Katsuyuki Noguchi

Phase scintillation observation during coronal sounding experiments with NOZOMI spacecraft

Radio occultation observations of the solar corona at solar offset distances of 12.8-36.9 RS were performed from December of 2000 to January of 2001, using the Nozomi spacecraft (Planet-B) of Japan. Phase scintillation spectra up to the frequency of ∼10 Hz were obtained in two-way mode with S -band uplink and X-band downlink. The spectra cannot be represented by a single power law especially for small offset distances. That is, at low frequencies (large scales) the spectra show slopes indicative of Kolmogorov, a local flattening occurs from ∼0.5 Hz (scale ∼600 km) to ∼ 3H z (∼100 km), and a steepening occurs again at higher frequencies. This three-component nature of the spectrum was observed with a single method for the first time, although the spectral shape is variable and the three-component feature is not always evident.

Application of the GPS network to estimate the effect of the terrestrial ionosphere on the radio occultation measurements of planetary ionospheres

A feasibility study to observe the low-density Martian nightside ionosphere and the cislunar electron layer by radio occultation technique is presented. Since the total electron contents (TECs) along the ray paths of radio occultation in the ionospheres of Mars and the Moon are comparable to the fluctuation of the TEC of the terrestrial ionosphere, the distortions of radio occultation data by the terrestrial ionosphere should be taken into account. Local time and seasonal dependences of the terrestrial TEC fluctuations are investigated using the GPS receiver network, showing that the terrestrial ionosphere is calm at night in winter and that this period is suitable for the radio occultation measurements of the planetary ionospheres. A method is also developed to estimate the terrestrial TEC fluctuation along the ray path of radio occultation from GPS TEC data.

Estimation of changes in the composition of the Martian atmosphere caused by CO2 condensation from GRS Ar measurements and its application to the rederivation of MGS radio occultation measurements

We propose a method to estimate seasonal changes in the composition of the Martian atmosphere, which is influenced by CO2 condensation due to the polar nights at southern high latitudes. The method relies on measurements of the Ar concentration obtained by the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey. We assume that the Martian atmosphere is composed of CO2, N2, and Ar, and is vertically well mixed. Since N2 and Ar do not condense even during the polar nights, the ratio of N2 and Ar remains constant, and the concentrations of N2 and CO2 can be estimated from Ar measurements. Estimates of the atmospheric composition were utilized for the rederivation of temperature and pressure profiles in the Mars Global Surveyor (MGS) radio occultation measurements (∼70 profiles of ∼20,000 profiles in the whole data set) at southern polar latitudes (90°S–75°S) during the autumn, winter, and spring seasons (Ls = 0°–240°). The rederived profiles indicated that use of the standard global composition overestimated the temperature by at least approximately 5 K at Ls = ∼120° (midwinter), when the largest CO2 depletion occurred and the CO2 volume mixing ratio fell to 78%. The occurrence and degree of CO2 supersaturation were several times higher and larger, respectively, in the rederived temperature profiles than in the original MGS profiles. This suggests that consideration of CO2 depletion during southern polar nights is needed when studying CO2 supersaturation using radio occultation profiles.

Properties of solar wind turbulence from radio occultation experiments with the NOZOMI spacecraft

Radio-sounding experiments using signals from the Japanese NOZOMI spacecraft to probe the circum solar plasma were performed from December 2000 through January 2001. They can be used to obtain information about the properties of the solar wind plasma in the region where it is accelerated at heliocentric distances of 12.8–36.9Rs (where Rs is the radius of the Sun). Measurements of the intensity and frequency of the received signals were carried out with high time resolution (∼0.05 s for the frequency and ∼0.0064 s for the intensity), making it possible to investigate the anisotropy of inhomogeneities and the spatial spectrum of the turbulence of the circum solar plasma. Analysis of these radio-sounding data has shown that the scintillation index and intensity of the frequency fluctuations decrease approximately according to a power law with increasing distance of the line of sight from the Sun. Measurements of the amplitude fluctuations and estimates of the solar wind velocity derived from spatially separated observations indicate the presence of small-scale inhomogeneities with sizes of the order of 50 km at heliocentric distances less than 25Rs, which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30Rs become close to isotropic.

Development of an interactive visual data mining system for atmospheric science

In atmospheric science, 3D visualization techniques have been mainly used to create impressive presentation in recent decades. However, from the viewpoint of utilize for visual data mining, 3D visualization methodology has difficulties in becoming wide spread because most conventional and established way is to make 2D diagrams consisting of two dimensions of a temporal transitional 3D grid. From these observations, we have been developing a quick look tool of atmospheric science data for 3d visual data mining. We expect that scientists can utilize this tool for finding out 2D diagrams from the data by using various 2D or 3D visualization methods, and become accustomed themselves to 3D visualization methods.

CISLUNAR PLASMA EXPLORATION WITH THE SELENE RADIO SCIENCE SYSTEM

Abstract The SELENE Project of Japan includes three spacecraft: a main orbiting satellite and two subsatellites. Among many scientific objectives of the Project is the exploration of plasma formations above the lunar surface. The thin plasma layer above the area of the Moon, presented to the Sun, was first detected in radio occultation experiments conducted during 1972 with the Soviet lunar satellite “Luna-19”. The SELENE Project radio science experiment will be realized by means of a dual S- and X- band transmitter installed onboard the orbiter and Earth based antennas. The effect of the thin plasma layer near the Luna can be hidden by the fluctuations of the total electron content of the Earths ionosphere. Dual spacecraft occultation configurations can remove partially the influence of the Earths ionosphere. Several dual spacecraft configurations are discussed and upper error limits of total electron content measurements are estimated.

First test of the NOZOMI radio science system in actual space flight

Abstract The Japanese NOZOMI spacecraft (S/C) launched on the 4th of July, 1998 was equipped with a coherent dual frequency Radio Science System (RSS) to explore the Martian atmosphere and the solar wind plasma by the radio sounding method. Several communication operations were performed in order to test and calibrate the RSS during the cruise phase of the flight to Mars. S- and X- band signals emitted by the S/C were received and recorded at the Usuda tracking station (Japan). Analysis of the data processing results revealed the influence of the S/C spin on all signal parameters. In particular the S/C spin shifted the frequencies of both the S- and X- band signals, destroying their coherency. It was shown that the effect of the spin stabilizing system can be removed from the final occultation results and the Nozomi S/C can be successfully used in radio occultation experiments.

Detectability of lunar plasma clouds from SELENE radio occultations

Abstract The SELENE mission includes a dual frequency S- and X-band occultation experiment for the exploration of plasma clouds above the lunar surface. The accuracy of the measurement is mainly limited by the influence of the turbulent media of the Earths ionosphere and the solar wind plasma. The mission scenario will make accessible only the region along the lunar limb. Still, the radio communication system of the SELENE mission, which consists of three orbiters, is able to provide other occultation configuration. The proposed types of radio occultation will allow exclusion of the effect of the Earths ionosphere and the solar wind plasma and occultation on both sides of the Moon. The sensitivity is expected to be better than 1014 el/M 2 . The occultation measurement together with the other SELENE mission experiments can present new information on the existence and the origin of the plasma clouds and the interaction of the solar wind with magnetic field anomalies on the surface of the Moon.

A study of BRDF over Tokyo for the spaceborne measurements of atmospheric trace gases

In the present study, we aim at developing an empirical model of BRDF over Tokyo, Japan, which is one of the most polluted areas in Asia, to evaluate the effect of the surface albedo on air-pollution monitoring from space. We used the RossThick-LiSparseReciprocal model with MODIS data to retrieve BRDF information. The BRDF had a strong dependence on season and local time, and the magnitude of the seasonal and local time change was up to 50%.

Local Time Dependence of the Thermal Structure in the Venusian Equatorial Upper Atmosphere: Comparison of Akatsuki Radio Occultation Measurements and GCM Results

Plain Language Summary Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35 degrees. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer. Akatsuki radio occultation measurements showed the local time dependence of the Venus atmosphere in the equatorial region. By comparing the measurements with a general circulation model, it is attributed to the upward propagating semidiurnal tide generated in the cloud layer. And then, we proposed a new method to estimate the zonal wind speed above the cloud layer, where any optical instruments cannot be measured, for the first time.