Tomohiko Imachi

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.

Low Frequency plasma wave Analyzer (LFA) onboard the PLANET-B spacecraft

The Low Frequency plasma wave Analyzer, LFA, on board the PLANET-B spacecraft has been developed to measure the Martian plasma waves. Two orthogonal electric dipole wire antennas, 50 m tip-to-tip, in the spacecraft spin plane are used to measure plasma waves, dc electric fields, and the spacecraft potential relative to the ambient plasma. The LFA has capability to measure the wave spectrum in the band from 10 Hz to 32 kHz, and to capture the signal waveform in the band from dc to 32 kHz by using a 4 MByte memory. The LFA scientific objectives are to explore the following: (1) Macroscopic plasma environment and boundaries from the solar wind to the ionosphere, (2) Microscopic plasma phenomena induced by the interaction between the solar wind and the Martian atmosphere and the moon Phobos, (3) Generation and propagation of electromagnetic waves, (4) Plasma densities and waves in the nightside ionosphere and tail, and (5) Comparison of Martian plasma waves with those of other planets such as non-magnetized Venus and magnetized Earth.

Equivalent Circuit Model for the Electric Field Sensitivity of a Magnetic Search Coil of Space Plasma

Magnetic search coils (MSCs) are sensitive to both magnetic and electric fields, but detecting electric fields is unnecessary for magnetic observations of plasma waves. However, it is important to evaluate both sensitivities for different geometries and electrostatic shields to avoid electric field pickup. An equivalent circuit model for the electric field sensitivity of an MSC in a collisionless isotropic cold plasma is developed here using electrical coupling through a sheath capacitance. That sensitivity is defined by a relationship between the MSC impedance and the sheath capacitance. To confirm the validity of the circuit model, the sensitivity to an electric field is measured by imposing an external electric field using charged parallel metallic plates in laboratory experiments. The coupling capacitance between the MSC and charged plates is equivalent to the sheath capacitance in a space plasma. The measured results show good agreement with an approximate expression deduced from the equivalent circuit model.

Radio-frequency power distribution measurement system using thin metamaterial absorber

A radio-frequency (RF) power distribution measurement system is developed. The 2-d distribution of an incident RF power is measured on the surface of a finite size metamaterial absorbers, which is designed to suppress undesired edge scattering. The system is evaluated by the measurement of an RF wave transmitted from a dipole antenna. The measured distributions of incident RF power are consistent with those obtained by EM simulations, demonstrating practical effectiveness of the technique to measure RF power distributions.

Development of a system for measuring power and phase distributions of radio waves

A system for measuring power and phase distributions of a radio wave incident on the surface of a thin metamaterial absorber is developed. By measuring the voltages on an array of lumped resistors interconnecting surface patches on a mushroom-type metasurface at its resonance frequency, incident power and phase distributions are obtained. The effectiveness of the technique is evaluated by measuring a radio wave transmitted from a dipole antenna, on the basis of the comparison between the simulation and the actual measurement.

Kanazawa-SAT^3: micro-satellite mission for monitoring x-ray transients coincide with gravitational wave events

We are developing a micro satellite, Kanazawa-SAT3 , to be launched in FY2019. The main purpose of the mission is to localize X-ray transients coincide with gravitational wave events, e.g. short gamma-ray bursts, and to investigate the formation of extreme space-time of black holes and the origin of relativistic jet. We are developing a wide field X-ray imaging detector as a mission instrument. It has a couple of 1-dimensional imaging systems with a random coded aperture mask and silicon strip detectors. In this paper, we introduce the mission overview and the current status of Kanazawa-SAT3 and the flight model performance.

Numerical analysis and visualization of spherical waves absorbed by a thin metamaterial absorber

A thin metamaterial absorber has been used for measuring 2-d radio-frequency (RF) field distributions incident on the absorber surface. A matrix of lumped resistors interconnecting the metal patches on a grounded substrate work as a sensor array to measure the 2-d power and phase distributions of the incident RF field at the resonance frequency. In this study the field distributions on the absorber surface are numerically analyzed using plane-wave expansion of a spherical wave in the radiating near-field zone of an antenna. Results of the numerical analysis is compared with simulations and actual experiments to evaluate its accuracy. Such a detailed field distribution can be used to visualize the wave propagation from RF antennas or noise sources.

Current status and planning of the Plasma Wave Experiment (PWE) onboard the ERG satellite

The ERG (Exploration of energization and Radiation in Geospace) project is a mission to study acceleration and loss mechanisms of relativistic electrons around the Earth. To achieve comprehensive observations of plasma/particles, fields, and waves, the Plasma Wave Experiment (PWE) is installed onboard the ERG satellite to measure electric field in the frequency range from DC to 10 MHz, and magnetic field in the frequency range from a few Hz to 100 kHz. Two CPU boards, one for electric field and another for magnetic field, are installed for the PWE and a variety of operational modes can be implemented. In the present paper, we introduce unique specifications of the PWE to meet the scientific objects of the ERG mission.

Radio-frequency source estimation using field distribution measured on metamaterial absorber surface

A thin metamaterial absorber has been used as a sensor array to measure the 2-d distribution of radio-frequency (RF) wave field incident and absorbed on its surface. The absorber is designed so that the incident wave is absorbed by a metasurface which is matched with the incident wave impedance. Absorption is achieved by lumped resistors interconnecting a 2-d dense array of mushroom-type square patches formed on a grounded dielectric substrate. The resistors inserted between the adjacent patches arranged in the row (x) and column (y) directions on the surface absorb the incident x- and y-polarizations, respectively, at the resonance frequency of the metasurface. In this case the x- and y-resistors act as small dipole sensors which output the voltages proportional to the x- and y-components of electric field of the incident wave, with their effective lengths given as a unit cell size. By monitoring the voltages on the array of the lumped resistors, 2-d amplitude and phase distributions of electric field are obtained on the absorber surface. The field distribution measured on the absorber surface is then used to extract the information on its sources. As the sensor array absorbs the incident wave with tiny reflection, it acts as an almost ideal antenna array which does not disturb the incident field distribution, where no mutual coupling between the array elements can be taken into account. At the same time the detailed field distributions are obtained with the spatial resolution much smaller than the wavelength of the incident wave, due to the electrically dense sensor (resistor) arrangement. Such an accurate and detailed field distribution is expected to give a useful information to localize the angle-of-arrivals of far-field sources, as well as the locations of near-field sources. In this study, as an example of such a source localization technique, we apply the MUSIC algorithm to estimate the locations of near-field sources. The performance of the technique is evaluated with simulations. Preliminary results show that the technique is effective to localize the RF (a few GHz) sources which are located about one meter away from a square-shaped absorber of several tens of centimeters per side.

Numerical and experimental evaluation of spherical wave absorption incident on a thin metamaterial absorber

It has been proposed that a thin metamaterial absorber could be used for measuring 2-d RF field distributions incident on the absorber surface. The structure of the absorber consists of a matrix of dense metal patches formed on the surface of a thin grounded dielectric substrate. The incident wave power is absorbed by lumped resistors inserted between the surface patches, which are matched with incident wave impedance at the resonance frequency of the metamaterial structure. The incident field distribution can be monitored by measuring the voltages induced on (or the amounts of power consumed by) the lumped resistors.