• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2010년도 한국우주과학회보 제19권1호
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• pp.40.2-40.2
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• 2010

It has been reported that geosynchronous magnetopause crossings are more frequently observed in the prenoon sector than in the postnoon sector, indicating a dawn-dusk magnetopause asymmetry during extreme solar wind conditions. Motivated by these observations, we investigate geosynchronous magnetic field variations normalized to SYM-H when sudden commencements (SC) are observed on the ground. From a statistical analysis of the geosynchronous magnetic field responses to SC events from 1997 to 2006, we found that the normalized SC amplitude at geosynchronous orbit is larger in the morning sector than in the afternoon sector. In order to examine if this morning-afternoon asymmetry at geosynchronous orbit occurs only during disturbed geomagnetic conditions, we compared the geosynchronous magnetic field strength obtained in the morning and afternoon during undisturbed intervals (Kp < 3). We found that the asymmetry appears under undisturbed geomagnetic conditions and it is not due to solar wind aberration. This indicates that the morning-afternoon asymmetry was not strongly affected by changes in solar wind condition. Using solar wind data, we discuss what causes the morning-afternoon asymmetry at geosynchronous orbit.

• 천문학회보
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• 제37권1호
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• pp.91.2-91.2
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• 2012

Kokubun (1983) reported the local time variation of normalized amplitude of sudden commencement (SC) with a strong day-night asymmetry at geosynchronous orbit with 81 SC events. Further careful inspection of Kokubun's local time distribution reveals that the normalized SC amplitudes in the prenoon sector are larger than those in the postnoon sector. That is, there is a morning-afternoon asymmetry in the normalized SC amplitudes. Until now, however, there are no studies on this SC-associated morning-afternoon asymmetry at geosynchronous orbit. Motivated by this previous observation, we investigate a large data set (422 SC events in total) of geosynchronous SC observations and confirm that the geosynchronous SC amplitudes is larger in the morning sector than in the afternoon sector. This morning-asymmetry is probably caused by the enhancement of partial ring current, which is located in the premidnight sector, due to solar wind dynamic pressure increase. We also examine the latitudinal and seasonal variations of the normalized SC amplitude. We find that the SC-associated geosynchronous magnetic field perturbations are dependent on the magnetic latitude and season of the year. This may be due to the location of the magnetopause and cross-tail currents enhanced during SC interval with respect to geosynchronous spacecraft position.

• 천문학회보
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• 제37권2호
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• pp.116.1-116.1
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• 2012

When an interplanetary (IP) shock passes over the Earth's magnetosphere, the geosynchronous magnetic field strength near the noon is always enhanced, while the geosynchronous magnetic field near the midnight decreases or increases. In order to understand what determines the positive or negative magnetic field response at nightside geosynchronous orbit to sudden increases in the solar wind dynamic pressure, we have examined 120 IP shock-associated sudden commencements (SC) using magnetic field data from the GOES spacecraft near the midnight (MLT = 2200~0200) and found the following magnetic field perturbation characteristics. (1) There is a strong seasonal dependence of geosynchronous magnetic field perturbations during the passage of IP shocks. That is, the SC-associated geosynchronous magnetic field near the midnight increases (a positive response) in summer and decreases (a negative response) in winter. (2) These field perturbations are dominated by the radial magnetic field component rather than the north-south magnetic field component at nightside geosynchronous orbit. (3) The magnetic elevation angles corresponding to positive and negative responses decrease and increase, respectively. These field perturbation properties can be explained by the location of the cross-tail current enhancement during SC interval with respect to geosynchronous spacecraft position.

• Journal of Astronomy and Space Sciences
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• 제30권2호
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• pp.95-100
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• 2013

In this paper we examine a total of 16 dipolarization events that were observed by THEMIS spacecraft in space close to geosynchronous orbit, r < ${\sim}7\;R_E$. For the identified events, we examine the characteristics of the plasma flows and associated bubbles as defined based on $pV^{5/3}$, where p is the plasma pressure and V the volume of unit magnetic flux. First, we find that the flow speed in the near-geosynchronous region is very low, mostly within a few tens of km/s, except for a very few events for which the flow can rise up to ~200 km/s but only very near the dipolarization onset time. Second, the bubble parameter, $pV^{5/3}$, decreases by a much smaller factor after the dipolarization onset than for the events in the farther out tail region. We suggest that the magnetic dipolarization in the near-geosynchronous region generates or is associated with only very weak plasma bubbles. Such bubbles in the near-geosynchronous region would penetrate earthward only by a small distance before they stop at an equilibrium position or drift around the Earth.

• 항공우주산업기술동향
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• 제6권2호
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• pp.116-124
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• 2008

한국항공우주연구원이 개발하고 있는 통신해양기상위성은 2009년 6월경에 발사될 예정이다. 지구 자전속도와 같은 속도로 돌기 때문에 24시간 기상 및 해양관측과 통신서비스가 가능한 국내에서 개발되는 최초의 정지궤도위성이다. 2008년 11월에 프랑스에서 해양 탑재체가 들어옴으로써 세 가지 탑재체를 장착할 수 있게 되었고 그 후에는 우주환경시험 등을 거쳐서 프랑스령 기아나 우주센터에서 발사될 전망이다. 본 논문에서는 통신해양기상위성이 세계적으로도 첫 번째 정지궤도 복합위성이 되기 때문에 국내 및 세계적으로 정지궤도 복합위성과 관련된 개발 동향을 살펴보고자 한다.

• Journal of Astronomy and Space Sciences
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• 제21권2호
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• pp.93-100
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• 2004

We have investigated characteristic solar wind dynamics associated with relativistic electron events at geosynchronous orbit. Most of the events for April, 1999 through December, 2002 are found to be accompanied by a prolonged solar quiet period which is characterized as low solar wind density, weak interplanetary magnetic field (IMF), and fast alfvenic fluctuations in IMF $B_z$. In a typical relativistic event, electron fluxes begin to increase by orders of magnitude when solar wind parameters drop to low values (e.g., $n_{sw}∼5 cm^{-3}$ and ｜$B_{IMF}$∼5 nT) after sharp peaks. Then the elevated electron fluxes stay at the high level during the solar quiet period. This observation may suggest the following scenario for the occurrence of a geosynchronous relativistic event: (ⅰ) Quiet solar winds can yield a stable and more dipole-like magnetospheric configurations in which the geosynchronous orbit locates well inside the trapping boundary of the energetic electrons. (ⅱ) If a large population of MeV electrons are generated (by whatever acceleration process(es)) in the inner magnetosphere, they can be trapped and effectively accumulated to a high intensity. (ⅲ) The high electron flux can persist for a number of days in the geosynchronous region as long as the solar wind dynamics stays quiet. Therefore the scenario indicates that the occurrence of a relativistic event would be a result of a delicate balance between the effects of electron acceleration and loss. In addition, the sensitive dependence of a relativistic event on the solar wind conditions makes the prediction of solar wind variability as important as understanding of electron acceleration processes in the forecast of a relativistic event.

• 한국위성정보통신학회논문지
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• 제10권4호
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• pp.128-135
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• 2015

현재까지 국내에서 궤도 상의 자국 우주 물체를 지상에서 추적할 수 있는 시스템은 한국천문연구원에서 개발 중인 우주물체 전자광학 감시체계가 유일하다. 이 시스템은 자국 저궤도 위성의 궤도력 산출과 현재 대한민국에 할당된 정지궤도 영역을 감시하는 기능을 보유하고 있다. 그러나 관측 대상이 확대되고 운영 조건의 변화에 따라, 국내 정지궤도 영역 상시감시가 가능한 저예산 보조관측 시스템은 전체 우주 감시시스템의 매우 효율적인 운영을 지원할 수 있다. 따라서 저예산 시스템의 관측정밀도 저하에 따른 궤도력 산출의 위험도 증가를 억제할 수 있는, 지구 정지궤도 영역 상시 감시용 전용 광학관측 시스템의 요구사항을 분석했다.

• 천문학회보
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• 제37권1호
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• pp.90.2-90.2
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• 2012

Geosynchronous electron flux dropouts are most likely due to fast drift loss of the particles to the magnetopause (or equivalently, the "magnetopause shadowing effect"). A possible effect related to the drift loss is the radial diffusion of PSD due to gradient of PSD set by the drift loss effect at an outer L region. This possibly implies that the drift loss can affect the flux levels even inside the trapping boundary. We recently investigated the details of such diffusion process by solving the diffusion equation with a set of initial and boundary conditions set by the drift loss. Motivated by the simulation work, we have examined observationally the energy spectrum and pitch angle distribution near trapping boundary during the geosynchronous flux dropouts. For this work, we have first identified a list of geosynchronous flux dropout events for 2007-2010 from GOES satellite electron measurements and solar wind pressures observed by ACE satellite. We have then used the electron data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements to investigate the particle fluxes. The five THEMIS spacecraft sufficiently cover the inner magnetospheric regions near the equatorial plane and thus provide us with data of much higher spatial resolution. In this paper, we report the results of our investigations on the energy spectrum and pitch angle distribution near trapping boundary during the geosynchronous flux dropout events and discuss implications on the effects of the drift loss on the flux levels at inner L regions.

• Journal of Astronomy and Space Sciences
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• 제28권1호
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• pp.27-36
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• 2011

The present study examines the morning-afternoon asymmetry of the geosynchronous magnetic field strength on the dayside (magnetic local time [MLT] = 06:00~18:00) using observations by the Geostationary Operational Environmental Satellites (GOES) over a period of 9 years from February 1998 to January 2007. During geomagnetically quiet time (Kp < 3), we observed that a peak of the magnetic field strength is skewed toward the earlier local times (11:07~11:37 MLT) with respect to local noon and that the geosynchronous field strength is larger in the morning sector than in the afternoon sector. That is, there is the morning-afternoon asymmetry of the geosynchronous magnetic field strength. Using solar wind data, it is confirmed that the morning-afternoon asymmetry is not associated with the aberration effect due to the orbital motion of the Earth about the Sun. We found that the peak location of the magnetic field strength is shifted toward the earlier local times as the ratio of the magnetic field strength at MLT = 18 (B-dusk) to the magnetic field strength at MLT = 06 (B-dawn) is decreasing. It is also found that the dawn-dusk magnetic field median ratio, B-dusk/B-dawn, is decreasing as the solar wind dynamic pressure is increasing. The morning-afternoon asymmetry of the magnetic field strength appears in Tsyganenko geomagnetic field model (TS-04 model) when the partial ring current is included in TS-04 model. Unlike our observations, however, TS-04 model shows that the peak location of the magnetic field strength is shifted toward local noon as the solar wind dynamic pressure grows in magnitude. This may be due to that the symmetric magnetic field associated with the magnetopause current, strongly affected by the solar wind dynamic pressure, increases. However, the partial ring current is not affected as much as the magnetopause current by the solar wind dynamic pressure in TS-04 model. Thus, our observations suggest that the contribution of the partial ring current at geosynchronous orbit is much larger than that expected from TS-04 model as the solar wind dynamic pressure increases.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2004년도 한국우주과학회보 제13권1호
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• pp.41-41
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• 2004

We report the results on the investigation of the association of solar wind dynamics and the occurrence of geosynchronous relativistic electron events. This study analyzed E>2MeV electron fluxes measured by GOES 10 satellite and solar wind parameters by ACE satellite for April, 1999 to December, 2002. Most of the relativistic events during the time period are found to be accompanied by the prolonged period of quiet solar wind dynamics which is characterized as low solar wind pressure, weak interplanetary magnetic field, and fast fluctuations in IMF Bz. (omitted)