). Specifically, the latitudinal structure has remained nearly constant. Furthermore, the general dependence of the radial intensity profile has remained the same throughout the years, though radial distance has slightly, though significantly, changed. This constancy implies that the spatial structure of both the particle distribution and magnetic field have varied little over the eighteen year span. The primary changes in the synchrotron radiation have been seen in the intensity of emission as a function of time. There are certain epochs (e.g., 1987) which show more emissivity than others (e.g., 1981, 1995) at all longitudes. When each epoch is longitudinally averaged, there may be an anti-correlation between the radial distance and corresponding peak intensities of the synchrotron radiation, as one might expect if radial diffusion is important. We examine these trends by comparing the data to plots of the total intensity at 13 cm (by Klein et al., in: Rucker, H.O., et al., Planetary Radio Emissions V. Austrian Acad. Sci. Press, Vienna, p. 221). Overall, variations in our 21-cm data are similar to those measured at 13 cm, but there appears to be a change in spectral index and perhaps in the spatial brightness distribution in 1992. We attribute this to a change in both the spatial and energy distribution of the relativistic electrons.