Faculty Profiles - OTSUKA, Yuichi

Papers - OTSUKA, Yuichi

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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.

DOI： 10.1016/S0273-1177(99)00913-8

Web of Science

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

Web of Science

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 Open Access

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

Web of Science

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

Web of Science

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

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

Web of Science

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

Web of Science

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

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

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

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

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

Web of Science

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar Reviewed Open Access

WL Oliver, Y Otsuka, M Sato, T Takami, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 102 ( A7 ) page： 14499 - 14512 1997.7

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

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we develop a climatology of wave propagation based on the observation of 58 daytime experiments conducted during 1986-1994. The thermosphere seems to be continuously swept by waves detectable by an incoherent scatter radar. These waves generally come for hours on end from a,consistent or slowly varying direction, which can be any direction on a given day. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times. On average, the horizontal phase trace speed remains near 240 m/s for all periods inspected (40-130 min). This speed matches the behavior expected for lossless waves with 150-200 km vertical wavelength. We find small variability in this relation for different times of day, seasons, solar and magnetic conditions, and directions of wave travel, though waves on disturbed days seem to travel moderately faster on solar minimum mornings.

DOI： 10.1029/97JA00491

Web of Science

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar Reviewed Open Access

Journal of Geophysical Research Vol. 102 page： 14,499-14,512 1997

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

Geophysical Research Letters Vol. 24 page： 2,287-2,290 1997

Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves Reviewed

Journal of Geophysical Research Vol. 100 page： 23,763-23,768 1995

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Papers - OTSUKA, Yuichi