Observation of Mercury’s sodium tail using Fabry–Perot Interferometer

We observed sodium emission from Mercury’s atmosphere using a Fabry–Perot Interferometer at Haleakala Observatory on June 14, 2006. The Fabry–Perot Interferometer was used as a wavelength-tunable filter. The spectra of the surface reflection were subtracted from the observed spectra because sodium emission is contaminated by the surface reflection of Mercury. The image obtained in our observation shows the sodium exosphere extended to the anti-solar direction. The lifetime of a sodium atom was estimated to be 1.6 × 10⁴ to 1.9 × 10⁵ s with an error by a factor of 3–4.     

Attempt to identify a source mechanism of Mercury’s sodium exosphere by a spectrometer using Fabry–Perot etalon

The Mercury’s Sodium Atmosphere Spectral Imager (MSASI) on BepiColombo (BC) will address a range of fundamental scientific questions pertaining to Mercury’s exosphere. The measurements will provide new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral range around sodium D2 emission (589 nm). A tandem Fabry–Perot etalon is used to achieve a compact design. We presents a design of the spectral analyzer using Fabry–Perot interferometer. We conclude that: (1) The MSASI optical design is practical and can be implemented without new or critical technology developments; (2) The thermally-tuned etalon design is based on concepts, designs and materials that have good space heritage.     

Optical performance of PHEBUS/EUV detector onboard BepiColombo

BepiColombo, a mission of ESA (European Space Agency) in cooperation with JAXA (Japan Aerospace Exploration Agency), will explore Mercury, the planet closest to the Sun. BepiColombo will launch in 2014 on a journey lasting up to six and a half years; the data gathering phase should occupy a one year nominal mission, with a possible extension of another year. The data which will be brought back from the orbiters will tell us about the Hermean surface, atmospheric composition, and magnetospheric dynamics; it will also contribute to understanding the history and formation of terrestrial planets. The PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) instrument will be flown on MPO: Mercury Planetary Orbiter, one of the two BepiColombo orbiters. The main purpose of the instrument is to reveal the composition and the distribution of the exosphere of Mercury through EUV (Extreme Ultraviolet: 55–155 nm) and FUV (Far Ultraviolet: 145–315 nm) measurements. A consortium composed of four main countries has been formed to build it. Japan provides the two detectors (EUV and FUV), Russia implements the scanning system, and France and Italy take charge of the overall design, assembly, test, integration, and also provide two small NUV (Near Ultraviolet) detectors (for the light from calcium and potassium molecules). An optical prototype of the EUV detector which is identical to the flight configuration has been manufactured and evaluated. In this paper, we show the first spectra results observed by the EUV channel optical prototype. We also describe the design of PHEBUS and discuss the possibility of detecting noble gases in Mercury’s exosphere taking the experimental results so far into account.     

Development of the EUV detector for the BepiColombo mission

An ultraviolet spectrometer, PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) that is loaded onto the Mercury Planetary Orbiter in the BepiColombo mission is under development. The instrument, basically consisting of two spectrophotometers (EUV: 50–150 nm, FUV: 145–330 nm) and one scanning mirror, aims at measuring emission lines from molecules, atoms and ions present in the tenuous atmosphere of Mercury. The detectors employ microchannel plates as 2-D photon-counting devices. In order to enhance the quantum detection efficiencies, the surface of the top microchannel plates of EUV detector is covered with photocathode. This method enables us to identify weak atmospheric signatures such as neon (73.5 nm) and argon (104.8 nm), which could not be detected with conventional detector systems. This paper presents measurements of the performance characteristics of potassium bromide and esium iodide photocathodes, which have been evaluated for use in the EUV channel.     

MEMORIS: a wide angle camera for the BepiColombo mission

MEMORIS (MErcury Moderate Resolution Imaging System) is a wide angle camera (WAC) concept for the ESA mission BepiColombo. The main scientific objectives consist of observing the whole surface of Mercury in the spectral range of 400–1000 nm, with a spatial resolution of 50 m per pixel at peri-Herm (400 km) and 190 m at apo-Herm (1500 km). It will obtain a map of Mercury in stereo mode allowing the determination of a digital elevation model with a panchromatic filter through two different channels. The camera will also perform multispectral imaging of the surface with a set of 8–12 different broad band filters. A third channel dedicated to limb observations will provide images of the atmosphere. MEMORIS will thus monitor the surface and the atmosphere during the entire mission, providing a unique opportunity to study the relationship between surface regions and the atmosphere, as suggested by ground-based observations and theory.     

Current status of the BepiColombo/MMO spacecraft design

This paper shows the current baseline of the conceptual design of the BepiColombo/MMO (mercury magnetospheric orbiter) spacecraft, which is conducted by the ISAS Mercury Exploration Working Group. The MMO is a spinning spacecraft of 200 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and subsystems such as thermal control, communication, power, etc. are described. The latest status of the development of critical technologies for the MMO and the outline of the international cooperation between ESA and ISAS are also presented.     

The BepiColombo/MMO model payload and operation plan

The Institute of Space and Astronautical Science (ISAS) of Japan plans to contribute the Mercury Magnetospheric Orbiter (MMO) to the BepiColombo program, the ESA Cornerstone mission to the planet Mercury. The principal objective of the MMO is to study the magnetic field and magnetosphere of Mercury. The ISAS Mercury exploration working group has performed the definition study of the MMO mission in cooperation with the ESA/ESTEC BepiColombo project team. This paper briefly reviews the scientific objectives, and describes the model payload and its operation plan.     

Mercury’s Birkeland current system

A consideration is given to the generation of field-aligned currents under different solar wind conditions. The preliminary results from a set of resistive MHD calculations indicate that the field-aligned current system could be significantly changed by the orientation of the interplanetary magnetic field. For most of the cases studied, the total current is less than or on the order of 10⁵ A. Even though this current is at least a factor of 10 smaller than its counter part at Earth, it might still produce some important dynamical effects with interesting consequence on the sporadic behavior of Mercury’s atomic sodium emission.     

Development of an extreme ultraviolet imaging spectrometer for the BepiColombo mission

Extreme and far ultraviolet imaging spectrometers are proposed for the low-altitude orbiter of the BepiColombo mission. The UV instrument, consisting of the two spectrometers with common electronics, aims at measuring (1) emission lines from molecules, atoms and ions present in the Mercury’s tenuous atmosphere and (2) the reflectance spectrum of Mercury’s surface. The instrument pursues a complete coverage in UV spectroscopy. The extreme UV spectrometer covers the spectral range of 30–150 nm with the field of view of 5.0°, and the spectrum from 130 to 430 nm is obtained by the far UV spectrometer. The extreme UV spectrometer employs multi-layer coating technology to enhance its sensitivity at particular emission lines. This technology enables us to identify small ionospheric signatures such as He II (30.4 nm) and Na II (37.2 nm), which could not be detected with conventional optics.     

Navigating BepiColombo during the weak-stability capture at Mercury

BepiColombo is scheduled for launch in August 2013 and to arrive after a nearly six-year long transfer at Mercury in June 2019. The trajectory has a number of challenging elements: a launch with Soyuz/Fregat into a geostationary transfer orbit, followed by a lunar flyby, long low-thrust arcs and five more planetary flybys (one at the Earth, two at Venus and two at Mercury). At arrival the low thrust arcs reduce the approach velocity so much that BepiColombo passes by the Sun–Mercury Lagrange points L₁ and L₂ and gets weakly captured in a highly eccentric orbit around Mercury in case the orbit insertion manoeuvre would fail.This paper describes the navigation strategy during the final phase. Five trajectory correction manouevres during the last 65 days requiring up to 20 m/s (3σ) are proposed. With this strategy it is possible to navigate BepiColombo safely through the weak-stability boundary of Mercury and to reach the target periherm with a precision of 11 km.     

Determination of an instantaneous Laplace plane for Mercury’s rotation

Mercury is the target of two space missions: MESSENGER, which carried out its first and second flybys of Mercury on January 14, 2008 and October 6, 2008, and the ESA/JAXA space mission BepiColombo, scheduled to arrive at Mercury in 2020. The preparation of these missions requires a good knowledge of the rotation of Mercury.     

On electromagnetic phenomena in Mercury’s magnetosphere

Mercury has a small but intriguing magnetosphere. In this brief review, we discuss some similarities and differences between Mercury’s and Earth’s magnetospheres. In particular, we discuss how electric and magnetic field measurements can be used as a diagnostic tool to improve our understanding of the dynamics of Mercury’s magnetosphere. These points are of interest to the upcoming ESA-JAXA BepiColombo mission to Mercury.     

Images of the Earth's aurora and geocorona from the dynamics explorer mission

Several results from analyses of auroral and geocoronal images from the Dynamics Explorer Mission are summarized. (1) The motion of the transpolar arc of a theta aurora is found to be correlated with the y-component of the interplanetary magnetic field. The arc motion is in the general direction of the y-component. (2) A sequence of global images of a small auroral substorm shows the initial development of intense luminosities in a relatively small spatial region, or ‘bright spot’, in the pre-midnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at lower latitudes and contiguous to the midnight boundary of the bright spot. This evolution of auroral luminosities is interpreted in terms of acceleration of electrons in the boundary layer of the magnetotail plasma sheet to produce the bright spot and subsequent injection into, and eastward drift within, the plasma sheet to form the diffuse area of lesser intensities. (3) A series of images of the Earth's geocorona in scattered solar Ly α emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the Earth's exosphere. A Chamberlain model provides an adequate fit to radial distances of 4.5 R_E, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly α intensities in the anti-solar direction.     

Observation of the double sodium layer over Haikou,China by Lidar

A Double Sodium Layer (DSL) structure was observed during the night of August 22, 2011 over Haikou, China (20°N, 110°E) by Na lidar. This DSL comprised a typical sodium layer at altitudes of 80~105km and a higher sodium layer at altitudes of 105~115km in about 0.5 h. A wavelength of 589nm dye laser pumped by a Nd: YAG laser was used to make the measurement. The backscattered fluorescence photons from the sodium layer were collected by a telescope with a primary mirror of 1000mm in diameter. The sodium density of these layers during the nighttime observation in the Mesosphere and Lower-Thermosphere (MLT) was studied.     

A review on the upper atmospheric sodium observations from India: Insights

This review article briefly brings out the historical development of atmospheric sodium (Na) measurements over India and the importance of coordinated measurements with multiple techniques to address physical processes in the Earth's upper atmosphere. These measurements were initiated in the early 1970s by observing Na airglow emission intensities with broad band airglow photometer from Mt. Abu, a low-latitude hill station in India. Considerable amount of night-to-night variations in nocturnal emission intensities of the Na airglow were observed. Later, investigations regarding the dependence with the magnetic activities from the equatorial and low latitudes were carried out and double-humped structures in the nocturnal variation of intensities were reported. With the advent of Na lidar at Gadanki around 2005, the measurements of atmospheric neutral Na atom concentration became possible and more frequent occurrences of sporadic Na layers over the magnetic low latitude station compared to other latitudes were detected indicating the role played by electrodynamics. Later, a possible relationship between E-region field aligned plasma irregularities and the concentration of neutral Na atoms was investigated using coordinated measurements of VHF radar and Na lidar. Further, simultaneous measurements with Na lidar and a narrow band airglow photometer with narrow field of view brought out the importance of coordinated observation wherein the characterization of gravity waves could be carried out and also revealed the importance of collisional quenching due to ambient molecules in the Na airglow emission processes. In addition, combining the ground based measurements of Na lidar and meteor wind radar along with satellite measurements made possible to hypothesize the over-turning Kelvin–Helmholtz billow in the Na layer manifests “C-type” or inverted lambda shape structures in the height-time-concentration map of neutral Na atoms. This review paper presents a synoptic view mostly based on the previously reported observations of Na airglow emission, Na lidar and coordinated Na airglow and Na lidar observations from the Indian sector and highlights the importance of simultaneous measurements of mesospheric Na and its emissions along with satellite-borne measurements to address interesting geophysical processes in the Earth's upper atmosphere.     

2010年5月4日夜间海口上空钠层密度的激光雷达观测研究

使用Nd:YAG激光器和泵浦染料激光器产生的589nm激光, 与中高层大气中钠原子产生共振, 利用直径为1000mm的望远镜接收钠层的光子回波. 研究中国海口市2010年5月4日夜间的中高层大气中钠层密度分布, 计算钠层的相对密度和绝对密度, 研究钠层密度峰值变化. 观测到突发钠层出现在23:52LT, 持续12min.     

Studying the evolution of the hot universe with the X-ray evolving universe spectroscopy mission – XEUS

Europe is one of the major partners building the International Space Station (ISS) and European industry, together with ESA, is responsible for many station components including the Columbus Orbital Facility, the Automated Transport Vehicle, two connecting modules and the European Robotic Arm. Together with this impressive list of contributions there is a strong desire within the ESA Member States to benefit from this investment by utilizing the unique capabilities of the ISS to perform world-class science. XEUS is one of the astronomical applications being studied by ESA to utilize the capabilities of the ISS. XEUS will be a long-term X-ray observatory with an initial mirror area of 6 m² at 1 keV that will be expanded to 30 m² following a visit to the ISS. The 1 keV spatial resolution is expected to be 2–5″ half-energy-width. XEUS will consist of separate detector and mirror spacecraft (MSC) aligned by active control to provide a focal length of 50 m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting 0.1–2.5 keV sensitivity will then be 4 × 10⁻¹⁸ erg cm⁻² s⁻¹, around 200 times better than XMM-Newton, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.     

Space telescope observations of the earth's upper atmosphere by stellar occultations

Stellar occultations provide a useful means of measuring the trace gas composition of the Earth's mesosphere with a sensitivity of order one part per billion. The operational details will differ from those of other astronomical observations by ST, because of the difficulties in guiding near the Earth's limb. Two specific trace gases of interest to atmospheric studies, C$\text{l}$ and C$\text{l}$O, are discussed in this paper.     

Particle acceleration by stochastic fluctuations and dawn-dusk electric field in the Earth’s magnetotail

This work is devoted to investigate the interaction between protons and stochastic time-dependent electromagnetic fields generated by oscillating clouds of finite size, randomly positioned in the x–y  -plane. The geometry of the system is two-dimensional and, beside the time-dependent electromagnetic fluctuations, a steady-state, dawn-dusk electric field, _Ey$E_y$, has been added along the y-direction. The simultaneous presence of the stochastic time-dependent fluctuations and of the constant electric field component in the same system gives rise to two types of acceleration mechanisms operating on test particles: a second order Fermi-like process and a direct acceleration. By performing a parametric study, we extensively study the contribution of the two processes to proton acceleration. The energy values reached by test particles in this simple model are in good agreement with those observed in the Earth’s magnetotail region. Possible applications to the problem of particle acceleration in the terrestrial magnetosphere are widely discussed, and guidelines for future works are drawn.     

Specification of the Earth’s plasmasphere with data assimilation

In this paper we report on initial work toward data assimilative modeling of the Earth’s plasmasphere. As the medium of propagation for waves which are responsible for acceleration and decay of the radiation belts, an accurate assimilative model of the plasmasphere is crucial for optimizing the accurate prediction of the radiation environments encountered by satellites. On longer time-scales the plasmasphere exhibits significant dynamics. Although these dynamics are modeled well by existing models, they require detailed global knowledge of magnetospheric configuration which is not always readily available. For that reason data assimilation can be expected to be an effective tool in improving the modeling accuracy of the plasmasphere. In this paper we demonstrate that a relatively modest number of measurements, combined with a simple data assimilation scheme, inspired by the ensemble Kalman filtering data assimilation technique does a good job of reproducing the overall structure of the plasmasphere including plume development. This raises hopes that data assimilation will be an effective method for accurately representing the configuration of the plasmasphere for space weather applications.