• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2008년도 International Symposium on Remote Sensing
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• pp.348-357
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• 2008

The classification of the effect of ionospheric disturbances on the radio occultation signal amplitude has been introduced based on an analysis of more than 2000 seances of radio occultation measurements per formed with the help of the CHAMP German satellite. The dependence of the histograms of variations in the radio occultation signal amplitude on the IMF variation index has been revealed. It has been indicated that it is possible to introduce the radio occultation index characterizing the relation between ionospheric disturbances and solar activity. An amplitude radio occultation (RO) method is proposed to study connection between the ionospheric and solar activity on a global scale. Sporadic amplitude scintillation observed in RO experiments contain important information concerning the seasonal, geographical, and temporal distributions of the ionospheric disturbances and depend on solar activity. The probability of strong RO amplitude variations (RO $S_4$ index greater than 0.2) in the CHAMP RO signals diminishes sharply with the weakening of solar activity from 2001 to 2008. The general number of RO events with strong amplitude variations can be used as an indicator of the ionospheric activity. We found that during 2001-2008 the daily globally averaged RO $S_{4a}$ index depends essentially on solar activity. The maximum occurred in January 2002, minimum has been observed in summer 2008. Different temporal behavoir of $S_{4a}$ index has been detected for polar (with latitude greater than $60^{\circ}$) and low latitude (moderate and equatorial) regions. For polar regions $S_{4a}$ index is slowly decreasing with solar activity. In the low latitude areas $S_{4a}$ index is sharply oscillating, depending on the solar ultraviolet emission variations. The different geographical behavoir of $S_{4a}$ index indicates different origin of ionospheric plasma disturbances in polar and low latitude areas. Origin of the plasma disturbances in the polar areas may be connected with influence of solar wind, the ultraviolet emission of the Sun may be the main cause of the ionospheric irregularities in the low latitude zone. Therefore, the $S_{4a}$ index of RO signal is important radio physical indicator of solar activity.

• Journal of Ecology and Environment
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• 제37권2호
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• pp.69-79
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• 2014

Soil organic carbon (SOC) in the Arctic is vulnerable to climate change. However, research on SOC stored in the high Arctic regions is currently very limited. Thus, this study was aimed at understanding the distribution and characteristics of SOC with respect to geomorphology and vegetation in Svalbard. In August 2011, soil samples were collected near the Vestre Lov$\acute{e}$nbreen moraine. Sampling sites were chosen according to altitude (High, Mid, and Low) and differences in levels of vegetation establishment. Vegetation coverage, aboveground biomass, and SOC contents were measured, and density-size fractionation of SOC was conducted. The SOC content was the highest in the Mid site ($126.9mg\;g^{-1}$) and the lowest in the High site ($32.1mg\;g^{-1}$), although aboveground biomass and vegetation coverage were not different between these two sites. The low SOC content measured at the High site could be related to a slower soil development following glacial retreat. On the other hand, the Low site contained a high amount of SOC despite having low vegetative cover and a high ratio of sand particles. These incompatible relationships between SOC and vegetation in the Low site might be associated with past site disturbances such as runoff from snow/glacier melting. This study showed that geomorphological features combined with glacier retreat or melting snow/glacier effects could have affected the SOC distribution and vegetation establishment in the high Arctic.

• Ocean and Polar Research
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• 제26권3호
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• pp.489-499
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• 2004

The climatological characteristics of the polar ionospheric currents obtained from the simultaneous observations of the ionospheric electric field and conductivity are examined. For this purpose, 43 and 109 days of measurements from the Chatanika and Sondrestrom incoherent scatter radars are utilized respectively. The ionospheric current density is compared with the corresponding ground magnetic disturbance. Several interesting characteristics about the polar ionosphere are apparent from this study: (1) The sun determines largely the conductance over the Sondrestrom radar, while the nighttime conductance distribution over the Chatanika radar is significantly affected by auroral precipitation. (2) The regions of the maximum N-S electric field over the Chatanika radar are located approximately at the dawn and dusk sectors, while they tend to shift towards dayside over the Sondrestrom radar. The N-S component over Son-drestrom is slightly stronger than Chatanika. However, the E-W component over Chatanika is negligible compared to that of Sondrestrom. (3) The E-W ionospheric current flows dominantly in the night hemisphere over Chatanika, while it flows in the sunlit hemisphere over Sondrestrom. The N-S current over Chatanika flows prominently in the dawn and dusk sectors, while a strong southward current flows in the prenoon sector over Sondrestrom. (4) The assumption of infinite sheet current approximation is far from realistic, underestimating the current density by a factor of 2 or more. It is particularly serious for the higher latitude region. (5) The correlation between ${\Delta}H\;and\;J_E$ is higher than the one between ${\Delta}D\;and\;J_N$, indicating that field-aligned current affects ${\Delta}D$significantly.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2010년도 제39회 하계학술대회 초록집
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• pp.259-259
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• 2010

Periodically polarity inverted (PPI) ZnO structures on (0001) Al2O3 substrates are demonstrated by plasmas assisted molecular beam epitaxy. The patterning and re-growth methods are used to realize the PPI ZnO by employing the polarity controlling method. For the in-situ polarity controlling of ZnO films, Cr-compound buffer layers are used.[1, 2] The region with the CrN intermediate layer and the region with the Cr2O3 and Al2O3 substrate were used to grow the Zn- and O-polar ZnO films, respectively. The growth behaviors with anisotropic properties of PPI ZnO heterostructures are investigated. The periodical polarity inversion is evaluated by contrast images of piezo-response microscopy. Structural and optical interface properties of PPI ZnO are investigated by the transmission electron microcopy (TEM) and micro photoluminescence ($\mu$-PL). The inversion domain boundaries (IDBs) between the Zn and the O-polar ZnO regions were clearly observed by TEM. Moreover, the investigation of spatially resolved local photoluminescence characteristics of PPI ZnO revealed stronger excitonic emission at the interfacial region with the IDBs compared to the Zn-polar or the O-polar ZnO region. The possible mechanisms will be discussed with the consideration of the atomic configuration, carrier life time, and geometrical effects. The successful realization of PPI structures with nanometer scale period indicates the possibility for the application to the photonic band-gap structures or waveguide fabrication. The details of application and results will be discussed.

• Ocean and Polar Research
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• 제32권4호
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• pp.397-409
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• 2010

Herein, we compared the production rate of UV-absorbing compounds (mycosporine-like amino acids) and carotenoids in two phytoplankton species--Phaeocystis pouchetii and Porosira glacialis--which are the dominant species in Polar Regions under artificial UV radiation conditions. P. pouchetii exposed to UVR and PAR evidenced reductions in the carbon fixation rate, and was not significantly influenced by differing light conditions. However, the concentrations of UV-absorbing compounds and photo-protective pigments of P. pouchetii were increased with increasing exposure time, but P. glacialis maintained constant levels during the UVR exposure experiment. The production rates of MAAs showed a reverse phase between the two phytoplankton species. The carbon fixation rate of P. pouchetii cells was inhibited by exposure to UV radiation, but the production rates of MAAs in P. pouchetii were increased under UV radiation exposure. The carbon fixation rate and production rate of MAAs in P. glacialis were simultaneously inhibited under UV radiation exposure conditions. These results provide us with new information regarding the processes involved in the production of UV-absorbing compounds and photoprotective pigments in two phytoplankton species.

• Ocean and Polar Research
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• 제26권1호
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• pp.11-22
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• 2004

Benthic pollution assessment based on macrobenthic community structure with environmental variables was carried out at twelve stations during two periods on a presumed pollution gradient in Gamak Bay. Univariate and multivariate methods were applied to investigate structural changes in the benthic communities. A clear gradient of pollution effects on the macrobenthic community was observed from the interior to the exterior of the bay. The community on the northwestern basin was severely disturbed due to a low level of hydrodynamics and a large amount of pollutant input from nearby cities. Exterior regions on the southern basin appeared to have the best benthic environmental characteristics among all stations according to most methods of analysis. Central ridge regions and two stations around the islets in the mouth of the bay exhibited intermediate levels of perturbation when compared to the more disturbed interior and undisturbed exterior regions. Pollution effects on the communities were attenuated at the southern area of the central ridge during spring compared to those of summer, where aquacultural farming was densely distributed. The environmental variables primarily correlated to the macrobenthic community structure were total organic carbon (C), organochlorine pesticides (OCPs), and tributyltins (TBTs), contents found on the surface sediment, as anthropogenic variables indicating organic materials.

• 천문학논총
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• 제7권1호
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• pp.39-50
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• 1992

A velocity inhomogeneity, which is the regional preponderence of either radial or tangential orbits, is searched with the new technique proposed by Kim (1992) for Coma, Hydra I, and Abell 2256 cluster of galaxies. Conspicuous inhomogeneities are found in the Coma and A2256 which X-ray isophotes are indicative for their underlying potentials being ellipitcal in shape, Even in their central regions, zones that are dominated by radial orbits are clearly distinguishable from that of the tangential orbits, and defining the cluster 'equator' as the direction of maximum elongation of the X-ray isophotes, radial orbits dominate along this direction whereas tangential orbits dominate the 'polar' zones. Merger events that are evidenced in X-ray observations occur in the equatorial zones of Coma and A2256, suggesting preponderence of radial orbits in the zones, which is in good agreement with their velocity structures. On the other hand, the inhomogeneity in Hydra I turns out to be insignificant in the central regions and this is just what is expected from a cluster whose X-ray isophotes is nearly circular. The velocity distribution in regions further out, however, shows significant inhomogeneity and this seems to support the previous results that this cluster is likely to have substructures and velocity anisotropy.

• 동력기계공학회지
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• 제17권5호
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• pp.30-37
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• 2013

For vessels operating in the cold climate regions, the ballast water inside or hopper tanks above the waterline may be frozen, starting at the top of the tank and at the side walls. Therefore, countermeasures against freeze-up of the ballast tank such as air-bubbling system, hot steam injecting system, heating coil system and water circulating system are taken to prevent freeze-up phenomenon; however, there are no rigorous investigations of anti-freezing to examine the effectiveness and validity of systems against freeze-up of the ballast tank, in which the temperatures are about -$25^{\circ}C$ (ambient air temperature) and $0^{\circ}C$ (sea water), respectively. In this paper, to ensure reasonable specifications for cold regions if the measures from the above-mentioned systems against freeze-up are effective, the phenomenon of ballast tank freeze-up is simulated and discussed in low temperature conditions. With the results using the commercial CFD code, CFX 14, the most cost-effective solution is conducted to prevent being frozen along the outer surface.

• 대한토목학회논문집
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• 제39권1호
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• pp.211-217
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• 2019

전 세계적인 경제 및 정치 이슈로 인한 원유가격의 급변으로 비전통 원유자원인 오일샌드에 대한 관심이 증가하고 있다. 주로 오일샌드 현장은 극한지에 위치하고 있다. 혹독한 환경인 극한지의 오일샌드 현장에서 원유를 생산하기 위해서는 빠르고 간단한 절차의 기초 설치가 필요하다. 하지만 플랜트 기초로 주로 사용되는 현장타설 및 항타말뚝은 저온 및 유기토 표층이 있는 극한지 환경에 설치하기가 적합하지 않다. 따라서 본 연구에서는 항타나 굴착없이 소형 회전관입 장비로 간편한 급속시공이 가능한 헬리컬 파일의 거동을 분석하고자 한다. 헬리컬 파일은 강관축에 헬릭스가 부착된 형태로 헬릭스의 개수 및 형상별 지지거동의 차이가 발생되므로, 헬릭스의 조합(개수, 직경)에 따라 지지력 거동의 차이를 수치해석적으로 분석하였다.

• 대한토목학회논문집
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• 제39권6호
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• pp.801-809
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• 2019

2009년 NASA에서 수행된 달 극지 탐사 미션을 통해 달 극지의 영구음영지역에 얼음 층이 존재한다는 증거가 발견되었다. 이후, 달 극지 지역 얼음 층 탐사를 위한 지반 특성 평가 연구들이 국내외적으로 활발히 이루어지고 있다. 본 연구에서는 달 영구음영지역의 지반온도변화를 예측하고, 얼음 층이 지반 온도 프로파일에 미치는 영향을 정량적으로 평가하고자 달 지반에 대한 비정상 상태 열 해석을 수행하였다. 수치해석결과를 통해 위도 86° 이상에서 달 지반 내부의 온도가 얼음승화 기준온도인 112 K 이하로 수렴한다는 것을 확인하였고, 이는 달 극지 내 얼음 층이 존재하고 있을 가능성이 높은 지역을 확률적으로 판단할 수 있는 근거가 되었다. 얼음 층이 매장되어 있는 깊이에 따라 지반 온도 프로파일에 미치는 영향정도가 다르게 나타났는데, 온도 편차가 큰 얕은 심도에 존재하는 얼음 층은 이질적인 온도분포 특성을 초래한다는 사실을 확인하였다. 또한, 본 연구는 매장된 얼음의 상을 보존하도록 하는 드릴 비트의 최대 허용 마찰열에 대해서 고찰하였다.