• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2004년도 한국우주과학회보 제13권1호
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• pp.93-93
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• 2004

We have examined the solar wind dynamic pressure enhancements during geomagnetic storm main phase. The Dst index has been used to identify more than 100 geomagnetic storms which occurred in the time interval of 1997 to 2001. We have selected only the events having the minimum Dst value less than -50 nT. In order to identify the pressure impact, we have looked at the low latitude ground H data as well as the solar wind pressure data themselves. (omitted)

• Journal of Astronomy and Space Sciences
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• 제20권2호
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• pp.95-100
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• 2003

This paper is for the investigation of the relationship between the geomagnetic disturbances and the relativistic electron events occurring at geosynchronous orbit. We have analyzed the electron fluxes of E > 2 MeV measured by GOES 10 satellite and the hourly Dst index for the period of April, 1999 to December, 2002. With the rigorous definition of the relativistic event, total 34 events were identified during the time period. Our statistical study showed that more than 50% of the total events occurred associated with weak (or sometimes virtually no) magnetic storms. And only ~ 20% of the events took place accompanied by a strong magnetic storm of $Dst_{min}$ < -100 nT. This result suggests that large geomagnetic storms may not be crucial for the occurrence of a relativistic event at geosynchronous orbit. We also found that there is no clear correlation between the maximum electron flux of an event and the associated minimum of Dst. Therefore any study on the physical mechanism (s) accounting for the relativistic events should take it into account that strong magnetic storms may not be necessarily required for the occurrence of a relativistic electron event at geosynchronous orbit.

• 천문학회보
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• 제38권1호
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• pp.63.1-63.1
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• 2013

To improve the forecast capability of geomagnetic storms, we consider the real time solar and near Earth conditions together, since the characteristics of CMEs can be modified during their transit from the Sun to the Earth, and the geomagnetic storms may be directly affected by not only solar events but also near Earth interplanetary conditions. Using 55 CME-Dst pairs associated with M- and X-class solar flares, which have clearly identifiable source regions during 1997 to 2003, we confirm that the peak values of negative magnetic field Bz and duskward electric field Ey prior to Dst minimum are strongly related with Dst index. We suggest the solar wind criteria (Bz<-5 nT or Ey>3 mV/m for t>2 hr) for moderate storm less than -50 nT by modifying the criteria for intense storms less than -100 nT proposed by Gonzalez and Tsurutani (GT, 1987). As the results, 90% (28/31) of the storms are correctly forecasted by our criteria. For 15 exceptional events that are incorrectly forecasted by only CME parameters, 12 cases (80%) can be properly forecasted by solar wind criteria. When we applying CME and solar wind conditions together, all geomagnetic storms (Dst<-50 nT) are correctly forecasted. Our results show that, the storm forecast capability of the 2~3 days advanced warning based on CME parameters can be improved by combining with the urgent warning based on the near Earth solar wind condition.

• Journal of Astronomy and Space Sciences
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• 제31권1호
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• pp.7-13
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• 2014

Variabilities in the solar wind cause disturbances throughout the heliosphere on all temporal and spatial scales, which leads to changeable space weather. As a view of space weather forecasting, in particular, it is important to know direct and indirect causes modulating the space environment near the Earth in advance. Recently, there are discussions on a role of the interaction of the solar wind with Mercury in affecting the solar wind velocity in the Earth's neighborhood during its inferior conjunctions. In this study we investigate a question of whether other parameters describing the space environment near the Earth are modulated by the inner planets' wake, by examining whether the interplanetary magnetic field and the proton density in the solar wind observed by the Advanced Composition Explorer (ACE) spacecraft, and the geomagnetic field via the Dst index and Auroral Electrojet index (AE index) are dependent upon the relative position of the inner planets. We find there are indeed apparent variations. For example, the mean variations of the geomagnetic fields measured in the Earth's neighborhood apparently have varied with a timescale of about 10 to 25 days. Those variations in the parameters we have studied, however, turn out to be a part of random fluctuations and have nothing to do with the relative position of inner planets. Moreover, it is found that variations of the proton density in the solar wind, the Dst index, and the AE index are distributed with the Gaussian distribution. Finally, we point out that some of properties in the behavior of the random fluctuation are to be studied.

• Journal of Astronomy and Space Sciences
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• 제20권1호
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• pp.53-62
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• 2003

SPS(Space Physics Sensor)는 1999년 발사된 다목적 실용 위성 1호(KOMPSAT-I)에 실린 관측 장비로서, 태양 활동 극대기인 2000년 6월부터 2001년 8월까지 지구 이온층에 관한 자료를 지구로 전송하였다. 이 자료 중 때때로 저위도 지역에서 급격한 플라즈마 밀도 증가 현상을 볼 수 있었다. 이러한 현상의 통계적 분포를 살펴본 결과, 지구 자기장이 약한 대서양 지역과 하와이 지역에서 발생 확률이 가장 높으며, 지자기 활동성 지수인 Dst나 태양 활동성을 나타내는 F10.7지수와는 특별한 상관관계가 없다는 사실을 확인할 수 있었다. 밀도 증가 지역 내의 전자 온도 변화는 개별 사건마다 증가, 유지, 또는 감소를 보이고 있으나, 온도가 급격히 감소하는 경우가 지배적이었다.

• Journal of Astronomy and Space Sciences
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• 제25권2호
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• pp.113-128
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• 2008

Solar wind dynamic pressure enhancements are known to cause various types of disturbances to the magnetosphere. In particular, dynamic pressure enhancements may affect the evolution of magnetic storms when they occur during storm times. In this paper, we have investigated the statistical significance and features of dynamic pressure enhancements during magnetic storm times. For the investigation, we have used a total of 91 geomagnetic storms for 2001-2003, for which the Dst minimum $(Dst_{min})$ is below -50 nT. Also, we have imposed a set of selection criteria for a pressure enhancement to be considered an event: The main selection criterion is that the pressure increases by ${\geq}50%\;or\;{\geq}3nPa$ within 30 min and remains to be elevated for 10 min or longer. For our statistical analysis, we define the storm time to be the interval from the main Dst decrease, through $Dst_{min}$, to the point where the Dst index recovers by 50%. Our main results are summarized as follows. $(i){\sim}$ 81% of the studied storms indicate at least one event of pressure enhancements. When averaged over all the 91 storms, the occurrence rate is ${\sim}$ 4.5 pressure enhancement events per storm and ${\sim}$ 0.15 pressure enhancement events per hour. (ii) The occurrence rate of the pressure enhancements is about three times higher for CME-driven storm times than for CIR-driven storm times. (iii) Only 21.1% of the pressure enhancements show a clear association with an interplanetary shock. (iv) A large number of the pressure enhancement events are accompanied with a simultaneous change of IMF $B_y$ and/or $B_z$: For example, 73.5% of the pressure enhancement events are associated with an IMF change of either $|{\Delta}B_z|>2nT\;or\;|{\Delta}B_y|>2nT$. This last finding suggests that one should consider possible interplay effects between the simultaneous pressure and IMF changes in many situations.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2008년도 한국우주과학회보 제17권2호
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• pp.24.1-24.1
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• 2008

We examine the eccentricity parameter (EP) of Coronal Mass Ejections (CMEs). For this, we select 298 front-side CMEs from SOHO LASCO CMEs whose speed is larger than 1000km/s and angular width is greater than $120^{\circ}$ during from 1997 to 2007. These are thought to be the most plausible candidate of geoeffective CMEs. We examine the relation between CMEs eccentricity parameter and the minimum value of the Dst index. We find that strong geomagnetic storms (Dst < -200nT) are well correlated with the EP from the scattered plot. We also find that CMEs have high geoeffectiveness when they occurred near the center of the solar disk with the small EP and they have the small speed with the small EP. These results indicate that the CME EP also can be an important indicator to forecast CME geoeffectiveness such as Earthward direction parameter (Moon et al. 2005, Kim et al. 2008).

• Journal of Astronomy and Space Sciences
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• 제26권1호
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• pp.9-24
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• 2009

태양주기 23의 태양흑점과 코로나질량방출(Coronal Mass Ejection, CME), 지자기폭풍과의 상호 연관성을 알아보기 위해 CME의 연도별 발생빈도 분포와 북-남 비대칭 분포, 태양흑점수와 면적의 연평균 분포와 북-남 비대칭 분포, 지자기폭풍의 연도별 발생빈도 분포와의 상호 상관관계를 알아보았다. 1996년부터 2007년까지의 SOHO/LASCO 목록 CME의 위치정보를 이용하여 북쪽 반구에서 발생한 CME와 남쪽 반구에서 발생한 CME의 연도별 발생빈도 분포를 알아내었다. 태양흑점수와 면적을 북쪽 반구와 남쪽 반구를 구분하여 그 연평균 분포를 구하였고 지자기 교란정도를 알 수 있는 Ap지수, Dst지수, aa지수를 이용하여 지자기폭풍의 연도별 발생빈도 분포를 구하였다. 이렇게 구한 각각의 분포간의 상호 상관관계를 구하였다. 또한 CME를 각 너비(Angular Width)와 속도(Linear Speed)에 따라 분류하여 흑점 분포, 지자기폭풍의 발생빈도 분포와 상관관계도 알아보았다. 그 결과 CME의 전체 발생빈도는 흑점수와 면적의 전체 분포와의 상관관계가 높았고 흑점수와 면적의 북-남 비대칭 분포와는 상관관계가 낮았다. CME의 북-남 비대칭 분포는 흑점의 북-남 비대칭 분포와 상관관계가 높았다. CME와 지자기폭풍 발생빈도 분포와의 상관관계를 살펴 본 결과 CME 전체나 북-남 비대칭 분포와는 상관도가 낮게 나왔다. 그러나 CME를 규모별로 나누어 그 총 발생빈도와의 상관관계를 알아 본 결과 Ap지수와 aa지수는 속도가 빠른 경우, Dst지수는 각너비나 속도별로 나눈 모든 경우에 상관도가 높게 나왔다. 흑점과 지자기폭풍 발생빈도의 경우 흑점수와 면적의 전체 분포와 지자기폭풍 사이의 상관관계가 높게 나타났다. 이 결과를 통해 CME의 발생빈도 분포는 흑점의 분포와 연관성이 높고 이 둘의 전체 분포와 지자기폭풍의 발생빈도 분포와의 연관성이 높은 것을 알 수 있다. 그리고 CME, 흑점의 북-남 비대칭 분포와 지자기폭풍의 발생빈도 분포의 경우 연관성이 낮은 것을 알 수 있다.

• 천문학회보
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• 제36권2호
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• pp.81.2-81.2
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• 2011

Motivated by recent attempts to derive geomagnetic activity from hourly mean data in long term studies, we test the recursive Kalman filter method to obtain the regular solar variation curve of the geomagnetic field. Using a simple algorithm, we are able to assign a quiet day curve to every day separately, without the need for additional input parameter(s) to define the geomagnetically quiet days. We derive a digital counterpart AhK of the analog range index Ak at the subauroral Sodankyl$\ddot{a}$ station and compare it to the earlier digital estimate Ah and the local Ak index. We find that the new method outperforms the former estimate in every aspect studied and provides a robust, straightforward manner of estimating and verifying the manually scaled Ak index, based on readily available hourly values. The model is independent of sampling; thus, for shorter term studies where high-sampling data are available, more accurate estimates can also be obtained when needed. Therefore, in contrast to other recent approaches, we do not provide a method to quantify irregular activity directly but derive the actual quiet day curves in the traditional manner. In future applications the same algorithm may be used to define a wide variety of geomagnetic indices (such as Ak, Dst, or AE).

• Journal of Astronomy and Space Sciences
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• 제18권1호
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• pp.27-32
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• 2001

A statistical study has been performed of the magnetic storm recovery phase using the Dst index for 102 storm events in the interval January 1996 to December 1998. In 43 cases (or 42%) out of our 102 events, the recovery phase exhibits fast recovery (kaking about 8 hours or less) at its initial stage or for the entire recovery period. Since this fast recovery can be explained by the fast charge exchange less of $O^{+}$ ions which mostly com from the ionosphere, and since a fraction of $H^{+}$ ions is of ionospheric origin as well, our statistical result supports the view that the source of ring current ions in many magnetic storms can be terrestrial.