Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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Three-channel imaging Fabry-Perot interferometer for measurement of mid-latitude airglow Reviewed

K Shiokawa, T Kadota, MK Ejiri, Y Otsuka, Y Katoh, M Satoh, T Ogawa

APPLIED OPTICS Vol. 40 ( 24 ) page： 4286 - 4296 2001.8

Language：English Publishing type：Research paper (scientific journal) Publisher：OPTICAL SOC AMER

We have developed a three-channel imaging Fabry-Perot interferometer with which to measure atmospheric wind and temperature in the mesosphere and thermosphere through nocturnal airglow emissions. The interferometer measures two-dimensional wind and temperature for wavelengths of 630.0 nm (OI, altitude, 200-300 km), 557.7 nm (OI, 96 km), and 839.9 mn (OH, 86 km) simultaneously with a time resolution of 20 min, using three cooled CCD detectors with liquid-N-2 Dewars. Because we found that the CCD sensor moves as a result of changes in the level of liquid N-2 in the Dewars, the cooling system has been replaced by thermoelectric coolers. The fringe drift that is due to changes in temperature of the etalon is monitored with a frequency-stabilized He-Ne laser. We also describe a data-reduction scheme for calculating wind and temperature from the observed fringes. The system is fully automated and has been in operation since June 1999 at the Shigaraki Observatory (34.8N, 136.1E), Shiga, Japan. (C) 2001 Optical Society of America.

DOI： 10.1364/AO.40.004286

Traveling ionospheric disturbances detected in the FRONT campaign Reviewed

A Saito, M Nishimura, M Yamamoto, S Fukao, M Kubota, K Shiokawa, Y Otsuka, T Tsugawa, T Ogawa, M Ishii, T Sakanoi, S Miyazaki

GEOPHYSICAL RESEARCH LETTERS Vol. 28 ( 4 ) page： 689 - 692 2001.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

The F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign was conducted to clarify the non-classical features of traveling ionospheric disturbances (TIDs) at mid-latitudes in May, 1998 and August, 1999. A cluster of all-sky CCD cameras and a GPS receiver network observed a wide area of the ionosphere over Japan to detect the spatial structure and temporal evolution of TIDs. The propagation direction of the nighttime TID detected during the FRONT campaign periods is restricted to the southwest. The time evolution of their amplitude indicates that the TID structure is intensified as it travels from high-latitudes to low-latitudes. The significant coincidence between the structures of 630 nm band airglow and total electron content indicates that the perturbations take place in the bottomside of the ionospheric F region. Coherent echoes from the field-aligned irregularities were observed by the MU radar in the nights when the TID activity was high.

DOI： 10.1029/2000GL011884

Multi-point observation of short-period mesospheric gravity waves over Japan during the FRONT campaign Reviewed

K Shiokawa, MK Ejiri, Y Otsuka, T Ogawa, M Kubota, K Igarashi, A Saito, T Nakamura

GEOPHYSICAL RESEARCH LETTERS Vol. 27 ( 24 ) page： 4057 - 4060 2000.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Simultaneous observations of short-period gravity waves were carried out using three all-sky cooled-CCD imagers of OH airglow at Moshiri (northern edge of Japan) and at Shigaraki and Bisei (middle part of Japan) during the FRONT campaign of May 1998. These stations were separated by horizontal distances of 250 - 1300 km, and they offered a unique opportunity to investigate the spatial extent of gravity waves in the mesopause region. The images from Shigaraki and Bisei showed short-period gravity waves (horizontal wavelength: 20-40 km, phase front: similar to east-west aligned) moving northward (phase velocity: 20-40 m/s) on May 19, 21 and 22. At Moshiri, similar east-west waves moving northward were observed on May 19 and 21, indicating a broad horizontal extent of the gravity waves. Waves at Moshiri were not evident on May 22, however. The horizontal wind measured by the MF radar at Wakkanai (near Moshiri) showed height profiles that were similar to those at Yamagawa (southern edge of Japan) on May 10 and 21 but quite dissimilar oil May 22. On the basis of these data, we suggest that short-period gravity waves in the mesopause region carl have a I,road spatial extent of more than 1000 km. The generation and propagation of internal gravity waves are discussed as a cause of the spatial extent of thc waves.

DOI： 10.1029/2000GL011917

Traveling ionospheric disturbances observed in the OI 630-nm nightglow images over Japan by using a multipoint imager network during the FRONT campaign Reviewed

M Kubota, K Shiokawa, MK Ejiri, Y Otsuka, T Ogawa, T Sakanoi, H Fukunishi, M Yamamoto, S Fukao, A Saito

GEOPHYSICAL RESEARCH LETTERS Vol. 27 ( 24 ) page： 4037 - 4040 2000.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19-22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200-600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83-137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be similar to 260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

DOI： 10.1029/2000GL011858

Comprehensive imaging observations of midlatitude ionospheric disturbances during storm time substorms Reviewed

K Shiokawa, Y Otsuka, T Ogawa, K Igarashi, S Miyazaki, FJ Rich, A Saito, Y Yumoto

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 105 ( A12 ) page： 27067 - 27080 2000.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

This paper reports comprehensive imaging observations of 630-nm airglow enhancements and related phenomena in the nighttime F region over Japan. We found two events during which the airglow intensity increases up to 300 R at the lower-latitude side of Japan associated with storm time substorms of August 26; 1998, and February 18, 1999. Total electron content (TEC) observations by more than 900 GPS receivers over Japan show a clear TEC enhancement of similar to 10-40 Ya at latitudes below 40 degrees, which coincides with the airglow enhancements and the substorms. On the other hand, 2-MHz virtual heights obtained by bottomside sounding at four ionosonde stations show equatorward moving large-scale structures! which do not coincide with the air glow enhancements. Vertical ion drift data at an altitude of similar to 800 km obtained by the Defense Meteorological Satellite Program satellites show upward ion motion in midlatitudes just. after the substorm onset for the February event. These observations suggest, that the midlatitude airglow and TEC enhancements are caused by upward electron motion (fountain) in the low latitudes due to penetration of eastward electric field associated with the storm time substorms.

DOI： 10.1029/2000JA900099

A study of the forenoon ionospheric F-2 layer behavior over the middle and upper atmospheric radar Reviewed

Zhang, SR, WL Oliver, S Fukao, Y Otsuka

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 105 ( A7 ) page： 15823 - 15833 2000.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

The middle and upper atmosphere (MU) radar incoherent Scatter observations of electron density N-e profiles for 1986-1995 at Shigaraki (34.85 degrees N, 136.10 degrees E) show a forenoon density depression (bite-out) in summer bo th on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F-2 peak for low solar activity and around the F-2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon N-e depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma; temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature T-e, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce N-e and result in a negative partial derivative N-e/partial derivative t in the morning hours in summer. Among those processes the morning enhancement of T-e is found to be most important for the presence of the forenoon bite-out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite-out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N-2] in summer leads to a smaller daytime background N-e and thus to an enhanced T-e, and the after-sunrise photochemical contributions (mainly the O+ production) are so small that partial derivative Ne/partial derivative t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite-out can be explained basically in terms of the low-electron-density background. (4) The height of formation of the forenoon bite-out is also related to the sunrise effect on photoionization.

DOI： 10.1029/2000JA000007

A climatology of middle and upper atmosphere radar observations of thermospheric winds Reviewed

S Kawamura, Y Otsuka, Zhang, SR, S Fukao, WL Oliver

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 105 ( A6 ) page： 12777 - 12788 2000.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time-of-day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) anti with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong. one in the early daylight hours and a weak one in the afternoon-evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while: HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady willd of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward ill winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.

The IRI's B parameters measured by the MU radar Reviewed

Zhang, SR, S Fukao, XY Huang, Y Otsuka

LOWER IONOSPHERE: MEASUREMENTS AND MODELS Vol. 25 ( 1 ) page： 101 - 104 2000

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON PRESS LTD

The IS measurements of power profile by the MU radar at Shigaraki (34.9 degrees N, 136.1 degrees E) are analyzed to give IRI's B-parameters, B0 B1 for 1986-1991. Empirical formulas are obtained to represent their variations as functions of local time and season at high solar activity. They are compared also with ionosonde ones for Wuchang (30.5 degrees N, 114.4 degrees E). (C) 1999 COSPAR. Published by Elsevier Science Ltd.

Annual variations of the ionosphere: A review based on MU radar observations Reviewed

N Balan, Y Otsuka, S Fukao, MA Abdu, GJ Bailey

LOWER IONOSPHERE: MEASUREMENTS AND MODELS Vol. 25 ( 1 ) page： 153 - 162 2000

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON PRESS LTD

A review of the annual variations of the ionosphere, which focuses on the physical mechanisms causing the well-known seasonal anomaly and equinoctial asymmetry, is presented. In this review, the electron density (Nc), electron and ion temperatures (Te and Ti). and field-parallel and field-perpendicular plasma velocities (V-parallel to and V-perpendicular to), measured by the MU radar in the 180-600 km altitude range during 1986-1994, are analysed to study the altitude dependence of the seasonal anomaly and equinoctial asymmetry. The meridional component of the thermospheric neutral wind velocity (U-theta) derived from V-parallel to and neutral densities obtained from MSIS-86 are used to investigate the relative importance of the chemical and dynamical processes causing the anomaly and asymmetry. The review concludes that, although the anomaly and asymmetry involve chemical and dynamical processes, the dynamical processes (mainly through the neutral wind) predominate in the asymmetry while the chemical processes predominate in the anomaly. (C) 1999 COSPAR. Published by Elsevier Science Ltd.

The height of the maximum ionospheric electron density over the MU radar Reviewed

Zhang, SR, S Fukao, WL Oliver, Y Otsuka

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS Vol. 61 ( 18 ) page： 1367 - 1383 1999.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Ionospheric F-2-layer peak height h(m)F(2) variations, as measured over 1986-1995 by the MU radar (34.85 degrees N, 136.1 degrees E) and as calculated with a theoretical model, are discussed. The diurnal variations of the measured peak height for different seasons and levels of solar activity are compared with those estimated from ionosonde M3000F(2) and IRI predictions. Also given are the measured ion drift velocities and meridional neutral winds needed to understand the dynamic behavior of the F-2-layer. It is found that: (1) h(m)F(2) is generally higher during periods of the solar maximum than during periods of the solar minimum, and higher in summer than in winter; (2) for the solar maximum, h(m)F(2) drops markedly in the morning and in the afternoon, while, for the solar minimum, the h(m)F(2) minimum occurs in the morning during summer and usually in the afternoon during winter. In general, the measured h(m)F(2) is well reproduced by our model when we use the observed drift velocities and plasma temperatures as inputs. Our modeling study shows that the neutral wind contributes strongly to the diurnal variation of h(m)F(2) in winter by lowering the ionization layer by day, particularly for the solar maximum; it also helps to enlarge the daynight difference of h(m)F(2) in summer. The northward electromagnetic drifts that usually cancel the neutral wind effect have only a minor effect for the location of the MU radar. Other features of the observed h(m)F(2) variations, e.g., the solar maximum-minimum difference, the summer-winter difference, and the morning and afternoon drops, are explained by the basic processes of O+ production, loss and diffusion, as influenced by the atomic oxygen concentration and neutral and plasma temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S1364-6826(99)00088-7

The height of the maximum ionospheric electron density over the MU radar Reviewed

Journal of Atmospheric and Solar-Terrestrial Physics Vol. 61 page： 1367-83 1999

MU radar observations of H+ ions in the topside ionosphere Reviewed

S Kawamura, Y Otsuka, WL Oliver, N Balan, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 103 ( A9 ) page： 20697 - 20704 1998.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

We report the detection of H+ ions in the topside ionosphere above the middle and upper atmosphere (MU) radar in Japan, the first such detection by a mesosphere-stratosphere-troposphere radar extended to perform as an incoherent scatter radar. Owing to the very limited signal-to-noise ratios of similar to 1% achievable in the topside ionosphere with this radar, long integration times of 45 min and statistical analysis of 10 years of data are employed to yield useful results. This sensitivity restriction limits the top altitude of observation to only similar to 400 km during solar-minimum winter but up to similar to 650 km during solar-maximum summer. These measurements are possible only at night, when we may assume that the electrons and ions have the same temperature. While the data scatter is large, climatological averages constructed for various solar-activity, seasonal, and magnetic activity cases show less than similar to 5% H+ at most under conditions of detectability, although it is exactly those conditions of undetectability which are expected to correspond to the highest proportions of H+. At solar maximum we see no detectable amounts of H+ up to 650 km altitude, except perhaps for a faint amount in winter. At solar minimum the data are good up to similar to 550 km during summer and equinox but to only similar to 400 km during winter. The H+ profiles for solar-minimum summer and equinox closely resemble each other and reach similar to 5% at 550 km altitude. We see no clearly discernible H+ in winter up to our 400 km altitude limit of detectability; we expect that there is substantial HS present above 400 km, but the total ion density is so low that we cannot measure it. We see no detectable change in H+ between magnetically quiet and disturbed periods. The H+ proportions seen above the MU radar are much smaller than those seen above Arecibo, which has a similar magnetic latitude but much lower geographic latitude, and are much more comparable to those amounts seen by the higher-latitude radars at Saint Santin and Millstone Hill. Comparisons with models suggest that at these low altitudes, H+ may well be near its charge-exchange-equilibrium value.

DOI： 10.1029/98JA00898

Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar Reviewed

Y Otsuka, S Kawamura, N Balan, S Fukao, GJ Bailey

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 103 ( A9 ) page： 20705 - 20713 1998.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85 degrees N, 136.10 degrees E, magnetic latitude 25 degrees N) in Japan are studied using the electron and ion temperature (T-e and T-i, respectively) data measured by the radar during nearly a full solar cycle (1986-1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of T-e and T-i is presented for the altitude range 200-550 km. The temperatures T-e and T-i are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season; while daytime T-e is highest in summer and lowest in equinox, daytime T-i is highest in equinox and lowest in summer. With solar activity, while daytime T-e decreases, the corresponding T-i increases. The diurnal variation of T-e is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in T-e are illustrated through model calculations using the Sheffield University plasmasphere-ionosphere model.

DOI： 10.1029/98JA01748

MST radar measurement of ionospheric F region winds: The "layer wind" technique Reviewed

WL Oliver, Y Otsuka, S Fukao

RADIO SCIENCE Vol. 33 ( 4 ) page： 941 - 948 1998.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

We develop an improved application of the mesosphere-stratospheretroposphere (MST) radar technique for the measurement of winds at ionospheric F region altitudes. While the experiment is basically one of incoherent scatter (IS), which yields very weak signals at the usual operating frequencies of MST radars, we develop an analysis technique that allows the signals to be integrated over the entire F region of the ionosphere to gain sufficient statistical accuracy to compute the "layer wind" accurately. This method is contrasted with the normal IS method of determining winds at discrete heights. The method is applied to data collected with the middle and upper atmosphere radar in Japan, though it is applicable to any MST radar having a comparable or even somewhat smaller power-aperture product and may also be adapted for improved results from mainline IS radars. Using this method, we are able to discuss confidently the geophysical behavior of the wind on individual days, whereas previously we were reluctant to discuss anything but climatological features from data averages.

DOI： 10.1029/98RS00658

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar Reviewed

N Balan, Y Otsuka, GJ Bailey, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 103 ( A5 ) page： 9481 - 9495 1998.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Annual variations of the ionosphere and thermosphere studied with the middle and upper atmosphere (MU) radar during the solar maximum period 1988-1992 show that the well-known seasonal anomaly in the electron density Ne exists only during daytime and at altitudes near the ionospheric peak and below. The observations also reveal the existence of equinoctial asymmetries in the ionosphere and thermosphere, with the asymmetry in the ionosphere changing its character with altitude during daytime. In the bottomside ionosphere the electron density Ne is slightly greater in September equinox than in March equinox. At higher altitudes the asymmetry reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 100%. The electron temperature Te exhibits equinoctial asymmetries almost opposite those in Ne. The ion temperature Ti shows a weak asymmetry, in phase with the asymmetry in Ne. The field-aligned and field-perpendicular plasma velocities V-parallel to and V-perpendicular to are also different in the two equinoxes. In the thermosphere the neutral wind and composition show consistent equinoctial asymmetries. The meridional component of the daytime poleward wind velocity (U-theta) derived from the field-parallel plasma velocity is weaker in March equinox than in September equinox by up to 20 m s(-1), and the values of the daytime [O]/[N-2] ratio obtained from MSIS-86 are larger in September equinox than in March equinox by about 20%. Model calculations carried out by incorporating the measured V-perpendicular to and U-theta into the Sheffield University plasmasphere-ionosphere model that uses MSIS-86 for neutral atmosphere show that the equinoctial asymmetries in the ionosphere arise mainly from the corresponding asymmetries in the thermosphere, with major contributions from neutral winds and minor contributions from composition.

DOI： 10.1029/97JA03137

Plasma temperature variations in the ionosphereover the middle and upper atmosphere radar Reviewed

Journal of Geophysical Research Vol. 103 page： 20,705-20,713 1998

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar Reviewed

Journal of Geophysical Research Vol. 103 page： 9,481-9,495 1998

MU radar observations of H+ ions in the topside ionosphere Reviewed

Journal of Geophysical Research Vol. 103 page： 20,697-20,704 1998

MST radar measurement of ionospheric F region winds:The "layer-wind" technique Reviewed

Radio Science Vol. 33 page： 941-948 1998

New aspects in the annual variation of the ionosphere observed by the MU radar Reviewed

N Balan, Y Otsuka, S Fukao

GEOPHYSICAL RESEARCH LETTERS Vol. 24 ( 18 ) page： 2287 - 2290 1997.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Annual variations of the daytime ionosphere studied with the MU radar during a full solar cycle show equinoctial asymmetry in density, temperature, and plasma velocity. The electron density (Ne) in the bottomside ionosphere is slightly greater in September equinox compared to March equinox, which arises from the asymmetry in thermospheric composition. At higher altitudes, the asymmetry in Ne reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 150%. The electron temperature (Te) shows an asymmetry, which is opposite to the asymmetry in Ne; the values of Te in the topside in March equinox are less than those in September equinox by up to 300K. The asymmetry in ion temperature (Ti) is weak (about 100K) and in phase with the asymmetry in Ne. The northward perpendicular plasma velocity (V-perpendicular to) is slightly greater in March equinox compared to September equinox. The meridional component of the thermospheric neutral wind velocity (U-theta), derived from the field-parallel plasma velocity V-parallel to, is up to 50% lower in March equinox compared to September equinox. The asymmetry in neutral winds accounts for the asymmetry in ionospheric density and temperature in the topside, with minor contribution from the asymmetry in plasma drifts.

DOI： 10.1029/97GL02184

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