• Journal of Astronomy and Space Sciences
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• v.26 no.4
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• pp.467-478
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• 2009

The super-geomagnetic storms called 2003 Halloween event globally occurred during the period of 29 through 31 which are the following days when the solar flares of X18 class exploded on 28 October 2003. The S4 index from GPS signal strength and the peak electron density ($NmF_2$) from GPS tomography method are analyzed according to the date. The occurrences of the cycle slip and scintillation in the GPS signals are 1,094 and 1,387 on 28 and 29 October, respectively and these values are higher than 604 and 897 on 30 and 31 October. These mean the ionospheric disturbances are not always generated by the period of geomagnetic storm. Therefore, GPS S4 index is useful to monitor the ionospheric disturbances. Behaviors of ionospheric electron density estimated from GPS tomography method are analyzed with the date. At UT = 18 hr, the maximum $NmF_2$ is shown on 28 October. It agrees with $NmF_2$ variation measured from Anyang ionosonde, and the GPS signal are better condition on 30 and 31 October than 28 October. In conclusion, GPS signal condition is relation with geomagnetic activities, and depend upon the variation of the electron density. We will study the long-term data to examine the relationship between the GPS signal quality and the electron density as the further works.

• Journal of Astronomy and Space Sciences
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• v.23 no.1
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• pp.55-70
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• 2006

There are many astronomical records of comets in Korean history books. We collected those of the comet that appeared in the winter of the year 1664. These records were written in Daily Reports of the Royal Observatory on Astronomical Events (星變測候單子), Final Reports of the Royal Observatory on An Astronomical Events (天變謄錄), the Daily Records of the Royal Secretariat of the Joseon Dynasty (承政院日記), and the Chronicle of the Joseon Dynasty (朝鮮王朝實錄). We used the Olbers' method for determination of preliminary orbit of the comet, and compared the result with the previous researches. In the Oblers' method the orbit is assumed to be parabolic (e=1). We obtained the following orbital elements: perifocal distance q=1.07\pm0.008AU, perifocal passage time $T=2329165.50\pm0.72$ days, argument of the perifocus $\omega=318^{\circ}.2\pm2^{\circ}.29$, longitude of ascending node $\Omega=85^{\circ}.23\pm2^{\circ}53(J1665)$(J1665), inclination $i=160^{\circ}.28\pm1^{|circ}.56$.

• Journal of Astronomy and Space Sciences
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• v.27 no.1
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• pp.43-54
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• 2010

This study was performed by the policy research project of Ministry of Land, Transport and Maritime Affairs, which title is "Developing safety standards and management of space radiation on the polar route". In this research, total six experiments were performed using Korean commercial flights (B747). Three of those are on the polar route and the other three are on the north pacific route. Space radiation exposure measured on the polar route is the average 84.7 uSv. The simulation result using CARI-6M program gives 84.9 uSv, which is very similar to measured value. For the departure flight using the north pacific route, the measured space radiation is the average 74.4 uSv. It seems that is not so different to use the polar route or not for the return flight because the higher latitude effect causing the increase of space radiation is compensated by the shortened flight time effect causing decreasing space radiation exposure.

• Journal of Astronomy and Space Sciences
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• v.40 no.4
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• pp.149-171
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• 2023

ShadowCam is a National Aeronautics and Space Administration Advanced Exploration Systems funded instrument hosted onboard the Korea Aerospace Research Institute (KARI) Korea Pathfinder Lunar Orbiter (KPLO) satellite. By collecting high-resolution images of permanently shadowed regions (PSRs), ShadowCam will provide critical information about the distribution and accessibility of water ice and other volatiles at spatial scales (1.7 m/pixel) required to mitigate risks and maximize the results of future exploration activities. The PSRs never see direct sunlight and are illuminated only by light reflected from nearby topographic highs. Since secondary illumination is very dim, ShadowCam was designed to be over 200 times more sensitive than previous imagers like the Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (LROC NAC). ShadowCam images thus allow for unprecedented views into the shadows, but saturate while imaging sunlit terrain.

• Journal of Astronomy and Space Sciences
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• v.27 no.3
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• pp.205-212
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• 2010

To better understand the physical processes that maintain the high-latitude lower thermospheric dynamics, we have identified relative contributions of the momentum forcing and the heating to the high-latitude lower thermospheric winds depending on the interplanetary magnetic field (IMF) and altitude. For this study, we performed a term analysis of the potential vorticity equation for the high-latitude neutral wind field in the lower thermosphere during the southern summertime for different IMF conditions, with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Difference potential vorticity forcing and heating terms, obtained by subtracting values with zero IMF from those with non-zero IMF, are influenced by the IMF conditions. The difference forcing is more significant for strong IMF $B_y$ condition than for strong IMF $B_z$ condition. For negative or positive $B_y$ conditions, the difference forcings in the polar cap are larger by a factor of about 2 than those in the auroral region. The difference heating is the most significant for negative IMF $B_z$ condition, and the difference heatings in the auroral region are larger by a factor of about 1.5 than those in the polar cap region. The magnitudes of the difference forcing and heating decrease rapidly with descending altitudes. It is confirmed that the contribution of the forcing to the high-latitude lower thermospheric dynamics is stronger than the contribution of the heating to it. Especially, it is obvious that the contribution of the forcing to the dynamics is much larger in the polar cap region than in the auroral region and at higher altitude than at lower altitude. It is evident that when $B_z$ is negative condition the contribution of the forcing is the lowest and the contribution of the heating is the highest among the different IMF conditions.

• Journal of Astronomy and Space Sciences
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• v.27 no.4
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• pp.329-335
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• 2010

We investigate the sources of the variation of the high-latitude thermospheric neutral mass density depending on the interplanetary magnetic field (IMF) conditions. For this purpose, we have carried out the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) simulations for various IMF conditions under summer condition in the southern hemisphere. The NCAR-TIEGCM is combined with a new empirical model that provides a forcing to the thermosphere in high latitudes. The difference of the high-latitude thermospheric neutral mass density (subtraction of the values for zero IMF condition from the values for non-zero IMF conditions) shows a dependence on the IMF condition: For negative $B_y$ condition, there are significantly enhanced difference densities in the dusk sector and around midnight. Under the positive-$B_y$ condition, there is a decrease in the early morning hours including the dawn side poleward of $-70^{\circ}$. For negative $B_z$, the difference of the thermospheric densities shows a strong enhancement in the cusp region and around midnight, but decreases in the dawn sector. In the dusk sector, those values are relatively larger than those in the dawn sector. The density difference under positive-$B_z$ condition shows decreases generally. The density difference is more significant under negative-$B_z$ condition than under positive-$B_z$ condition. The dependence of the density difference on the IMF conditions in high latitudes, especially, in the dawn and dusk sectors can be explained by the effect of thermospheric winds that are associated with the ionospheric convection and vary following the direction of the IMF. In auroral and cusp regions, heating of thermosphere by ionospheric currents and/or auroral particle precipitation can be also the source of the dependence of the density difference on the IMF conditions.

• Journal of Astronomy and Space Sciences
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• v.32 no.3
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• pp.181-187
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• 2015

Storm sudden commencements (SSCs) occur due to a rapid compression of the Earth's magnetic field. This is generally believed to be caused by interplanetary (IP) shocks, but with exceptions. In this paper we explore possible causes of SSCs other than IP shocks through a statistical study of geomagnetic storms using SYM-H data provided by the World Data Center for Geomagnetism - Kyoto and by applying a superposed epoch analysis to simultaneous solar wind parameters obtained with the Advanced Composition Explorer (ACE) satellite. We select a total of 274 geomagnetic storms with minimum SYM-H of less than -30nT during 1998-2008 and regard them as SSCs if SYM-H increases by more than 10 nT over 10 minutes. Under this criterion, we found 103 geomagnetic storms with both SSC and IP shocks and 28 storms with SSC not associated with IP shocks. Storms in the former group share the property that the strength of the interplanetary magnetic field (IMF), proton density and proton velocity increase together with SYM-H, implying the action of IP shocks. During the storms in the latter group, only the proton density rises with SYM-H. We find that the density increase is associated with either high speed streams (HSSs) or interplanetary coronal mass ejections (ICMEs), and suggest that HSSs and ICMEs may be alternative contributors to SSCs.

• Journal of Astronomy and Space Sciences
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• v.34 no.4
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• pp.237-244
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• 2017

Magnetic flux ropes, often observed during intervals of interplanetary coronal mass ejections, have long been recognized to be critical in space weather. In this work, we focus on magnetic flux rope structure but on a much smaller scale, and not necessarily related to interplanetary coronal mass ejections. Using near-Earth solar wind advanced composition explorer (ACE) observations from 1998 to 2016, we identified a total of 309 small-scale magnetic flux ropes (SMFRs). We compared the characteristics of identified SMFR events with those of normal magnetic cloud (MC) events available from the existing literature. First, most of the MCs and SMFRs have similar values of accompanying solar wind speed and proton densities. However, the average magnetic field intensity of SMFRs is weaker (~7.4 nT) than that of MCs (~10.6 nT). Also, the average duration time and expansion speed of SMFRs are ~2.5 hr and 2.6 km/s, respectively, both of which are smaller by a factor of ~10 than those of MCs. In addition, we examined the geoeffectiveness of SMFR events by checking their correlation with magnetic storms and substorms. Based on the criteria Sym-H < -50 nT (for identification of storm occurrence) and AL < -200 nT (for identification of substorm occurrence), we found that for 88 SMFR events (corresponding to 28.5 % of the total SMFR events), substorms occurred after the impact of SMFRs, implying a possible triggering of substorms by SMFRs. In contrast, we found only two SMFRs that triggered storms. We emphasize that, based on a much larger database than used in previous studies, all these previously known features are now firmly confirmed by the current work. Accordingly, the results emphasize the significance of SMFRs from the viewpoint of possible triggering of substorms.

• Journal of Astronomy and Space Sciences
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• v.21 no.4
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• pp.283-294
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• 2004

New BV RI photometric observations of the contact binary AH Tau were performed with the 61 cm reflector and a 2K CCD camera at the Sobaeksan Optical Astronomy Observatory during seven nights from September to December, 2001. A total of 144 times of minima observed up to date, including three times of minima obtained from our observation, were analyzed. It is found that the orbital period of AH Tau has varied in a cyclic way superposed on a secular period decrease. The rate of the secular period decrease is calculated to be $1^s$ .04 per century, implying that a mass of about $3.8{\times}10^{-8}M{\odot}/yr$ from the more massive primary flows into the secondary if a conservative mass transfer is assumed. Assuming that the sinusoidal period variation is produced by a light-time effect due to an unseen third body, the resultant semi-amplitude, period, and eccentricity for the deduced light-time orbit are obtained as 35.4 years, 0.014 day and 0.52, respectively. The mass of the third-body is calculated as a tout $0.24M{\odot}$ when the third body is assumed to be coplanar with AH Tau system.

• Journal of Astronomy and Space Sciences
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• v.23 no.1
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• pp.71-90
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• 2006

This study is about , which constitutes Honcheonui(an armillary sphere) put of $\ulcorner$Uigijipseol$\lrcorner$ (Volume I) together with and -dealt with earlier than this subject. The study's construction on the text of is organized into 15 categories, including installation, observations, instructions for use, astronomical theories and formulas, etc. This study provides easy-to-understand illustrations of the figures shown in the original and contains detailed descriptions of the related calculation procedures. In the 'Instructions for Use of Honcheonui' discussed here, the readers are introduced to astronomical computation systems, mainly based on spherical trigonometry and proportional methods. The section also provides systematic and detailed reviews of astronomical theories and calculation procedures, allowing you to assess the level of astronomy knowledge at that time.