Y. Otsuka

Nagoya University, Nagoya, Aichi, Japan

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Publications (233)372.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper reports simultaneous observations of ionospheric scintillation during an auroral substorm that were made using an all-sky full-color digital single-lens reflex (DSLR) camera (ASC) and a Global Positioning System (GPS) ionospheric scintillation and total electron content monitor (GISTM) in Tromsø (69.60 N, 19.20 E), Norway. On the night of November 19, 2009, a small substorm occurred in northern Scandinavia. The ASC captured its temporal evolution from the beginning of the growth phase to the end of the recovery phase. The amplitude scintillation, as monitored by the S4 index from the GISTM, did not increase in any substorm phase. By contrast, phase scintillation, as measured by the σ φ index, occurred when discrete auroral arcs appeared on the GPS signal path. In particular, the phase scintillation was significantly enhanced for a few minutes immediately after the onset of the expansion phase. During this period, bright and discrete auroral forms covered the entire sky, which implies that structured precipitation on the scale of a few kilometers to a few tens of kilometers dominated the electron density distribution in the E region. Such inhomogeneous ionization structures probably produced significant changes in the refractive index and eventually resulted in the enhancement of the phase scintillation.
    11/2014;
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    ABSTRACT: Space-borne imagers are able to observe the airglow structures with wide field-of-views regardless of the tropospheric condition that limits the observational time of the ground-based imagers. Concentric wave structures of the O2 airglow in 762-nm wavelength were observed over North America on June 1, 2013 from the International Space Station. This was the first observation in which the entire image of the structure was captured from space, and its spatial scale size was determined to be 1,200 km radius without assumptions. The apparent horizontal wavelength was 80 km and the amplitude in the intensity was approximately 20% of the background intensity. The propagation velocity of the structure was derived as 125 ± 62 m/s and atmospheric gravity waves were estimated to be generated for 3.5 ± 1.7 hours. Concentric structures observed in this event were interpreted to be generated by super cells that caused a tornado in its early phase.
    Geophysical Research Letters. 09/2014;
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    ABSTRACT: Auroral patches in diffuse auroras are very common features in the post-midnight local time. However, the processes that produce auroral patches are not yet well understood. In this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches. These examples were observed at Athabasca, Canada (geomagnetic latitude: 61.7°N), and Tromsø, Norway (67.1°N). Captured in sequences of images, the auroral fragmentation occurs as finger-like structures developing latitudinally with horizontal scale sizes of 40–100 km at ionospheric altitudes. The structures tend to develop in a north–south direction with speeds of 150–420 m/s without any shearing motion, suggesting that pressure-driven instability in the balance between the earthward magnetic-tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation. Therefore, these observations indicate that auroral fragmentation associated with pressure-driven instability is a process that creates auroral patches. The observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low-energy ambient plasma.
    Journal of Geophysical Research: Space Physics. 09/2014;
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    ABSTRACT: In this paper, we analyze vertical ExB drifts obtained from the Doppler shifts of the daytime 150 km radar echoes from two radar stations located off the magnetic equator, namely, Gadanki in India and Kototabang in Indonesia, and compare those with corresponding CINDI observations onboard the C/NOFS satellite and the Scherliess-Fejer model in an effort to understand to what extent the low latitude vertical ExB drifts of the 150 km region represent the F region vertical ExB drifts. The radar observations were made during 9–16 LT in January, June, July and December 2009. A detailed comparison reveals that vertical ExB drifts observed by the radars at both locations agree well with those of CINDI and differ remarkably from those of the model. Importantly, the model and observed drifts show large disagreement when the observed drifts are either large or downward. Further, while the CINDI as well as the radar observations from the two longitudes are found to agree with each other on the average, they differ remarkably on several occasions when compared on a one-to-one basis. The observed difference in detail is due to measurements made in different volumes linked with latitudinal and/or longitudinal differences and underlines the role of neutral dynamics linked with tides and gravity waves in the two longitude sectors on the respective vertical ExB drifts. The results presented here are the first of their kind and are expected to have wider applications in furthering our understanding on fine-scale longitudinal variabilities in the ionosphere in general and ionospheric electrodynamics in the Indian and Indonesian sectors in particular.
    Journal of Geophysical Research 04/2014; 119:3777-3788. · 3.17 Impact Factor
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    ABSTRACT: The total electron content (TEC) in the equatorial and low-latitude ionosphere over Brazil was monitored in two dimensions by using 2011 data from the ground-based global navigation satellite system (GNSS) receiver network operated by the Brazilian Institute for Geography and Statistics. It was possible to monitor the spatial and temporal variations in TEC over Brazil continuously during both day and night with a temporal interval of 10 min and a spatial resolution of about 400 km. The daytime equatorial ionization anomaly (EIA) and post-sunset plasma enhancement (PS-EIA) were monitored over an area corresponding to a longitudinal extension of 4000 km in South America. Considerable day-to-day variation was observed in EIA and PS-EIA. A large latitudinal and longitudinal gradient of TEC indicated a significant ionospheric range error in application of the GNSS positioning system. Large-scale plasma bubbles after sunset were also mapped over a wide range. Depletions with longitudinally separated by more than 800 km were observed. They were extended by more than 2000 km along the magnetic field lines and drifted eastward. It is expected that 2-dimensional TEC mapping can serve as a useful tool for diagnosing ionospheric weather, such as temporal and spatial variation in the equatorial plasma trough and crest, and particularly for monitoring the dynamics of plasma bubbles.
    Advances in Space Research 01/2014; · 1.18 Impact Factor
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    ABSTRACT: [1] During a 2-h interval from 2240 to 2440 UT on November 12, 2012, regions of increased 630.0 nm airglow emission were simultaneously detected by dual all-sky imagers in the polar cap, one at Longyearbyen, Norway (78.1 N, 15.5E) and the other at Resolute Bay, Canada (74.7 N, 265.1E). The Resolute Bay incoherent scatter radar observed clear enhancements of the F region electron density up to 1012 m− 3 within these airglow structures, which indicates that these are optical manifestations of polar cap patches propagating across the polar cap. During this interval of simultaneous airglow imaging, the nightside/dawnside (dayside/duskside) half of the patches was captured by the imager at Longyearbyen (Resolute Bay). This unique situation enabled us to estimate the dawn-dusk extent of the patches to be around 1500 km, which was at least 60–70% of the width of the anti-sunward plasma stream seen in the SuperDARN convection maps. In contrast to the large extent in the dawn-dusk direction, the noon-midnight thickness of each patch was less than 500 km. These observations demonstrate that there exists a class of patches showing cigar-shaped structure. Such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation.
    Geophysical Research Letters. 12/2013;
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    ABSTRACT: [1] We present the first reported gravity wave patterns in the mesopause region caused by a typhoon in the troposphere. On 10 December 2002, concentric rings of gravity waves in OH airglow were observed simultaneously by all-sky imagers in the Optical Mesosphere and Thermosphere Imager system in Japan, located at Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.9°N, 136.1°E), and Sata (31.0°N, 130.7°E). The airglow structures, which were well defined and formed a coherent wave pattern expanding concentrically, were identified over 8 h (2135–2947 LT). We estimate the horizontal wavelength, horizontal phase speed, and wave period as 34.5 km, 50.2 m s−1, and 11.5 min, respectively. Infrared cloud images from the Geostationary Meteorological Satellite show that the center of the rings estimated from the airglow data corresponds to a spiral band of Typhoon Pongsona (T0226). This unique event provides new insight into coupling between the lower and upper atmosphere.
    Geophysical Research Letters. 11/2013; 40(22).
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    ABSTRACT: Unusual ionospheric variations were observed in the M9.0 Tohoku-oki earthquake on 11 March 2011. Among various kinds of features in the ionosphere, significant depletion of total electron content (TEC) near the epicenter was observed after the earthquake. Although previous studies have suggested that the coseismic ionospheric variations are associated with atmospheric perturbation caused by vertical displacement of the sea surface, the mechanism of the TEC depletion has not been fully understood. In this paper, a two-dimensional nonlinear nonhydrostatic compressible atmosphere-ionosphere model is employed to investigate the ionospheric variations in the vicinity of the epicenter. The simulation results reveal that an impulsive pressure pulse produced by a sudden uplift of the sea surface leads to local atmospheric expansion in the thermosphere and that the expansion of the thermosphere combined with the effect of inclined magnetic field lines in the ionosphere causes the sudden TEC depletion above the epicenter region.
    Geophysical Research Letters 10/2013; 40(19):5009-5013. · 3.98 Impact Factor
  • K.J.W. Lynn, Y. Otsuka, K. Shiokawa
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    ABSTRACT: A form of range-time plots derived from ionograms taken from a standard digital ionosondes, situated at the low latitude sites of Vanimo, Port Moresby and Darwin, exhibit bursts of spread F at the center of descending and ascending off-angle reflectors. This particular type of event has since been identified with the passage of optically imaged ionospheric plasma depletions (bubbles) over a Darwin ionosonde. This paper describes the process for producing this form of range-time display and its relationship to ionospheric height, satellite traces and range spread F as seen on individual ionograms. First hop satellite traces are proposed to be via direct reflection from the steep electron density gradients at the base of bubbles while second hop satellite traces then involve a single additional ground reflection. Measurements of night equatorial drift velocity were made from the range-time displays and found to be in the range 20–220 m/s peaking at approximately 90–100 m/s in good agreement with values derived from drift measurements made by a variety of other types of equipment.
    Journal of Atmospheric and Solar-Terrestrial Physics 06/2013; 98:105–112. · 1.42 Impact Factor
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    ABSTRACT: The paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS-TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm-time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources.
    Journal of Geophysical Research 05/2013; 118(5):2660-2669. · 3.17 Impact Factor
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    ABSTRACT: [1] We present observational evidence of gravity wave ducting and show, for the first time, that the ducting occurs over large horizontal distances in the mesopause region. An optical network of four all-sky imagers in Japan identified two-dimensional patterns of small-scale gravity wave bands in OH airglow images on 13 June 2004. The wave signatures clearly showed northward propagation of more than 1800 km over a wide range of latitudes. The horizontal wavelength, horizontal phase speed, and wave period were estimated from the airglow data as 33.4 km, 42.8 m s−1, and 13.4 min, respectively. The wave structure lasted for the whole 5-h airglow observation period. Simultaneous MF radar wind data and TIMED/SABER measurements suggested that the wave was trapped and ducted at the airglow height. The ducting likely contributed to the remarkable coherence of the wave as it propagated northward.
    Geophysical Research Letters 02/2013; 40(3):601-605. · 3.98 Impact Factor
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    ABSTRACT: In this paper, we study for the first time the daytime vertical E × B drift velocities from Gadanki and Kototabang using the Doppler shifts of the 150-km echoes observed during 2008-2010, a period of low solar activity. Drift velocities are mostly positive and confined to 35 m s-1 at both the locations, except for Gadanki where on a few occasions negative drift velocities have been observed in the afternoon hours. Drift velocities generally show a decreasing trend with local time and the largest drift is generally observed in the forenoon hours consistent with extensively reported observations and models of E × B drift. Drift velocities from Gadanki and Kototabang compared exceeding well on some days and differed remarkably on many days despite the fact that they are longitudinally separated by only 20°. The day-to-day variation in the drift velocity could be as high as 15 m s-1 at Gadanki and 7 m s-1 at Kototabang. Seasonal mean drifts over Gadanki are found to be generally larger than those of Kototabang. The observations have been compared in detail with those reported earlier based on ground- and satellite- based observations and also with the Scherliess-Fejer model. The observed differences in the drifts at the two locations, including the downward drifts, have been discussed in the light of current understanding of the longitudinal variability of E × B drift.
    Journal of Geophysical Research 11/2012; 117(A11):11312-. · 3.17 Impact Factor
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    ABSTRACT: We have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromsø site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010-2011 (FP02-FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 × 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to -80°C. The large incident angle (maximum: 1.3°-1.4°) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromsø by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s-1 and from 4 to 27 m s-1 for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02-FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s-1 to more than 50 m s-1 depending on airglow intensity.
    Earth, Planets, and Space. 11/2012; 64(11):1033-1046.
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    ABSTRACT: We report on nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3°E geographic latitude: 0.2°S and geomagnetic latitude: 10.6°S) during a 7-year period from October 2002 to October 2009. MSTIDs were observed in 630-nm nighttime airglow images by using a highly sensitive all-sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 ± 170 m/s, 42 ± 11 min, and 790 ± 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within ±30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere.
    Journal of Geophysical Research 10/2012; 117(A10):10324-. · 3.17 Impact Factor
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    ABSTRACT: We report for the first time the rapid oscillating motion of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) based on airglow imaging observations at Tromsø (magnetic latitude: 67.1°N), Norway on 8 December, 2009. The MSTIDs appeared in 630-nm airglow images at 1530 UT as wave-like structures south of Tromsø with a horizontal wavelength of ˜200 km and a phase surface of north to south. They moved eastward with velocities of 30-60 m/s. The velocity was faster in the poleward-side of the MSTIDs, forming a northeast-southwest phase surface at later times. This phase surface direction is opposite to that of midlatitude MSTIDs. The MSTIDs show sudden oscillations and phase jump in the east-west direction with a timescale of ˜10 min at 1730 UT. The oscillations were associated with an auroral brightening observed at the poleward edge of the images and small magnetic field perturbations observed by ground magnetometers. The Doppler measurement of the 630-nm airglow by a Fabry-Perot interferometer at Tromsø showed a stable southeastward thermospheric wind with a velocity of ˜150 m/s. These observations indicate that the MSTID oscillations were linked to auroral electric field in the ionosphere, implying that the observed MSTIDs are ionospheric plasma structures. We suggest that the observed MSTIDs were created by atmospheric gravity waves at the beginning, left as fossil plasma structures even after the gravity wave packet dissipated in the thermosphere, moved eastward according to the background electric field driven by the F-region dynamo, and oscillated associated with the auroral electric field.
    Journal of Geophysical Research 10/2012; 117(A10):10316-. · 3.17 Impact Factor
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    ABSTRACT: We investigated the relationship between post-midnightF-region field aligned irregularities (FAIs) andF-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2°S, 100.3°E; geomagnetic latitude 10.4°S) and an ionosonde installed at Chumphon, Thailand (10.7°N, 99.4°E; geomagnetic latitude 3.3°N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of theF-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of theE-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that theF-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and theF-layer altitude. Both the post-midnight FAIs and the uplift of theF-layer were frequently seen around midnight between May and August. The seasonal variation of the midnightF-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of theF-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of theF-layer observed at Chumphon. The result shows that the uplift of theF-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the upliftedF-layer.
    Journal of Geophysical Research 08/2012; 117(A8):8337-. · 3.17 Impact Factor
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    ABSTRACT: The paper presents the medium-term quasi periodic (˜9-27 day) response of middle and low-latitude ionosphere to solar [F10.7) and geomagnetic (Kp-index) forcing. The ionospheric response is examined by wavelet analysis of the relative deviations of TEC over Japan for the period of time 2000-2008. It is found that the ˜27-day rTEC oscillations correlate well with the same oscillations of the solar index F10.7 particularly in the solar maximum and its early declining phase (2001-2005). During the declining phase of solar activity (for example, year of 2005) the Kp-index variability exhibits additionally strong oscillations with periods 13.5- and 9-days. Similar oscillations are found in rTEC as well but they do not follow the geomagnetic forcing as faithfully as those associated with F10.7. During solar minimum the quasi periodic rTEC variability is shaped mainly by the recurrent geomagnetic activity. An attempt is made to investigate the latitudinal dependence of the ˜9-27-day rTEC response over Japan as well as the phase relationship between the forcing and response.
    Journal of Geophysical Research 08/2012; 117(A8):8330-. · 3.17 Impact Factor
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    ABSTRACT: The Solar-Terrestrial Environment Laboratory, Nagoya University, has operated the Optical Mesosphere Thermosphere Imagers (OMTIs). The OMTIs consist of 13 all-sky cooled-CCD imagers, five Fabry-Perot interferometers (FPIs), three meridian scanning photometers, and four airglow temperature photometers, to measure two-dimensional pattern, Doppler wind, and temperature through airglow emissions, at Norway, Australia, Indonesia, Thailand, far-eastern Russia, four stations in Japan, and two stations in Canada. In this presentation, we review these automated observation of OMTIs. Then we focus on an event of plasma bubble dissipation observed by airglow imagers at geomagnetic conjugate points at Darwin, Australia (12.4S, 131.0E, magnetic latitude (MLAT): -22.1N) and Sata, Japan (31.0N, 130.7E, MLAT: 21.2N) on August 8, 2002. The plasma bubble was observed in 630-nm airglow imagers at both stations from 15 UT (00 LT) to 18UT (03 LT) and dissipate at 18-19 UT (03-04 LT) in the field-of-view of the imagers. The ionosonde at Darwin show spread-F signatures at 16-22 UT even after the bubble disappearance. A Fabry-Perot interferometer at Shigaraki, Japan indicates decrease of eastward thermospheric wind from ~50 m/s (15 UT) to 0 m/s (18 UT) consistent with slowing down of eastward motion of the bubble. The ionosonde shows that the ionospheric height increases from 18 to 19 UT when the bubble dissipated. The 630-nm airglow intensity decreases associated with this ionospheric height raise. We discuss possible cause of this sudden dissipation of plasma bubbles based on these observations.
    07/2012;
  • Y. Otsuka, K. Shiokawa, T. Ogawa
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    ABSTRACT: We report simultaneous observations of an equatorial plasma bubble and a Medium-Scale Traveling Ionospheric Disturbance (MSTID) in 630-nm airglow images taken with an all-sky airglow imager at Shigaraki (34.9°N, 136.1°E dip angle of the geomagnetic field ∼49°), Japan. Clear depletion of the 630-nm airglow intensity was observed as the equatorial plasma bubble propagated eastward, whereas the MSTID, which had a wavefront aligned from northwest to southeast, propagated southwestward. This result indicates that MSTIDs do not propagate at the same velocity as the ambient plasma, which is clearly shown by the eastward motion of the plasma bubbles. We found that the airglow depletion caused by the plasma bubble disappeared when the plasma bubble encountered the MSTID. The plasma depletion could be filled with ambient rich plasma that moved into the plasma-depleted region by E × B drift associated with the MSTID, indicating that MSTIDs are accompanied by electric field perturbations.
    Geophysical Research Letters 07/2012; 39(14):14105-. · 3.98 Impact Factor
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    ABSTRACT: An All-Sky imager at Resolute Bay, Canada observed many cases of Poleward Moving Sun Aligned Arcs (PMSAAs) during the Winter of 2010. Three Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN), intersecting the field of view of the imager, showed rapid fluctuations of signal amplitude and phase (scintillation) associated with many of the PMSAAs. These fluctuations were intermittent (durations <10 min) and seen near simultaneously on many available ray-paths irrespective of the orientation of the ray-path with the arc and its motion. This observation is contrary to the well accepted hypothesis that scintillation producing irregularity is formed only in certain parts of the plasma structure. Spectral analysis of the amplitude and phase scintillation associated with PMSAAs showed different spectral slope compared to other forms of scintillations. These results along with the fact that scintillation is seen on all available ray-paths suggests a different mechanism for the generation of the scintillation. Implication of the results for the understanding of the scintillation producing irregularities will be discussed.
    07/2012;

Publication Stats

2k Citations
372.29 Total Impact Points

Institutions

  • 2000–2014
    • Nagoya University
      • Solar-Terrestrial Environment Laboratory
      Nagoya, Aichi, Japan
    • National Institute for Space Research, Brazil
      • Aeronomy Division
      São José dos Campos, São Paulo, Brazil
  • 2012
    • Clemson University
      • Department of Physics and Astronomy
      Anderson, Indiana, United States
  • 2006
    • Osaka University
      • Institute of Scientific and Industrial Research
      Suika, Ōsaka, Japan
    • King Mongkut's Institute of Technology Ladkrabang
      Krung Thep, Bangkok, Thailand
  • 2005
    • National Institute of Information and Communications Technology
      Edo, Tōkyō, Japan
  • 2002
    • The University of Sheffield
      Sheffield, England, United Kingdom
  • 1997–2000
    • Kyoto University
      Kioto, Kyōto, Japan
  • 1999
    • Boston University
      Boston, Massachusetts, United States