Atsushi Kumamoto

Lunar Radar Sounder Observations of Subsurface Layers Under the Nearside Maria of the Moon

Observations of the subsurface geology of the Moon help advance our understanding of lunar origin and evolution. Radar sounding from the Kaguya spacecraft has revealed subsurface layers at an apparent depth of several hundred meters in nearside maria. Comparison with the surface geology in the Serenitatis basin implies that the prominent echoes are probably from buried regolith layers accumulated during the depositional hiatus of mare basalts. The stratification indicates a tectonic quiescence between 3.55 and 2.84 billion years ago; mare ridges were formed subsequently. The basalts that accumulated during this quiet period have a total thickness of only a few hundred meters. These observations suggest that mascon loading did not produce the tectonics in Serenitatis after 3.55 billion years ago. Global cooling probably dominated the tectonics after 2.84 billion years ago.

Asymmetry of occurrence‐frequency and intensity of AKR between summer polar region and winter polar region sources

The statistical analyses of intensity and occurrence-frequency of auroral kilometric radiation (AKR) observed by the plasma wave and sounder experiment (PWS) onboard the Akebono (EXOS-D) satellite have been carried out for the seven-year period from March 1989 to February 1996. The results show clear seasonal variations of AKR intensity and occurrence-frequency both in the northern and southern polar regions. AKR exhibits an increase in the intensity and occurrence-frequency of intense emissions in the winter polar regions; thus there is a distinct north-south asymmetry of AKR. This tendency of asymmetry feature of AKR emission sources becomes more prominent in the high frequency range. These results have given strong confirmation to the GEOTAIL observation reported by Kasaba et al. [1997]. The origin of this asymmetry of AKR emissions are plausibly related with the seasonal dependence of the acceleration processes of the auroral electrons as has been suggested by Newell et al. [1996].

Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft

The Lunar Radar Sounder (LRS) on-board the SELENE lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (∼5 MHz) frequency range. Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the Moon, but also of the Moon’s regional and global thermal history of the Moon and of the origin of the Earth-Moon system. In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz. This paper provides the basic function parameter of the LRS system based on the final function test and proposes observation targets and data analysis that will provide important information leading to a greater understanding of the tectonics and thermal history of the Moon.

SC related electric and magnetic field phenomena observed by the Akebono satellite inside the plasmasphere

Electric and magnetic field variations inside the plasmasphere associated with SCs identified on the ground are analyzed based on the Akebono satellite observations which have been carried out more than 13 years since March 1989. 126 electric field observation data corresponding to SCs show abrupt change of intensity as well as direction within a few minutes inside the plasmasphere. Temporal variations of the electric field showed a bipolar waveform with the amplitude range of 0.2–38 mV/m. The electric field signature is followed by a dumping oscillation with the period of Pc3–4 ranges. The magnetic field variations of 33 SCs also show an abrupt increase of 0.2–65 nT within a few minutes, which indicate the compression of the magnetosphere due to the discontinuity of solar wind. The initial excursion of the electric field during SCs tends to be directed westward. The amplitude does not show a dependence on magnetic local time that has been observed outside the plasmasphere. The magnitude of the electric field variations tends to be proportional with the power of 0.6 to the magnetic field variation in the plasmasphere. The Poynting vector of the initial SC impulse is directed toward the earth, which suggests that energy of magnetic disturbances associated with SCs propagates toward the earth inside the plasmasphere with the refraction due to the plasma density gradient. One of the most interesting results from the present study is that a DC offset of the Ey component of the electric field appears after the initial electric field impulse associated with SCs. This signature is interpreted to be a magnetospheric convection electric field penetration into the inner plasmasphere (L = 2.5). The intensity of the offset of the Ey field gradually increases by 0.5–2.0 mV/m about 1–2 minutes after the onset of the initial electric field impulse and persists about 10–30 minutes.

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

Electrodynamics in the duskside inner magnetosphere and plasmasphere during a super magnetic storm on March 13-15, 1989

Variations of cold plasma density distribution and large-scale electric field in the inner magnetosphere and plasmasphere during a geomagnetic storm were investigated by using the observation data of the Akebono satellite which has been carried out for more than 15 yeas since March, 1989. We focus on the super geomagnetic storm on March 13–15, 1989, for which the maximum negative excursion of the Dst index was −589 nT. During the main phase of the magnetic storm, the strong convection electric field with a spatially inhomogeneous structure appears in the inner magnetosphere between L = 2.0 and 7.0. The averaged intensity of the electric field was in a range of about 2.5–9.2 mV/m. The spatial distribution in the magnetic equatorial region indicates that the magnitude within an L-value range of 2.2–7.0 is much larger than that observed at L = 7.0–10.0. Associated with the appearance of the strong convection electric field, the cold plasma density near the trough region around L = 3.0–6.0 was enhanced with one or two order magnitude, compared with that in the magnetically quiet condition. This implies that a mount of the ionospheric plasma may be supplied from the topside ionosphere into the trough and plasmasphere regions by the frictional heating due to the fast plasma convection in the ionosphere as pointed out by previous studies on the enhancements of plasma density in these regions, based on incoherent scatter radar and total electron content (TEC) observations (e.g., Yeh and Foster, 1990; Foster et al., 2004). During the recovery phase of the magnetic storm, the convection electric field observed in the inner magnetosphere and plasmasphere regions recovers within 3-4 days almost up to the level of the magnetically quiet condition.

Ionization ledge structures observed in the equatorial anomaly region by using PPS system on-board the Ohzora (EXOS-C) satellite

To verify an additional ionization layer predicted in the equatorial ionosphere, topside ionograms obtained by the Planetary Plasma Sounder (PPS) system on-board the Ohzora (EXOS-C) satellite were analyzed. Based on the analysis of the PPS data of 8 passes in March and 11 passes in May, 1987, the ionization ledge observed in the local noon time period shows similar nature as it has been theoretically predicted for the F3 layer by Balan and Bailey (1995). It was noted that some peaks of the ledge structure were located on the field line of higher latitude region than the field line of the crest of the equatorial anomaly.

Plasma wave observation using waveform capture in the Lunar Radar Sounder on board the SELENE spacecraft

The waveform capture (WFC) instrument is one of the subsystems of the Lunar Radar Sounder (LRS) on board the SELENE spacecraft. By taking advantage of a moon orbiter, the WFC is expected to measure plasma waves and radio emissions that are generated around the moon and/or that originated from the sun and from the earth and other planets. It is a high-performance and multifunctional software receiver in which most functions are realized by the onboard software implemented in a digital signal processor (DSP). The WFC consists of a fast-sweep frequency analyzer (WFC-H) covering the frequency range from 1 kHz to 1 MHz and a waveform receiver (WFC-L) in the frequency range from 10 Hz to 100 kHz. By introducing the hybrid IC called PDC in the WFC-H, we created a spectral analyzer with a very high time and frequency resolution. In addition, new techniques such as digital filtering, automatic filter selection, and data compression are implemented for data processing of the WFC-L to extract the important data adequately under the severe restriction of total amount of telemetry data. Because of the flexibility of the instruments, various kinds of observation modes can be achieved, and we expect the WFC to generate many interesting data.

Detectability of subsurface interfaces in lunar maria by the LRS/SELENE sounding radar: Influence of mineralogical composition

[1] The Lunar Radar Sounder (LRS) of the SELENE mission has detected horizontal subsurface features at depths of a few hundreds of meters within all major lunar maria. We have mapped these features at global scale and found a heterogeneous geographical distribution, which correlates negatively with the maps of TiO 2 and FeO obtained from UV-VIS measurements by the Clementine probe. High concentration of ilmenite (FeTiO 3 ) in the basaltic mare lava can explain this correlation, this mineral being a strong absorber for low frequency electromagnetic waves. Thus, absence of detection of subsurface interfaces by LRS on a large portion of lunar maria does not necessarily imply that these interfaces are actually absent.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.