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1968년 ~ 2024년까지 1,211 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
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651
- LEE DONG-HUN
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.101-107
- 2003
- 원문 바로보기
The magnetosphere is often perturbed by impulsive input such as interplanetary shocks and solar wind discontinuities. We study how these initial perturbations are propagating within the magnetosphere over various latitude regions by adopting a three-dimensional numerical dipole model. We examine the wave propagation on a meridional plane in a time-dependent manner and compare the numerical results with multi-satellite and ground observations. The dipole model is used to represent the plasmasphere and magnetosphere with a realistic Alfven speed profile. It is found that the effects of refraction, which result from magnetic field curvature and inhomogeneous Alfven speed, are' found to become important near the plasmapause. Our results show that, when the disturbances are assumed at the subsolar point of the dayside magnetosphere, the travel time becomes smaller to the polar ionosphere compared to the equatorial ionosphere.
652
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.37-44
- 2003
- 원문 바로보기
In this paper, we review recent studies on the magnetic helicity changes of solar active regions by photospheric horizontal motions. Recently, Chae(200l) developed a methodology to determine the magnetic helicity change rate via photospheric horizontal motions. We have applied this methodology to four cases: (1) NOAA AR 8100 which has a series of homologous X-ray flares, (2) three active regions which have four eruptive major X-ray flares, (3) NOAA AR 9236 which has three eruptive X-class flares, and (4) NOAA AR 8668 in which a large filament was under formation. As a result, we have found several interesting results. First, the rate of magnetic helicity injection strongly depends on an active region and its evolution. Its mean rate ranges from 4 to <TEX>$17 {\times} 10^{40}\;Mx^2\;h^{-1}$</TEX>. Especially when the homologous flares occurred and when the filament was formed, significant rates of magnetic helicity were continuously deposited in the corona via photospheric shear flows. Second, there is a strong positive correlation between the magnetic helicity accumulated during the flaring time interval of the homologous flares in AR 8100 and the GOES X-ray flux integrated over the flaring time. This indicates that the occurrence of a series of homologous flares is physically related to the accumulation of magnetic helicity in the corona by photospheric shearing motions. Third, impulsive helicity variations took place near the flaring times of some strong flares. These impulsive variations whose time scales are less than one hour are attributed to localized velocity kernels around the polarity inversion line. Fourth, considering the filament eruption associated with an X1.8 flare started about 10 minutes before the impulsive variation of the helicity change rate, we suggest that the impulsive helicity variation is not a cause of the eruptive solar flare but its result. Finally, we discuss the physical implications on these results and our future plans.
653
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.111-121
- 2003
- 원문 바로보기
In order to explore the cosmic ray acceleration at the cosmological shocks, we have performed numerical simulations of one-dimensional, plane-parallel, cosmic ray (CR) modified shocks with the newly developed CRASH (Cosmic Ray Amr SHock) numerical code. Based on the hypothesis that strong Alfven waves are self-generated by streaming CRs, the Bohm diffusion model for CRs is adopted. The code includes a plasma-physics-based 'injection' model that transfers a small proportion of the thermal proton flux through the shock into low energy CRs for acceleration there. We found that, for strong accretion shocks with Mach numbers greater than 10, CRs can absorb most of shock kinetic energy and the accretion shock speed is reduced up to <TEX>$20\%$</TEX>, compared to pure gas dynamic shocks. Although the amount of kinetic energy passed through accretion shocks is small, since they propagate into the low density intergalactic medium, they might possibly provide acceleration sites for ultra-high energy cosmic rays of <TEX>$E\ll10^{18}eV$</TEX>. For internal/merger shocks with Mach numbers less than 3, however, the energy transfer to CRs is only about <TEX>$10-20\%$</TEX> and so nonlinear feedback due to the CR pressure is insignificant. Considering that intracluster medium (ICM) can be shocked repeatedly, however, the CRs generated by these weak shocks could be sufficient to explain the observed non-thermal signatures from clusters of galaxies.
654
- AHN BYUNG-HO
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.93-99
- 2003
- 원문 바로보기
Various attempts have been made to explain the: pronounced seasonal and universal time (UT) variations of geomagnetic indices. As one of such attempts, we analyze the hourly-averaged auroral electroject indices obtained during the past 20 years. The AU and AL indices maximize during summer and equinoctial months, respectively. By normalizing the contribution of the solar conductivity enhancement to the AU index, or to the eastward electrojet, it is found that the AU also follows the same semiannual variation pattern of the AL index, suggesting that the electric field is the main modulator of the semiannual magnetic variation. The fact that the variation pattern of the yearly-mean AU index follows the mirror image of the AL index provides another indication that the electric field is the main modulator of magnetic disturbance. The pronounced UT variations of the auroral electrojet indices are also noted. To determine the magnetic activity dependence, the probability of recording a given activity level of AU and AL during each UT is examined. The UT variation of the AL index, thus obtained, shows a maximum at around 1200-1800 UT and a minimum around 0000-0800 UT particularly during winter. It is closely associated with the rotation of the geomagnetic pole around the rotational axis, which results in the change of the solar-originated ionospheric conductivity distribution over the polar region. On the other hand the UT variation is prominent during disturbed periods, indicating that the latitudinal mismatch between the AE stations and the auroral electrojet belt is responsible for it. Although not as prominent as the AL index, the probability distribution of the AU also shows two UT peaks. We confirm that the Dst index shows more prominent seasonal variation than the AE indices. However, the UT variation of the Dst index is only noticeable during the main phase of a magnetic storm. It is a combined result of the uneven distribution of the Dst stations and frequent developments of the partial ring current and substorm wedge current preferentially during the main phase.
655
- HIEI E.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.45-47
- 2003
- 원문 바로보기
A white light flare was observed at the limb on 16 August 1989 in He 10830 <TEX>${\AA}$</TEX> spectra, H<TEX>$\alpha$</TEX> slit jaw photo-grams, and white light filter-grams of <TEX>${\lambda}=5600{\AA}{\pm}800{\AA}$</TEX>. The kernels of the white light flare are not spatially related with Ha brightenings, suggesting that the flare energy would be released at the photosphere.
656
- KIM SANG CHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.13-19
- 2003
- 원문 바로보기
We present a study of the old open cluster Trumpler 5 (Tr 5), based on the CDS archival data. From the color-magnitude diagrams of Tr 5, we have found the positions of main-sequence turn-off (MSTO) and red giant clump (RGC) stars. Using the mean magnitude of the RGC stars, we have estimated the reddening toward Tr 5, E(B - V) = 0.60 <TEX>$\pm$</TEX> 0.10. Using the stars common in two data sets and the theoretical isochrones of Padova group, we have estimated the distance modulus <TEX>$V_o - M_v = 12.64 {\pm} 0.20 (d = 3.4 {\pm} 0.3 kpc)$</TEX>, the metallicity [Fe/H) = -0.30 <TEX>$\pm$</TEX> 0.10, and the age of 2.4 <TEX>$\pm$</TEX> 0.2 Gyr (log t = 9.38). These metallicity and distance values are consistent with the relation between the metallicity and the Galactocentric distance of other old open clusters, for which we obtain the slope of <TEX>${\Delta}[Fe/H]/ R_{gc} = -0.064 {\pm} 0.010\;dex\;kpc^{-1}$</TEX>.
657
- PEARSON CHRIS
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.249-260
- 2003
- 원문 바로보기
ASTRO-F is the next generation Japanese infrared space mission of the Institute of Space and Astronautical Science. ASTRO-F will be dedicated to an All Sky Survey in the far-infrared in 4 bands from 50-200microns with 2 additional mid-infrared bands at 9microns and 20microns. This will be the first all sky survey in the infrared since the ground breaking IRAS mission almost 20 years ago and the first ever survey at 170microns. The All Sky Survey should detect 10's of millions of sources in the far-infrared bands most of which will be dusty luminous and ultra-luminous star forming galaxies, with as many as half lying at redshifts greater than unity. In this contribution, the ASTRO-F mission and its objectives are reviewed and many of the mission expectations are discussed.
658
- CHOI CHUL-SUNG
- Journal of the Korean astronomical society = 천문학회지
- 36, n.2
- pp.73-73
- 2003
- 원문 바로보기
Proof correction to the equation in the third paragraph of the DISCUSSION AND CONCLUSION has not been carried faithfully to the published version of the paper. The corrected equation should read <TEX>${\approx}10^{-3}\;M_8^4/^3(N_{\ast}/10^6\;pc^{-3})({\sigma}/300 km\;s^{-l})^{-l}(r/r_t)\;yr^{-1}$</TEX>, where Ms is the mass of the SMBH in units of <TEX>$10^8\;M_{\bigodot}$</TEX>, <TEX>$\sigma$</TEX> is the virial velocity of the stars, <TEX>$r_t$</TEX> is the tidal radius of the SMBH. This estimates the frequency that a star would pass within a sphere with the radius r from the SMBH, rather than the frequency of the tidal disruption event. Therefore, it increases with the mass of the SMBH. However, the loss cone effect should also be taken into account, which reduces the actual event rate. Here, we adopted a factor of one hundred to consider the deficiency from the isotrophic rate. The authors sincerely regret this error.
659
- SANDERS D. B.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.149-158
- 2003
- 원문 바로보기
Deep surveys at mid-infared through submillimeter wavelengths indicate that a substantial fraction of the total luminosity output from galaxies at high redshift (z > 1) emerges at wavelengths 30 - 300<TEX>${\mu}m$</TEX>. In addition, much of the star formation and AGN activity associated with galaxy building at these epochs appears to reside in a class of luminous infrared galaxies (LIGs), often so heavily enshrouded in dust that they appear as 'blank-fields' in deep optical/UV surveys. Here we present an update on the state of our current knowledge of the cosmic evolution of LIGs from z = 0 to z <TEX>$\~$</TEX> 4 based on the most recent data obtained from ongoing ground-based redshift surveys of sources detected in ISO and SCUBA deep fields. A scenario for the origin and evolution of LIGs in the local Universe (z < 0.3), based on results from multiwavelength observations of several large complete samples of luminous IRAS galaxies, is then discussed.
660
- AHN SANG-HYEON
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.145-148
- 2003
- 원문 바로보기
P-Cygni type Ly<TEX>$\alpha$</TEX> from starburst galaxies, either nearby galaxies or Lyman Break galaxies, are believed to be formed by galactic outflows such as galactic supershells or galactic superwinds. We develope a Monte Carlo code to calculate the Ly<TEX>$\alpha$</TEX> line transfer in a galactic supershell which is expanding and formed of uniform and dusty neutral hydrogen gas. The escape of Ly<TEX>$\alpha$</TEX> photons from the system is achieved by a number of back-scatterings. A series of emission peaks are formed by back-scatterings. When we observe P-Cygni type Ly<TEX>$\alpha$</TEX> emissions of starforming galaxies, we can usually see merely singly-peaked emission. Hence the secondary and the tertiary emission humps should be destroyed. In order to do this, dust should be spatially more extended into the inner cavity than neutral supershell. We find that the kinematic information of the expanding supershell is conserved even in dusty media. We discuss the astrophysical applications of our results.