Browsing by Author "Yamamoto, Mamoru"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Restricted First observation of the upper stratospheric vertical wind velocities using the Jicamarca VHF radar(American Geophysical Union, 1993-10) Maekawa, Yasuyuki; Fukao, Shoichiro; Yamamoto, Mamoru; Yamanaka, Manabu D.; Tsuda, Toshitaka; Kato, Susumu; Woodman Pollitt, Ronald FranciscoThe Jicamarca VHF radar (50 MHz) has detected atmospheric echoes in the so-called "gap region" from 30 to 60 km heights. These echoes are, for the first time, discriminated from clutter echoes, using both co-polarized ( co-pol) and cross-polarized (xpol) arrays to monitor the clutter component which may enter the antenna sidelobes. The atmospheric scatterings in this region are shown to be composed of refractivity layers with thickness as of 1-2 km as in other middle atmospheric height ranges, suggesting that these scatterings are caused by thin turbulent layers. The height profile of infcrred vertical wind velocities indicates a wavy structure. The wave amplitude, as a whole, increases with height, suggesting the activity of upward-propagating gravity waves.Item Open Access Gravity waves observed by the Jicamarca VHF radar in the equatorial upper stratosphere(ISEA, 1995) Maekawa, Yasuyuki; Soichiro, Fukao; Yamamoto, Mamoru; Yamanaka, Manabu D.; Tsuda, Toshitaka; Kato, Susumu; Woodman Pollitt, Ronald FranciscoInternal gravity waves are known to play an important role in dynamics of the middle atmosphere. They can transport momentum flux from lower atmosphere to upper atmosphere and sometimes break mean flows or large-scale long-period motions due to dissipation process associatcd with wave flow interaction. However, an entire profile of short period gravity waves has not yet been obtained, since it has been very difficult to detect extremely weak atmospheric echoes from so-called "gap region" of 30-60 km heights. Recently, the Jicamarca VHF radar system in Peru ( Woodman and A.Guillen, 1974), which is one of the largest high-power VHF /UHF radars in the world, has been greatly improved in height resolution, and reliable atmospheric echoes have been, for the first time, detected in the gap regían by the Jicamarca radar with the high-altitude resolution (500 m). This paper presents new observational results on the upper stratospheric short-period vertical wind oscillations, which have never been resolved by the VHF /UHP. radars other than the present revised Jicamarca radar.Item Restricted Mid‐latitude E region field‐aligned irregularities observed with the MU radar(American Geophysical Union, 1991-09) Yamamoto, Mamoru; Fukao, Shoichiro; Woodman Pollitt, Ronald Francisco; Ogawa, Tadahiko; Tsuda, Toshitaka; Kato, SusumuFine structures E region field-aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9N, 136.1E; geomagnetic latitude 25.0N). The 3.2-m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field-perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time-height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi-periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi-periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi-periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.Item Restricted VHF radar interferometry measurements of vertical velocity and the effect of tilted refractivity surfaces on standard doppler measurements(American Geophysical Union, 1991-03) Palmer, Robert D.; Larsen, Miguel F.; Woodman Pollitt, Ronald Francisco; Fukao, Shoichiro; Yamamoto, Mamoru; Tsuda, Toshitaka; Kato, SusumuAt VHF wavelengths, aspect sensitivity may result in an apparent beam direction that is off vertical even for a nominally vertically pointing beam direction if the refractivity surfaces responsible for the scatter are tilted with respect to the horizontal plane. Middle and upper atmosphere radar measurements obtained by using the system in a standard multireceiver configuration typical for radar interferometry (RI) and spaced antenna measurements have been analyzed for evidence of such effects. The analysis is based on the linear variation of the cross-spectral phase as a function of the radial velocity in the frequency doma.Jofor the RI cross spectra. I rue-verucal velocity escimates are obtained by using the fact that the phase difference between two antennas should be equal to zero when the echoes are being received from the vertical direction. The tilt angles of the refractivity surfaces were obtained from the phase of the cross-correlation function at zero lag, and the radial velocity in that direction was determined from the cross spectra. The results indicate that the vertical velocity derived from standard Doppler analyses is actually the velocity perpendicular to the refractivity surfaces and thus can be biased by the projection of the horizontal wind along the effective pointing direction.