Hirotaka Mori

Step-frequency radar

The step‐frequency radar is proposed as a means of detecting targets imbedded in high loss media. The radar derives information on target distance by exploiting the phase and amplitude characteristics of the returned signal. With a given frequency change, the phase angle change from a faroff target is larger, and therefore its phasor rotates faster, than that from a closeby target. Distance information is conveniently derived by applying the Fast Fourier Transform algorithm. The radar is able to offset an arbitrary distance and zoom to the targets, thereby obtaining fine resolution. With Wiener filtering, the resolution of the radar is high enough to identify the type, as well as the orientation, of the buried targets. A subtraction feature has also been incorporated which allows scattering from unwanted targets to be cancelled out from that of the wanted targets. The radar was successfully demonstrated for the detection and identification of various targets buried in sand, moist clay soils, underwater, a...

Development of a high-resolution imaging riometer for the middle and upper atmosphere observation program at Poker Flat, Alaska

Abstract An imaging riometer equipped with a 16 × 16-element square array antenna was developed and was installed at Poker Flat, Alaska, to obtain high-resolution two-dimensional images of the cosmic radio noise absorption at 38.2 MHz. This system scans the viewing region of a 200 × 200 km 2 square of the radio sky at the 90 km altitude every second, using 208 available antenna beams among 256 antenna phasing patterns. The maximum horizontal spatial resolution is 11 km at 90 km near the zenith over the antenna, which will enable us to resolve fine structures of auroral arcs with widths of the order of tens of kilometers or less, leading to more detailed investigation of auroral absorption dynamics than the existing imaging riometers. After the system check with the full system that was temporarily installed at the CRL Yamagawa observatory, Japan, in 1994, the final installation was completed and the operation started at Poker Flat, Alaska in September 1995.

Drift motion of ionospheric arc-like absorption regions observed with a 256-beam imaging riometer in Alaska

We studied the local time dependence of the direction of cosmic noise absorption drift using a large data set obtained with a 16 × 16-antenna imaging riometer in Poker Flat, Alaska (65.1°N, 147.5° W in geographic coordinates; 65.4°N, 100.7° in geomagnetic coordinates). The drift velocities were determined using regression lines for the absorption peaks, which were automatically detected. Based on the characteristics of the motion of the drift region, the events were categorized into three magnetic local time (MLT) groups: I (13–19 MLT), II (21-01 MLT) and III (01–07 MLT). Most events in group I move southwestward with steady velocities. Events in group II had various velocities and directions. Events in group III also had various velocities, but the tilt angles of their vector velocities from the eastward axis were between −90° and 90°, meaning that they moved eastward. These results are consistent with some found in previous studies. A polar plot of the drift had a pattern with features similar to that of magnetospheric convection, which has been reported on several studies: equatorward in the pre-midnight sector and sunward in the morning and evening sectors.

Night-time electron temperature troughs in the equatorial topside ionosphere revealed from RPA experiments on the ISS-b satellite

Abstract Night-time electron temperature (Te), electron density (Ne) and the mean ion mass (Mi) of the topside ionosphere at altitudes of about 1100 km have been obtained from the RPA experiments on ISS-b. Global maps of these quantities are derived by adopting the spherical surface harmonic functions fitted to the satellite data by means of a least-square method. The night-time map of Te shows a marked equatorial trough which is limited to a 100° longitudinal sector and a 20° latitudinal extent. The center of the trough is located a few degrees ‘summerward’ of the dip equator. The longitude of the trough is strongly controlled by the magnetic declination. This longitude changes rapidly around autumnal equinox from the region of maximum westward declination to that of eastward declination; the opposite change occurs around the time of vernal equinox. The maps of Mi and Ne in low latitudes have features apparently related to the Te trough. These features can be explained in terms of field-aligned interhemispheric plasma flow driven by the neutral air wind.

Evolution of the solar wind acceleration region during 1990-1994

The single-station measurements of interplanetary scintillation (IPS) at 2GHz and 8GHz using the Kashima radio telescope are used to study the distribution of the solar wind velocity and density fluctuations near the sun. Wind velocities derived from our IPS data with the IPS co-spectrum method show a radial increase in the distance range between 10 and 30 Rs (solar radii). From the scintillation index analysis, it is found that the radial fall of density fluctuations in the solar wind is described by the power-law function. A series of Kashima IPS observations reveals that a pronounced change in velocity and turbulence level occurs at the polar region of the sun during 1990–1994. That is, the high-speed wind and the reduced-turbulence region develop there as the solar activity declines. This fact is consistent with the long-term evolution of the coronal magnetic structure inferred from He1083nm observations.