Measurements of bidirectional reflectance of ordinary chondrite for muses-C in-situ detection

MUSES-C, a Japanese sample return mission, is targeting a small near Earth asteroid, 1998SF36, which is considered an S-type asteroid and is similar in spectroscopy to LL class ordinary chondrite meteorite ([Binzel et al., 2001]). Although this mission will bring us detailed photometric data, that is, disk-resolved bidirectional reflectance data of the asteroid, there were few bidirectional reflectance data of ordinary chondrite meteorites. For the purpose of comparison with the data obtained by the in-situ observation, we have performed measurements of bidirectional reflectance of ordinary chondrite samples.

Here we summarize the result of our laboratory measurements of the bidirectional reflectance and compare them with the scattering property of 1998SF36. Although the geometric albedo of 1998SF36 is higher than the typical value of S-type asteroids, however, the laboratory data of ordinary chondrite are similar to or brighter than the model disk-resolved reflectance of 1998SF36 derived from disk-integrated ground-based data. We found in our laboratory data that there is a positive correlation between the surface roughness and the strength of the opposition effect. A stronger opposition effect in the reflectance of 1998SF36 than the laboratory data may therefore indicate that the surface of the asteroid has rougher structure than our laboratory samples.     

An empirical model of the plasma environment around Mercury

Since the flyby observations by Mariner 10 in 1974 and 1975, Mercury has been one of the most interesting objects for space physics and planetary exploration. The MESSENGER and BepiColombo missions now plan to revisit this planet. In order to design plasma instruments for the BepiColombo mission, we have estimated electron and ion fluxes around Mercury with an empirical model, which has been developed for the Earth’s magnetotail. The solar wind data needed as input parameters are derived from Helios observations. The result shows that our predicted electron fluxes at aphelion agree well with the Mariner-10 data. It is also noted that ion instruments must cover a very wide dynamic range of proton fluxes. However, the applicability of the Earth’s magnetospheric model to Mercury is, in itself, an important issue for comparative magnetospheric studies.     

Report on the Kiso cometary dust trail survey

Cometary dust trails were first observed by IRAS; they are widely known to be the origins of meteoric showers. A new window has been opened for the study of dust trails, using ground-based observations. We succeeded in obtaining direct images of the 22P/Kopff dust trail with the Kiso 1.05-m Schmidt telescope. Following this initial success, we have continued to perform a dust trail survey at Kiso. As a result of this survey, we have detected dust trails along the orbit of six periodic comets, between February 2002 and March 2004. The optical depth of these dust trails are 10⁻⁹ to 10⁻⁸, which is consistent with IRAS measurements. In this paper, we describe the observations and data reduction procedures, and report the brief result obtained between February 2002 and March 2004.     

The spaceborne lidar experiment

Lidar is one of the key sensors to understand global atmospheric environment, so lidar is one of the candidates to be flown on future earth observation satellites. But spaceborne lidar is our first trial and we have some technical issues to resolve before establishing operational sensor. So we start development of the Spaceborne Lidar Experimental Model as a first step of lidar program of NASDA. The Spaceborne Lidar Experimental Model will be launched in early 2001.

The purpose of the Spaceborne Lidar Experiment is technological demonstration of system design and critical components to apply to future operational sensor. And we demonstrate availability of lidar observation data and scientific value.

There are some critical points to resolve such as (1) thermal design of laser oscillator which remove locally-generated-heat effectively and (2) mirror design with little distortion.     

Development of simultaneous imaging polarimeter for asteroids

We are currently developing a polarimeter to study surface physical properties of asteroids. To enhance polarimetric accuracy and observational efficiency, we newly devised the polarimeter whose measurements can provide the two Stokes parameters Q and U, simultaneously. The test-observations of the prototype polarimeter have been carried out in December 2003 and January 2004, mounted on the 101 cm telescope at Bisei Astronomical Observatory, Okayama, Japan. In the observations, unpolarized and polarized standard stars were observed to measure an instrumental polarization and its uncertainty. As a result, an instrumental polarization of 5.06 ± 0.18% has been measured.     

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.     

Jikiken(EXOS-B) observation of siple transmissions

A Japanese magnetospheric satellite Jikiken (EXOS-B) was used to observe Siple transmissions and VLF emissions triggered by the Siple signals. Energetic particle fluxes were also observed at the same time. The first experiments were made during July, August and September in 1979 and the second experiments were made during December in 1979 and January in 1980. The Siple triggered emissions were observed in August. This report reviews the preliminary results.     

The Cold-Dense Plasma Sheet: A Geotail Perspective

GEOTAIL observations of the low-latitude boundary layer (LLBL) in the tail-flanks show that they are the region where the cold-dense plasma appears with stagnant flow signatures accompanied by bi-directional thermal electrons (< 300 eV). It is concluded from these facts that the tail-LLBL is the site of capturing the cold-dense plasma of the magnetosheath origin on to the closed field lines of the magnetosphere. There are also cases that strongly suggest that the cold-dense plasma entry from the flanks can be significant to fill a substantial part of the magnetotail. In such cases, the cold-dense plasma is not spatially restricted to a layer attached to the magnetopause (that is, the LLBL), but continues to well inside the magnetotail, constituting the cold-dense plasma sheet. Inspired by the fact that these remarkable cases are found for northward interplanetary magnetic field (IMF), a statistical study on the status of the near-Earth plasma sheet is made. The results show that the plasma sheet becomes significantly colder and denser when the northward IMF continues than during southward IMF periods, and that the cold-dense status appears most prominently near the dawn and dusk flanks. These are consistent with the idea that, during northward IMF periods, the supply of cold-dense ions to the near-Earth tail from the flanks dominates over the hot-tenuous ions transported from the distant tail.     

Ion dynamics of magnetic reconnection in the magnetotail

We discuss the kinetic processes of plasma thermalization, acceleration, and mixing in magnetic reconnection. Non-Maxwellian, gyrotropic ion distribution functions such as anisotropic ion beams in the plasma sheet boundary layer (PSBL) and counter-streaming ions (CSIs) in the plasma sheet are often observed during a plasmoid passage of a satellite in the Earth's magnetotail. Non-gyrotropic ion distribution functions are also sometimes observed just after the passage of the plasmoid. We study the behavior of non-Maxwellian ion distribution functions observed by GEOTAIL. We further study theoretically the ion dynamics by using a particle-in-cell simulation, and discuss the role of non-Maxwellian distribution functions in magnetic reconnection.