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Found 11 entries in the Bibliography.
Showing entries from 1 through 11
| 2021 |
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This paper reports that plasma density depletions appearing at middle latitudes near sunrise survived until afternoon on 29 May 2017 during the recovery phase of a geomagnetic storm. By analyzing GPS data collected in Japan, we investigate temporal variations in the horizontal two-dimensional distribution of total electron content (TEC) during the geomagnetic storm. The SYM-H index reached −142 nT around 08 UT on 28 May 2017. TEC depletions extending up to approximately 38°N along the meridional direction appeared over Japan around 05 LT (LT = UT + 9 hours) on 29 May 2017, when TEC rapidly increased at sunrise due to the solar extreme ultraviolet (EUV) radiation. The TEC depletions appeared sequentially over Japan for approximately 8 hours in sunlit conditions. At 06 LT on 29 May, when the plasma depletions first appeared over Japan, the background TEC was enhanced to approximately 17 TECU, and then decreased to approximately 80\% of the TEC typical of magnetically quiet conditions. We conclude that this temporal variation of background plasma density in the ionosphere was responsible for the persistence of these plasma depletions for so long in daytime. By using the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2), we have evaluated how plasma production and ambipolar diffusion along the magnetic field may affect the rate of plasma depletion disappearance. Simulation shows that the plasma density increases at the time of plasma depletion appearance; subsequent decreases in the plasma density appear to be responsible for the long-lasting persistence of plasma depletions during daytime. The plasma density depletion in the top side ionosphere is not filled by the plasma generated by the solar EUV productions because plasma production occurs mainly at the bottom side of the ionosphere. Otsuka, Yuichi; Shinbori, Atsuki; Sori, Takuya; Tsugawa, Takuya; Nishioka, Michi; Huba, Joseph; Published by: Earth and Planetary Physics Published on: YEAR: 2021 DOI: 10.26464/epp2021046 Ionosphere; GPS; ionospheric irregularity; plasma bubble; SAMI2 |
| 2019 |
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The characteristics of global electron density variations in the ionosphere during a geomagnetic storm on November 7 and 8, 2004, were investigated using total electron content (TEC) obtained from the global navigation satellite system (GNSS). The regions of enhanced TEC over North America, Europe, and Japan first appeared in the mid-latitude regions. The TEC enhancements over North America showed a rapid longitudinal expansion and reached a wide longitudinal extent during the initial and main phases of the geomagnetic storm. TEC enhancements were simultaneously observed in both North America and Japan at 05:00 UT on November 8. Observation data from the Defense Meteorological Satellite Program showed a slight enhancement of electron density at 850 km below the equatorward boundary of the mid-latitude trough (45\textendash48\textdegreeN in geomagnetic latitude) over the Pacific Ocean. This electron density variation may correspond to the TEC enhancements observed in both Japan and North America. These results imply that an enhanced TEC region existed between North America and Japan. The TEC enhancement in Japan appeared with a magnetic conjugacy in the Southern hemisphere, indicating one of the characteristics of storm-enhanced density (SED). Moreover, TEC enhancements simultaneously appeared from Japan to Central Asia at 11:00 UT on November 8, corresponding to the early recovery phase of the geomagnetic storm. From the above results, it is suggested that SED phenomena can be simultaneously generated over a wide longitudinal width (~100\textdegree). The longitudinal extent of this SED event is 2.5\textendash5.0 times longer than those reported by previous studies. Sori, T.; Shinbori, A.; Otsuka, Y.; Tsugawa, T.; Nishioka, M.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2019 YEAR: 2019 DOI: 10.1029/2019JA026713 |
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Sori, T; Shinbori, A; Otsuka, Y; Tsugawa, T; Nishioka, M; Published by: Journal of Geophysical Research: Space Physics Published on: |
| 2015 |
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Climatology of equatorial plasma bubble observed by MyRTKnet over the years 2008--2013 Malaysia Real-Time Kinematics GNSS Network (MyRTKnet) which consists of 78 GPS receivers was used to investigate the occurrence of equatorial plasma bubble (EPB) along 96\textdegreeE-120\textdegreeE longitude. In this study, we present the monthly occurrence rate of EPB along the geographical longitudes of 96\textdegreeE-120\textdegreeE for a half of solar cycle period (2008-2013). A 2D map of rate of TEC change index (ROTI) projected at 300 km altitude was derived from the signal paths between GPS satellites and the receivers. A ROTI keogram for one day period was obtained from the east-west cross section of the 2D ROTI maps at 4\textdegreeN for every 5 min. The occurrence day of EPB was determined from the keogram by the existence of ROTI larger than 0.1 TECU/min within the 96\textdegreeE-120\textdegreeE longitude. The results show that the occurrence of EPB along the 96\textdegreeE-120\textdegreeE has maximum during equinoctial months and is consistent with previous studies. The occurrence rate of EPB during equinoctial months shows similar characteristics in low and high solar activity due to the broad observational coverage of the MyRTKnet. In contrast, the occurrence rate of EPB during solstice months shows significant relation with solar activity. Solstice months recorded high occurrence rate of EPB in high solar activity that might be attributed to post-midnight irregularities. Buhari, S.; Abdullah, M.; Yokoyama, T.; Hasbi, A.; Otsuka, Y.; Nishioka, M.; Bahari, S.A.; Tsugawa, T.; Published by: Published on: 08/2015 YEAR: 2015 DOI: 10.1109/IconSpace.2015.7283752 |
| 2014 |
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High-density GPS receivers located in Southeast Asia (SEA) were utilized to study the two-dimensional structure of ionospheric plasma irregularities in the equatorial region. The longitudinal and latitudinal variations of tens of kilometer-scale irregularities associated with equatorial plasma bubbles (EPBs) were investigated using two-dimensional maps of the rate of total electron content change index (ROTI) from 127 GPS receivers with an average spacing of about 50\textendash100 km. The longitudinal variations of the two-dimensional maps of GPS ROTI measurement on 5 April 2011 revealed that 16 striations of EPBs were generated continuously around the passage of the solar terminator. The separation distance between the subsequent onset locations varied from 100 to 550 km with 10 min intervals. The lifetimes of the EPBs observed by GPS ROTI measurement were between 50 min and over 7 h. The EPBs propagated 440\textendash3000 km toward the east with velocities of 83\textendash162 m s-1. The longitudinal variations of EPBs by GPS ROTI keogram coincided with the depletions of 630 nm emission observed using the airglow imager. Six EPBs were observed by GPS ROTI along the meridian of Equatorial Atmosphere Radar (EAR), while only three EPBs were detected by the EAR. The high-density GPS receivers in SEA have an advantage of providing time continuous descriptions of latitudinal/longitudinal variations of EPBs with both high spatial resolution and broad geographical coverage. The spatial periodicity of the EPBs could be associated with a wavelength of the quasiperiodic structures on the bottomside of the F region which initiate the Rayleigh-Taylor instability. Buhari, S.; Abdullah, M.; Hasbi, A.; Otsuka, Y.; Yokoyama, T.; Nishioka, M.; Tsugawa, T.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.1210.1002/2014JA020433 |
| 2011 |
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Bagiya, Mala; Iyer, K.; Joshi, H.; Thampi, Smitha; Tsugawa, Takuya; Ravindran, Sudha; Sridharan, R.; Pathan, B.; Published by: Journal of Geophysical Research Published on: Jan-01-2011 YEAR: 2011 DOI: 10.1029/2010JA015845 |
| 2009 |
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Medium-scale traveling ionospheric disturbances (MSTIDs) whose peak-to-peak amplitude was larger than 20 TECU (=1016el/m2) were observed at midlatitude during the geomagnetic storm on 10 November 2004. This amplitude was more than 10 times larger than that of the average MSTID. High-resolution data of the GPS Earth Observation Network (GEONET) clarified the characteristic of the total electron content (TEC) disturbances over Japan on 10 November 2004. The disturbances started around 1000 UT in the central part of Japan. The maximum of TEC temporal change was 7.2 TECU in 30 s. The disturbances had several wave fronts which extended from northwest to southeast and propagated from northeast to southwest. TEC data around Japan revealed that the disturbances were mainly observed from 18°N/S to 34°N/S of the geomagnetic latitude in the both hemispheres. Since those characteristics were similar to those of MSTIDs in spite of the unusual large amplitude, the MSTIDs are referred as “super-MSTIDs” in this paper. TEC variations of the super-MSTIDs were also observed at 460 km altitude by the GRACE satellite. The ion density fluctuations of the super-MSTIDs were observed in situ by the CHAMP and DMSP-F15 satellites, which flew at 360 km and 850 km, respectively. It is found that the plasma density variations of the super-MSTIDs occurred mainly above 360 km altitude. The characteristics that distinguish the event from plasma bubbles are its successive wave fronts, constant northwest-southeast direction along which the wave fronts stretched, and late local time of the occurrence. It is found that the uplift of the ionosphere around sunset excited the super-MSTIDs at midlatitudes. The uplift was attributed to the strong eastward electric field during the geomagnetic storm. Nishioka, M.; Saito, A.; Tsugawa, T.; Published by: Journal of Geophysical Research: Space Physics Published on: YEAR: 2009 DOI: https://doi.org/10.1029/2008JA013581 |
| 2007 |
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Tsugawa, T.; Zhang, S.-R.; Coster, A.; Otsuka, Y.; Sato, J.; Saito, A.; Zhang, Y.; Paxton, L.; Published by: Journal of Geophysical Research Published on: Jan-01-2007 YEAR: 2007 DOI: 10.1029/2007JA012415 |
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Low-latitude total electron content enhancement at low geomagnetic activity observed over Japan Kutiev, Ivan; Otsuka, Yuichi; Saito, Akinori; Tsugawa, Takuya; Published by: Journal of Geophysical Research: Space Physics Published on: |
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Tsugawa, T; Zhang, S-R; Coster, AJ; Otsuka, Y; Sato, J; Saito, A; Zhang, Y; Paxton, LJ; Published by: Journal of Geophysical Research: Space Physics Published on: |
| 2006 |
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Summer-Winter Hemispheric Asymmetry of Sudden Increase in Ionospheric Total Electron Content Tsugawa, T; Zhang, S; Coster, A; Otsuka, Y; Sato, J; Saito, A; Zhang, Y; Paxton, L; Published by: Published on: |
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