研究者詳細 - 野澤　悟徳

論文 - 野澤　悟徳

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Velocity of E-region HF echoes under strongly-driven electrojet conditions 査読有り

Gorin, J. D., A. V. Koustov1, R. A. Makarevich2, J.-P. St.-Maurice1, and S. Nozawa

Ann. Geophys. 30 巻頁： 235-250 2012年1月

Fine structure of sporadic sodium layer observed with a sodium lidar at Tromsoe, Norway 査読有り

Tsuda, T. T., S. Nozawa, T. D. Kawahara, T. Kawabata, N. Saito, S. Wada, C. M. Hall, S. Oyama, Y. Ogawa, S. Suzuki, T. Ogawa, T. Takahashi, H. Fujiwara, R. Fujii, N. Matuura, and A. Brekke

Geophys. Res. Lett. 38 巻 2011年9月

Angular dependence of pump-induced bottom- and top-side ionospheric plasma turbulence at EISCAT 査読有り

Kosch, M., E. Mjolhus, M. Ashrafi, M. Rietveld, T. Yeoman, S. Nozawa

J. Geophys. Res. 116 巻 2011年3月

記述言語：英語掲載種別：研究論文（学術雑誌）

We experimentally observe the location and angular size of the high-frequency (HF) radio window in the bottomside ionosphere, which permits radio wave propagation to the topside ionosphere, with high angular resolution at the European Incoherent Scatter (EISCAT) facility. HF pump-induced ion line enhancements were observed by the EISCAT UHF incoherent scatter radar on the ionospheric bottomside and topside.

DOI： 10.1029/2010JA016014

Meso-scale observations of Joule heating near an auroral arc and ion-neutral collision frequency in the polar cap E-region 査読有り

Kosch, M., Ho-Ching Iris Yiu, Callum Anderson, Takuo Tsuda, Yasunobu Ogawa, Satonori Nozawa, Anasuya Aruliah, Victoria Howells, Lisa Baddeley, Ian McCrea, James Wild

J. Geophys. Res. 116 巻 2011年3月

On the statistical relation between ion upflow and naturally enhanced ion-acoustic lines observed with the EISCAT Svalbard radar 査読有り

Ogawa, Y., S. C. Buchert, I. Haggstrom, M. T. Rietveld, R. Fujii, S. Nozawa, and H. Miyaoka

J. Geophys. Res. 116 巻 2011年3月

Horizontal structure of sporadic E layer observed with a rocket-borne magnesium ion imager 査読有り

Kurihara, J., Y. Koizumi-Kurihara, N. Iwagami, T. Suzuki, A. Kumamoto, T. Ono, M. Nakamura, M. Ishii, A. Matsuoka, K. Ishisaka, T. Abe, and S. Nozawa

J. Geophys. Res. 115 巻 ( A12318 ) 頁： 10.1029/2009JA014926 2010年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

To study the spatial structure of midlatitude sporadic E (Es) layers, the ultraviolet resonant scattering by magnesium ions (Mg+) in an Es layer was observed during the evening twilight with the Magnesium Ion Imager (MII) on the sounding rocket launched from the Uchinoura Space Center in Kagoshima, Japan. The in situ electron density measured by an onboard impedance probe showed that the Es layer was located at an altitude of 100 km during both the ascent and descent of the flight. Simultaneous observation with a ground-based ionosonde at Yamagawa identified the signature of horizontally “patchy" structures in the Es layer. The MII successfully scanned the horizontal Mg+ density perturbations in the Es layer and found that they had patchy and frontal structures. The horizontal scale and alignment of the observed frontal structure is generally consistent with a proposed theory. To our knowledge, this is the first observation of the two-dimensional horizontal structure of Mg+ in an Es layer.

An analysis of pump-induced artificial ionospheric ion upwelling at EISCAT 査読有り

Kosch, M., Y. Ogawa, M. Rietveld, S. Nozawa, and R. Fujii

J. Geophys. Res. 115 巻 ( A12317 ) 頁： 10.1029/2010JA015854 2010年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

Ion outflow from the high‐latitude ionosphere is a well‐known phenomenon and an important source of plasma for the magnetosphere. It is also well known that pumping the ionosphere with high‐power high‐frequency radio waves causes electron heating. On a few occasions, this has been accompanied by artificially induced ion upwelling. We analyze such a controlled experiment at EISCAT up to 600 km altitude. The pump‐enhanced electron temperatures reached up to ∼4000 K above 350 km, and ion upwelling reached up to ∼300 m/s above 500 km altitude. The pump‐induced electron pressure gradient can explain the ion velocity below 450 km. Between 450 and 600 km the electron pressure gradient correlates equally with ion acceleration and ion velocity, which represents the transition altitude to free ion acceleration. The electron gas pressure gradient can explain ion upwelling, at least up to 600 km altitude. In addition, such active experiments open the possibility to estimating the F layer ion‐neutral collision frequency and neutral density with altitude from ground‐based observations.

Lower-thermospheric wind fluctuations measured with an FPI in pulsating aurora at Tromsoe 査読有り

Oyama, S., K. Shiokawa, J. Kurihara, T. T. Tsuda, S. Nozawa, Y. Ogawa, Y. Otsuka, and B. J. Watkins

Ann. Geophys. 28 巻 ( 10 ) 頁： 1847-1857 2010年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

Simultaneous observations were conducted with 1 Introduction a Fabry-Perot Interferometer (FPI) at a wavelength of
557.7 nm, an all-sky camera at a wavelength of 557.7 nm, and the European Incoherent Scatter (EISCAT) UHF radar during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA-2) campaign in January 2009. This paper concentrated on two events during periods of pulsating au- rora. The lower-thermospheric wind velocity measured with the FPI showed obvious fluctuations in both vertical and hor- izontal components. Of particular interest is that the location of the fluctuations was found in a darker area that appeared within the pulsating aurora. During the same time period, the EISCAT radar observed sporadic enhancements in the F-region backscatter echo power, which suggests the pres- ence of low-energy electron (1 keV or lower) precipitation coinciding with increase in amplitude of the electromagnetic wave (at the order of 10 Hz or higher). While we have not yet identified the dominant mechanism causing the fluctuations in FPI-derived wind velocity during the pulsating aurora, the frictional heating energy dissipated by the electric-field per- turbations may be responsible for the increase in ionospheric thermal energy thus modifying the local wind dynamics in the lower thermosphere.

Tidal waves in the polar lower thermosphere using the EISCAT long run data set obtained in September 2005 査読有り

Nozawa, S, Y. Ogawa, S. Oyama, H. Fujiwara, T. Tsuda, A. Brekke, C. Hall , Y. Murayama, S. Kawamura, H. Miyaoka, and R. Fujii

J. Geophys. Res. 115 巻 2010年8月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromsø (69.6°N, 19.2°E) over ~23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day-to-day variations (~5–30 ms-1) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17–22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be ~58 km and ~76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than ~24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms-1) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5–6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere.

Solar activity dependence of ion upflow in the polar ionosphere observed with the EISCAT Tromsø UHF radar 査読有り

Ogawa, Y., S. C. Buchert, A. Sakurai, S. Nozawa, R. Fujii

J. Geophys. Res., 115 巻頁： doi:10.1029/2009JA014766 2010年7月

記述言語：英語掲載種別：研究論文（学術雑誌）

The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromsø UHF radar between 1984 and 2008. In agreement with other work we find that the upward ion flux is generally higher when solar activity is high than when it is low. Ion upflow events and also the upward velocity behave the opposite: they are more frequently seen and higher, respectively, at times of low solar activity. In any year about 30–40% ion upflow is accompanied by ~500 K higher electron temperature than the background temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger when solar activity is low than when it is high. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. Ions start to flow up at above 450 km altitude when solar activity was high, and lower, at 300–500 km altitude, at low solar activity. It is suggested that the solar activity influences long-term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2 density peak, and ion-neutral collision frequencies in the thermosphere and ionosphere.

Links between a stratospheric sudden warming and thermal structures and dynamics in the high-latitude mesosphere, lower thermosphere 査読有り

Kurihara, J., Y. Ogawa, S. Oyama, S. Nozawa, M. Tsutsumi, C. M. Hall, Y. Tomikawa, and R. Fujii

Geophys. Res. Lett. 37 巻 ( L13806 ) 頁： doi:10.1029/2010GL043643 2010年7月

記述言語：英語掲載種別：研究論文（学術雑誌）

We analyzed neutral winds, ambipolar diffusion coefficients, and neutral temperatures observed by the Nippon/Norway Tromsø Meteor Radar (NTMR) and ion temperatures observed by the European Incoherent Scatter (EISCAT) UHF radar at Tromsø (69.6°N, 19.2°E), during a major stratospheric sudden warming (SSW) that occurred in January 2009. The zonal winds at 80–100 km height reversed approximately 10 days earlier than the zonal wind reversal in the stratosphere and the neutral temperature at 90 km decreased simultaneously with the zonal wind reversal at the same altitude. We found different variations between geomagnetically quiet nighttime ion temperatures at 101–110 km and 120–142 km for about 10 days around the SSW. Our results from the ground-based observations agree well with the satellite observations shown in an accompanying paper. Thus, this study indicates that a SSW is strongly linked to thermal structure and dynamics in the high-latitude mesosphere, lower thermosphere, and ionosphere.

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign 査読有り

Kurihara, J., S. Oyama , S. Nozawa , T. Tsuda , R. Fujii , Y. Ogawa , H. Miyaoka , N. Iwagami , T. Abe , K.-I. Oyama , M. Kosch , A.L. Aruliah , E. Griffin , K. Kauristie

J. Geophys. Res. 114 巻頁： A12306, doi:10.1029/2009JA014392 2009年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

A coordinated observation of the atmospheric response to auroral energy
input in the polar lower thermosphere was conducted during the Dynamics and
Energetics of the Lower Thermosphere in Aurora (DELTA)
campaign. N2 rotational temperature was measured with a rocket-borne
instrument launched from the Andoya Rocket Range, neutral winds were
measured from auroral emissions at 557.7 nm with a Fabry-Perot
Interferome-ter (FPI) at Skibotn and the KEOPS, and ionospheric parameters
were measured with the European Incoherent Scatter (EISCAT) UHF radar at
Tromso. Altitude profiles of the passive energy deposition rate and the
particle heating rate were estimated using data taken with the EISCAT radar.
The local temperature enhancement derived from the difference between the
observed N2 rotational temperature and the MSISE-90 model neutral
temperature were 70-140 K at 110-140 km altitude. The temperature increase
rate derived from the estimated heating rates, however, cannot account for
the temperature enhancement below 120 km, even considering the contribution
of the neutral density to the estimated heating rate. The observed upward
winds up to 40 m s¡1 seem to respond nearly instantaneously to changes in
the heating rates. Although the wind speeds cannot be explained by the
estimated heating rate and the thermal expansion hypothesis, the present
study suggests that the generation mechanism of the large vertical winds
must be responsible for the fast response of the vertical wind to the
heating event.

On the source of the polar wind in the polar topside ionosphere: First results from the EISCAT Svalbard radar 査読有り

Ogawa, Y., I. Häggström, S. C. Buchert, K. Oksavik, S. Nozawa, M. Hirahara, A. P. van Eyken, T. Aso, and R. Fujii

Geophysical Research letters 36 巻頁： L24103, doi:10.1029/2009GL041501 2009年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

We present quantitative radar observations of both hydrogen ion (H+) and oxygen ion (O+) upflow in the topside polar ionosphere using measurements that were recently carried out with the EISCAT Svalbard Radar and the Reimei satellite. H+ upflow was clearly observed equatorward of the cusp above 500 km altitude. Within the cusp the H+ density was very low, and the upflow was dominated by O+ ions, but on closed field lines the H+ became the larger contributor to the upward flux above about 550 km. The total flux seemed to be conserved, and so below 550 km altitude O+ (with a small upward velocity of ~50 m s–1) appeared to determine the upward flux which was then maintained by H+ in the topside ionosphere. We also found that the H+ density in the topside polar ionosphere was several times higher than current predictions of ionospheric models like IRI2001

Ion heating in high-speed flow channel within the duskside cell of the polar-cap ion convection under large IMF-By 査読有り

Maeda, S, Y. Ogawa, K. Hosokawa, S. Nozawa, S. Oyama, T. Tsuda, and A. Brekke

J. Geophys. Res 114 巻頁： 10.1029/2009JA014300 2009年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

F region strong sunward ion flow embedded in the duskside cell of expanding polar cap ion convection and coincidental increase in the ion temperature were observed from about 73 degrees to 69 degrees in geomagnetic latitude between 16: 00 and 16: 48 MLT on 19 August, 2006 from an experiment using the European Incoherent Scatter (EISCAT) radars at Tromso, Sodankyla and Longyearbyen together with the CUTLASS HF radars. The spatiotemporal structure of the convection map obtained by the SuperDARN HF radars showed that the high-speed flow channel elongated in 14-17 MLT was moving equatorward. The flow channel appeared during an interval of negative interplanetary magnetic field B-z associated with large positive B-y. In order to evaluate ion frictional heating and resultant ion temperature increase quantitatively, the experiment was conducted to estimate the ion velocity vector and the ion temperature at the common scattering volume of the EISCAT radars. While the flow channel moved equatorward, the ion temperature increase became less evident at higher part of latitudes within the flow channel, which means that ion frictional heating was depressed where the neutral gases had already responded to the high-speed ions. The increase in the ion temperature at particular geomagnetic latitude continued for about 20 min, and then ceased while the ion velocity was still enhanced. Ion frictional heating lasted until the neutral wind was accelerated to the equivalent level of the surrounded ion flow.

Relations between proton auroras, intense electric field, and ionospheric electron density depletion 査読有り

Fujii, R., Y. Iwata, S. Oyama, S. Nozawa, and Y. Ogawa

J. Geophys. Res. 114 巻頁： 10.1029/2009JA014319 2009年9月

記述言語：英語掲載種別：研究論文（学術雑誌）

A case study with simultaneous European Incoherent Scatter and optical auroral observations was conducted in order to determine characteristics of the magnetosphere-ionosphere coupling from the viewpoint of the electrodynamics in the ionosphere. Particularly focused on were the relationships between ionospheric electron density depletion, perpendicular electric fields, and proton auroras. Intense electron density depletion was observed in the E and F regions poleward and in the vicinity of a thin equatorward moving arc. This depletion was associated with an intense, equatorward perpendicular electric field close to similar to 80 mV/m and most likely with a downward field-aligned current (FAC), but it did not accompany detectable proton aurora. Hence the downward FA electric field in the lower magnetosphere associated with this depletion was weak or absent. The motion of the FAC system with the depletion and the arc presumably enabled the downward FAC to obtain enough current carriers as ionospheric electrons were lost by the evacuation process. The evacuation process associated with the downward FAC was, however, efficient enough to establish the depletion. On the other hand, a widely distributed proton aurora observed immediately after the depletion was associated with an intense, equatorward perpendicular electric field close to 90 mV/m, enhanced electron density, and most likely a downward FAC. No electron precipitation was associated with this proton aurora. Thus the electron density enhancement, providing the downward current carriers, had to be caused by the ionization of precipitating protons presumably accelerated by downward field-aligned electric fields in the lower magnetosphere.

Spatial evolution of frictional heating and the predicted thermospheric wind effects in the vicinity of an auroral arc measured with the Sondrestrom incoherent-scatter radar and the Reimei satellite 査読有り

Oyama, S., T. T. Tsuda, T. Sakanoi, Y. Obuchi, K. Asamura, M. Hirahara, A. Yamazaki, Y. Kasaba, R. Fujii, S. Nozawa, and B. J. Watkins

J. Geophys. Res 114 巻頁： 10.1029/2009JA014091 2009年7月

記述言語：英語掲載種別：研究論文（学術雑誌）

Simultaneous observation with the Sondrestrom incoherent-scatter (IS) radar and the Reimei satellite was conducted on 3 October 2007. The objective was to measure horizontal patterns of the ionospheric structure in the vicinity of an auroral arc. The IS radar was scanned azimuthally with a fixed elevation angle, and the satellite narrow-view camera (557.7 nm) was directed downward taking pictures every 0.12 s. A stationary auroral arc was captured at 05: 17 UT (02: 54 MLT) with both instruments simultaneously. Gross ionospheric features measured with the IS radar around the arc were in good agreement with the expected magnetosphere-ionosphere current system. Of particular interest was the horizontal pattern in the ion speed and temperature in the F region. The ion speed within the arc was close to zero; by contrast the larger ion speed (350-400 m s(-1)) on the poleward side was parallel to the arc and almost no horizontal shear within about 20 km width perpendicular to the arc. This area was separated into two parts by the ion temperature: one was associated with clear enhancements in excess of 1200 K, and another was with more moderate enhancements (less than similar to 1000 K). The widths of the two areas were approximately 10 km each. The horizontal shear seen in the ion temperature suggested the presence of a narrow thermospheric wind shear of about 10 km width. This paper suggests that the possible cause for the thermospheric wind shear was ion drag associated with localized soft particle precipitation or F-region ionization.

Characteristics of ion upflow and downflow observed with the European Incoherent Scatter Svalbard radar 査読有り

Ogawa, Y. S. C. Buchert, R. Fujii, S. Nozawa, and A. P. van Eyken

J. Geophys. Res. 114 巻頁： 10.1029/2008JA013817 2009年5月

記述言語：英語掲載種別：研究論文（学術雑誌）

We have investigated how geomagnetic activity, the solar wind (SW), and the interplanetary magnetic field (IMF) influence the occurrence of the F-region/topside ionospheric ion upflow and downflow. Occurrence of dayside ion upflow observed with the European Incoherent Scatter Svalbard radar (ESR) at 75.2 degrees magnetic latitude is highly correlated with the SW density, as well as with the strength of the IMF By component. We suggest that this correlation exists because the region where ion upflow occurs is enlarged owing to SW density and IMF By magnitude, but it does not move significantly in geomagnetic latitude. The occurrence frequency of dayside ion upflow displays peaks versus the geomagnetic activity index (Kp), SW velocity, and negative IMF Bz component; that is, ion upflow is less frequently seen at the highest values of these parameters. Dayside ion downflow in the F-region/topside ionosphere occurs only when the Kp index and/or SW velocity are high or when IMF Bz is largely negative. The ion downflow is likely due to ballistic return of the ion upflow. We suggest that the region of ion upflow not only becomes larger but also moves equatorward with increasing Kp, SW velocity, and negative IMF Bz. The ESR can so be poleward of the upflow region and observe ions convecting poleward and returning ballistically downward.

*Acceleration mechanism of high-speed neutral wind observed in the polar lower thermosphere 査読有り

Tsuda, T. T., Satonori Nozawa , Shin Oyama , Tetsuo Motoba , Yasunobu Ogawa , Hiroyuki Shinagawa , Nozomu Nishitani , Keisuke Hosokawa , Natsuo Sato , Mark Lester , Ryoichi Fujii

J. Geophys. Res. 114 巻頁： doi:10.1029/2008JA013867 2009年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

e analyzed data obtained with the European incoherent scatter (EISCAT) Svalbard Radar (ESR) at Longyearbyen (78.2 degrees N, 16.0 degrees E in geographic coordinates, 75.2 degrees Lambda in invariant latitude) to advance our understanding of the acceleration mechanism of the lower thermospheric neutral wind when the ionospheric convection becomes enhanced. The Advanced Composition Explorer satellite observed a southward turning in the interplanetary magnetic field at 0843 UT on 16 June 2005. At 0900 UT, the F region ion velocity and the lower thermospheric neutral wind ( at 118-km altitude) observed with the ESR began to accelerate significantly in the westward and northwestward directions, respectively. The neutral wind was remarkably accelerated within 1 hour from 0900 UT, and its speed became a value of similar to 500 m s(-1) at 1000 UT. The wind speed was significantly higher than a typical wind speed at 118 km. We evaluated the ion drag contribution on the generation of the high-speed neutral wind. Our evaluation verifies that the ion drag force could not generate the high- speed neutral wind within such a short time (similar to 1 hour). This result implies that the major driving force was the horizontal pressure gradient force induced by the Joule heating. We deduced the contribution of the pressure gradient force based on a quantitative estimation of the Joule-heating-induced pressure gradient from the ESR and the Super Dual Auroral Radar Network data. We concluded that the pressure gradient force was the most probable force in this event.

Coordinated EISCAT Svalbard radar and Reimei satellite observations of ion upflows and suprathermal ions 査読有り

Ogawa, Y., K. Seki, M. Hirahara, K. Asamura, T. Sakanoi, S. C. Buchert, Y. Ebihara, Y. Obuchi, A. Yamazaki, I. Sandahl, S. Nozawa, and R. Fujii

J. Geophys. Research 113 巻頁： doi:10.1029/2007JA012791 2008年

The Pedersen current carried by electrons: A non-linear response of the ionosphere to magnetospheric forcing 査読有り

Buchert, S., T. Tsuda, R. Fujii, and S. Nozawa

Annales Geophysicae 26 巻頁： 2837–2844 2008年

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