研究者詳細 - 大塚　雄一

論文 - 大塚　雄一

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Annual variations of the ionosphere: A review based on MU radar observations 査読有り

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

LOWER IONOSPHERE: MEASUREMENTS AND MODELS 25 巻 ( 1 ) 頁： 153 - 162 2000年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り

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

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS 61 巻 ( 18 ) 頁： 1367 - 1383 1999年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り

Journal of Atmospheric and Solar-Terrestrial Physics 61 巻頁： 1367-83 1999年

MU radar observations of H+ ions in the topside ionosphere 査読有り Open Access

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 103 巻 ( A9 ) 頁： 20697 - 20704 1998年9月

Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar 査読有り

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 103 巻 ( A9 ) 頁： 20705 - 20713 1998年9月

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

WL Oliver, Y Otsuka, S Fukao

RADIO SCIENCE 33 巻 ( 4 ) 頁： 941 - 948 1998年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り

N Balan, Y Otsuka, GJ Bailey, S Fukao

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 103 巻 ( A5 ) 頁： 9481 - 9495 1998年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り Open Access

Journal of Geophysical Research 103 巻頁： 20,697-20,704 1998年

MST radar measurement of ionospheric F region winds:The "layer-wind" technique 査読有り

Radio Science 33 巻頁： 941-948 1998年

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar 査読有り

Journal of Geophysical Research 103 巻頁： 9,481-9,495 1998年

Plasma temperature variations in the ionosphereover the middle and upper atmosphere radar 査読有り

Journal of Geophysical Research 103 巻頁： 20,705-20,713 1998年

New aspects in the annual variation of the ionosphere observed by the MU radar 査読有り

N Balan, Y Otsuka, S Fukao

GEOPHYSICAL RESEARCH LETTERS 24 巻 ( 18 ) 頁： 2287 - 2290 1997年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り Open Access

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

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 102 巻 ( A7 ) 頁： 14499 - 14512 1997年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読有り Open Access

Journal of Geophysical Research 102 巻頁： 14,499-14,512 1997年

New aspects in the annual variation of the ionosphere observed by the MU radar 査読有り

Geophysical Research Letters 24 巻頁： 2,287-2,290 1997年

Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves 査読有り

Journal of Geophysical Research 100 巻頁： 23,763-23,768 1995年

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