研究者詳細 - 西谷　望

論文 - 西谷　望

分割表示 96 件中 61 - 80 件目／全件表示 >>

A New Empirical Model of the Subauroral Polarization Stream

Kunduri, BSR; Baker, JBH; Ruohoniemi, JM; Nishitani, N; Oksavik, K; Erickson, PJ; Coster, AJ; Shepherd, SG; Bristow, WA; Miller, ES

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 123 巻 ( 9 ) 頁： 7342 - 7357 2018年9月

Temporal and Spatial Variations of Storm Time Midlatitude Ionospheric Trough Based on Global GNSS-TEC and Arase Satellite Observations

Shinbori, A; Otsuka, Y; Tsugawa, T; Nishioka, M; Kumamoto, A; Tsuchiya, F; Matsuda, S; Kasahara, Y; Matsuoka, A; Ruohoniemi, JM; Shepherd, SG; Nishitani, N

GEOPHYSICAL RESEARCH LETTERS 45 巻 ( 15 ) 頁： 7362 - 7370 2018年8月

Attenuation of decameter wavelength sky noise during x-ray solar flares in 2013-2017 based on the observations of midlatitude HF radars

Berngardt, OI; Ruohoniemi, JM; Nishitani, N; Shepherd, SG; Bristow, WA; Miller, ES

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS 173 巻頁： 1 - 13 2018年8月

The ERG Science Center

Miyoshi, Y; Hori, T; Shoji, M; Teramoto, M; Chang, TF; Segawa, T; Umemura, N; Matsuda, S; Kurita, S; Keika, K; Miyashita, Y; Seki, K; Tanaka, Y; Nishitani, N; Kasahara, S; Yokota, S; Matsuoka, A; Kasahara, Y; Asamura, K; Takashima, T; Shinohara, I

EARTH PLANETS AND SPACE 70 巻 2018年6月

Characterization of Short-Wave Fadeout Seen in Daytime SuperDARN Ground Scatter Observations

Chakraborty, S; Ruohoniemi, JM; Baker, JBH; Nishitani, N

RADIO SCIENCE 53 巻 ( 4 ) 頁： 472 - 484 2018年4月

Spreading Speed of Magnetopause Reconnection X-Lines Using Ground-Satellite Coordination

Zou, Y; Walsh, BM; Nishimura, Y; Angelopoulos, V; Ruohoniemi, JM; McWilliams, KA; Nishitani, N

GEOPHYSICAL RESEARCH LETTERS 45 巻 ( 1 ) 頁： 80 - 89 2018年1月

Influence of Auroral Streamers on Rapid Evolution of Ionospheric SAPS Flows

Gallardo-Lacourt, B; Nishimura, Y; Lyons, LR; Mishin, EV; Ruohoniemi, JM; Donovan, EF; Angelopoulos, V; Nishitani, N

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 122 巻 ( 12 ) 頁： 12406 - 12420 2017年12月

Ground-based instruments of the PWING project to investigate dynamics of the inner magnetosphere at subauroral latitudes as a part of the ERG-ground coordinated observation network

Shiokawa, K; Katoh, Y; Hamaguchi, Y; Yamamoto, Y; Adachi, T; Ozaki, M; Oyama, SI; Nosé, M; Nagatsuma, T; Tanaka, Y; Otsuka, Y; Miyoshi, Y; Kataoka, R; Takagi, Y; Takeshita, Y; Shinbori, A; Kurita, S; Hori, T; Nishitani, N; Shinohara, I; Tsuchiya, F; Obana, Y; Suzuki, S; Takahashi, N; Seki, K; Kadokura, A; Hosokawa, K; Ogawa, Y; Connors, M; Ruohoniemi, JM; Engebretson, M; Turunen, E; Ulich, T; Manninen, J; Raita, T; Kero, A; Oksanen, A; Back, M; Kauristie, K; Mattanen, J; Baishev, D; Kurkin, V; Oinats, A; Pashinin, A; Vasilyev, R; Rakhmatulin, R; Bristow, W; Karjala, M

EARTH PLANETS AND SPACE 69 巻 2017年11月

Special Issue "Global data systems for the study of solar-terrestrial variability"

Watanabe, T; Iyemori, T; Shiokawa, K; Zhang, J; Kanekal, S; Nishitani, N

EARTH PLANETS AND SPACE 69 巻 2017年11月

SC-Associated Electric Field Variations in the Magnetosphere and Ionospheric Convective Flows

Kim, SI; Kim, KH; Kwon, HJ; Jin, H; Lee, E; Jee, G; Nishitani, N; Hori, T; Lester, M; Wygant, JR

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 122 巻 ( 11 ) 頁： 11044 - 11057 2017年11月

Propagation and evolution of electric fields associated with solar wind pressure pulses based on spacecraft and ground-based observations

Takahashi, N; Kasaba, Y; Nishimura, Y; Shinbori, A; Kikuchi, T; Hori, T; Ebihara, Y; Nishitani, N

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 122 巻 ( 8 ) 頁： 8446 - 8461 2017年8月

First imaging and identification of a noctilucent cloud from multiple sites in Hokkaido (43.2-44.4 N), Japan 査読有り

Hidehiko Suzuki, Kazuyo Sakanoi, Nozomu Nishitani, Tadahiko Ogawa, Mitsumu K. Ejiri, Minoru Kubota, Takenori Kinoshita, Yasuhiro Murayama and Yasushi Fujiyoshi

Earth, Planets and Space 68:182 巻 2016年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

Simultaneous imaging observations of a noctilucent cloud (NLC) from five sites in Hokkaido, Japan (43.17-45.36 N), were successfully carried out using digital cameras in the early hours of the morning (around 02:00 LST) on June 21, 2015. This is the first NLC event that has been captured from multiple sites in Japan. The simultaneous images obtained from multiple sites made it possible to calculate the exact altitude (=83.9 +- 0.1 km) and spatial distribution (47.5-50.0 N and 143.0-147.5 E) of the NLC by triangulation and image correlation methods. Based on a comparison of atmospheric parameters of the upper mesosphere provided by satellites and a middle-frequency (MF) radar in northern Hokkaido (Wakkanai) with the cloud distribution obtained from the Aeronomy of Ice in the Mesosphere satellite, this particular event is considered to be the result of southward advection of the NLC from a higher-latitude (i.e., colder) region. Anomalies in the upper mesospheric temperature of the northern hemispheric summer in 2015 were examined using AURA satellite data, because this is the first NLC event that has been identified in Japan. However, no remarkable temperature variations relative to other years were found in upper mesosphere. Based on a comparison between the NLC period and the record of sky conditions archived by the Japan Meteorological Agency, a high percentage of cloud (especially low-level) cover during the summer in Hokkaido cannot be ruled out as a possible reason why the NLC had not previously been sighted in Hokkaido.

DOI： 10.1186/s40623-016-0562-6

Application of ground scatter returns for calibration of HF interferometry data 査読有り

Ponomarenko, P., N. Nishitani, A.V. Oinats, T. Tsuya and J.-P. St.-Maurice

Earth Planets Space 67 巻 ( 138 ) 2015年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

Information on the vertical angle of arrival (elevation) is crucial in determining propagation modes of high-frequency (HF, 3-30 MHz) radio waves travelling through the ionosphere. The most advanced network of ionospheric HF radars, SuperDARN (Super Dual Auroral Radar Network), relies on interferometry to measure elevation, but this information is rarely used due to intrinsic difficulties with phase calibration as well as with the physical interpretation of the measured elevation patterns. In this work, we propose an empirical method of calibration for SuperDARN interferometry. The method utilises a well-defined dependence of elevation on range of ground scatter returns. "Fine tuning" of the phase is achieved based on a detailed analysis of phase fluctuation effects at very low elevation angles. The proposed technique has been successfully applied to data from the mid-latitude Hokkaido East SuperDARN radar. It can also be used at any other installation that utilises HF interferometry.

DOI： doi: 10.1186/s40623-015-0310-3

Occurrence characteristics and lowest speed limit of Sub-Auroral Polarization Stream (SAPS) observed by the SuperDARN Hokkaido East radar 査読有り

Nagano, H., N. Nishitani, and T. Hori

Earth Planets Space 67 巻 ( 126 ) 2015年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

We investigate the characteristics of the subauroral polarization stream (SAPS), with focus on the relationship between geomagnetic parameters and occurrence characteristics of SAPS. This study's observations were made using the Super Dual Auroral Radar Network (SuperDARN) Hokkaido East radar, which can observe the Far East region of Russia and has been in operation since 2006. In particular, we identify the lowest limit of SAPS speed, which has not been discussed in previous literature, in order to examine the lowest threshold of electric field able to generate SAPS as a result of magnetosphere-ionosphere (M-I) coupling. In order to conduct a comprehensive investigation of SAPS occurrence characteristics, we analyzed events with wider ranges of velocity and magnetic latitude (MLAT) than those in previous studies. As a result of quantitative estimation, we found two categories of westward flows that were reasonably separated using a speed threshold of 150-200 m/s. For the faster flows above the speed threshold, there is a clear correlation between MLAT and SYM-H geomagnetic index, whereas for the slower flows, there is no such correlation. The faster flows are considered to be SAPS, whereas the slower flows are probably associated with mid-latitude F-region ionospheric irregularities not directly related to storms or substorms. This slowest limit of SAPS gives us a minimum electric field of 7.5-10 mV/m that generates SAPS. However, this field strength is not strong enough to cause frictional heating, which is generally considered to be a crucial mechanism for generating SAPS. This result suggests that frictional heating is not always necessary to generate SAPS.

DOI： 10.1186/s40623-015-0299-7

Statistical study of medium-scale traveling ionospheric disturbances using SuperDARN Hokkaido ground backscatter data for 2011 査読有り

Oinats, A.V., V.I. Kurkin, and N. Nishitani

Earth Planets Space 67 巻 ( 22 ) 2015年2月

記述言語：英語掲載種別：研究論文（学術雑誌）

We describe an automated technique to determine parameters of traveling ionospheric disturbances (TIDs) using the Super Dual Auroral Radar Network (SuperDARN) high frequency (HF) radar data. The technique is based on the analysis of minimum ground backscatter range variations corresponding to different radar beams. Using this technique, we processed the SuperDARN Hokkaido radar data for 2011 and revealed statistical distributions of medium-scale TID (MSTID) azimuth and apparent horizontal velocity. We found four peaks with a distinct diurnal and seasonal dependence in the MSTID azimuth occurrence rate distributions. Northeast MSTID azimuths (20° to 50°) are typical of the summer and equinox morning hours; southeast azimuths (100° to 140°) prevail in the winter daytime; southwest azimuths (190° to 220°) are typical mostly in the summer and equinox nighttime and in the equinox evening; northwest azimuths (280° to 320°) are typical of the summer daytime and evening. The apparent horizontal velocities are generally within the 100 to 160 m/s range. The obtained results agree well with earlier studies by other researchers. However, there are also certain differences. The summer daytime northwestward MSTIDs are not indicated in the earlier studies. The nighttime horizontal velocities are 1.5 to 2 times higher than those in the daytime. Furthermore, winter velocity values are about 1.5 times higher than those in other seasons. These differences might be associated with the peculiarities of the data recorded by different facilities, or the features of the processing techniques, and require further investigation for their interpretation.

DOI： 10.1186/s40623-015-0192-4

Pi2 pulsation simultaneously observed in the E and F region ionosphere with the SuperDARN Hokkaido radar 査読有り

Teramoto, M., N. Nishitani, V. Pilipenko, T. Ogawa, K. Shiokawa, T. Nagatsuma, A. Yoshikawa, D. Baishev, and K. T. Murata

J. Geophys. Res. 119 巻頁： 3444-3462 2014年5月

記述言語：英語掲載種別：研究論文（学術雑誌）

We investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high-temporal (8 s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field-aligned irregularities in the sporadic Es region. They had the same period of 110 s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated ~8.0mV/m
in the F region and ~2.0mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were 56°and 45°, and the amplitude ratios were 2.7 × 105m/s and 8.4 × 105 m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfvén waves must be taken in account.

DOI： 10.1002/2012JA018585

Hokkaido HF radar signatures of periodically reoccurring nighttime MSTIDs detected at short ranges 査読有り

Koustov, A.V., K. N. Yakymenko, N. Nishitani, and P. V. Ponomarenko

J. Geophys. Res. 119 巻頁： 1200-1218 2014年2月

記述言語：英語掲載種別：研究論文（学術雑誌）

The Super Dual Auroral Radar Network Hokkaido HF radar often detects periodically reoccurring E region echoing regions propagating toward or away from it. In this work, we consider 117 of such events identified for 2008-2012. These are shown to occur at nighttime, preferentially during summer, although significant number for events was found for winter. Statistics for the local time of occurrence, magnitude of the speed and polarity of progressions, and temporal and spatial periodicities are presented. We show that the power of echoes is linearly related to their Doppler velocity which makes it possible to identify the events on both power and Doppler velocity plots. Other peculiar characteristics of echoes are discussed. The onset of
events is associated with gravity waves propagation through the radar field of view.

DOI： 10.1002/2013JA019422

Study of mid-latitude ionospheric convection during quiet and disturbed periods using the SuperDARN Hokkaido radar 査読有り

Zou, Y., and N. Nishitani

Advances in Space Research 54 巻頁： 473-480 2014年1月

Northward-propagating nighttime medium-scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido HF radar and GEONET 査読有り

Ichihara, A., N. Nishitani, T. Ogawa, and T. Tsugawa

Advances in Polar Science 頁： in press 2013年

Study of ionospheric disturbances during solar flare events with the SuperDARN Hokkaido Radar 査読有り

Watanabe, D., and N. Nishitani

Adv. Polar Sci. 24 巻頁： 12-18 2013年

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