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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
페이지 62/122
611
- SAKURAI T.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.7-12
- 2003
- 원문 바로보기
We have studied the magnetic helicity of active regions by using the data from (1) the photo-electric magnetograph of the Okayama Observatory (1983-1995) and (2) the video magnetograph of NAOJ/Mitaka (1992-2000). The latitude distribution of helicity showed a tendency that the regions in the north (south) hemisphere have negative (positive) helicities, respectively, which is already known as the hemispheric sign rule. If we look into the sign of helicity as a function of time, the sign rule was less definite or was reversed sometimes in the sunspot minimum phase. We also studied the relation between the magnetic helicity and the sunspot tilt angles, and found that these two quantities are positively correlated, which is opposite to the expectation of a theoretical model. The implications of this cycle-phase dependence of helicity signs and the correlation between magnetic he Ii city and sunspot tilt angles are discussed.
612
- CHAE JONGCHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.13-20
- 2003
- 원문 바로보기
Observations have indicated that magnetic reconnect ion may occur frequently in the photosphere and chromosphere as well as in the solar corona. The observed features include cancelling magnetic features seen in photospheric magnetograms, and different kinds of small-scale activities such as UV explosive events and EUV jets. By integrating the observed parameters of these features with the Sweet-Parker reconnect ion theory, an attempt is made to clarify the nature of chromospheric magnetic reconnection. Our results suggest that magnetic reconnect ion may be occurring at many different levels of the photosphere and chromosphere without a preferred height and at a faster speed than is predicted by the Sweet-Parker reconnect ion model using the classical value of electric conductivity. Introducing an anomalous magnetic diffusivity 10-100 times the classical value is one of the possible ways of explaining the fast reconnect ion as inferred from observations.
613
- FLETCHER ANDRE B.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.177-187
- 2003
- 원문 바로보기
Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from <TEX>$\~$</TEX>1 to <TEX>$\~$</TEX>10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs - which have been recorded over both 'short' human (<TEX>$10^0-10^9\;s$</TEX>) and 'long' cosmic (<TEX>$10^{13} - 10^{17}\;s$</TEX>) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.
614
- KOO BON-CHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.43-48
- 2003
- 원문 바로보기
We introduce and describe performance of the 6-meter telescope of Seoul Radio Astronomy Observatory (SRAO). All the softwares and instruments except the antenna structure and its driving system are developed for ourselves. The SIS mixer type receiver resulted in the receiver noise temperature less than 50 K (DSB) over the whole 3-mm radio window. An autocorrelation spectrometer, developed first in Korea, provides maximum 50 MHz band width over 1024 channels. Antenna surface is measured and adjusted using template method and radio holography which resulted in a superb surface accuracy bet-ter than 30<TEX>${\mu}m$</TEX>. Accordingly, the aperture and beam efficiences amount to <TEX>$70\%$</TEX> and <TEX>$75\%$</TEX>, respectively, largely independent of frequency in the 85 - 115 GHz range. It is also found that telescope pointing errors are less than 10' in both azimuth and elevation and that antenna gain is almost constant against elevation greater than <TEX>$20^{\circ}$</TEX>, without adjusting sub-reflector position. The SRAO 6-meter telescope is now fully operational and all these characteristics verify that observations are carried out with high precision and fidelity.
615
- KIM DONG-WOO
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.213-222
- 2003
- 원문 바로보기
We review recent observational results on early type galaxies obtained with high spatial resolution Chandra data. With its unprecedented high spatial resolution, Chandra reveals many intriguing features in early type galaxies which were not identified with the previous X-ray missions. In particular, various fine structures of the hot ISM in early type galaxies are detected, for example, X-ray cavities which are spatially coincident with radio jets/lobes, indicating the interaction between the hot ISM and radio jets. Also point sources (mostly LMXBs) are individually resolved down to Lx = a few x <TEX>$10^{37}\;erg\;sec^{-1}$</TEX> and it is for the first time possible to unequivocally investigate their properties and the X-ray luminosity function. After correcting for incompleteness, the XLF of LMXBs is well reproduced by a single power law with a slope of -1.0 - -1.5, which is in contrast to the previous report on the existence of the XLF break at Lx, Eddington = 2 x <TEX>$10^{38}\;erg\;sec^{-1}$</TEX> (i.e., Eddington luminosity of a neutron star binary). Carefully considering both detected and undetected, hidden populations of point sources we further discuss the XLF of LMXBs and the metal abundance of the hot ISM and their impact on the properties of early type galaxies.
616
- DING M. D.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.49-54
- 2003
- 원문 바로보기
We introduce the two-dimensional spectral observations of solar flares using the Solar Tower Tele-scope of Nanjing University, China. In particular, we introduce three typical events and the methods used to analyze the data. (1) The flare of November 11, 1998, which is a limb flare. We derive the temperature and density within the flaring loop using non-LTE calculations. The results show that the loop top may be hotter and denser than other parts of the loop, which may be a result of magnetic reconnect ion above the loop. (2) The flare of March 10, 2001, which is a white-light flare that shows an emission enhancement at the near infrared continuum. We propose a model of non-thermal electron beam heating plus backwarming to interpret the observations. (3) The flare of September 29, 2002, which shows unusual line asymmetries at one flare kernel. The line asymmetries are caused by an upward moving plasma that is accelerated and heated during the flare development.
617
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.1-12
- 2003
- 원문 바로보기
Nonthermal particles can be produced due to incomplete thermalization at collisionless shocks and further accelerated to very high energies via diffusive shock acceleration. In a previous study we explored the cosmic ray (CR) acceleration at cosmic shocks through numerical simulations of CR modified, quasi-parallel shocks in 1D plane-parallel geometry with the physical parameters relevant for the shocks emerging in the large scale structure formation of the universe (Kang & Jones 2002). Specifically we considered pancake shocks driven by accretion flows with <TEX>$U_o = 1500 km\;s^{-l}$</TEX> and the preshock gas temperature of <TEX>$T_o = 10^4 - 10^8K$</TEX>. In order to consider the CR acceleration at shocks with a broader range of physical properties, in this contribution we present additional simulations with accretion flows with <TEX>$U_o = 75 - 1500 km\;s^{-l}$</TEX> and <TEX>$T_o = 10^4K$</TEX>. We also compare the new simulation results with those reported in the previous study. For a given Mach number, shocks with higher speeds accelerate CRs faster with a greater number of particles, since the acceleration time scale is <TEX>$t_{acc}\;{\propto}\;U_o^{-2}$</TEX>. However, two shocks with a same Mach number but with different shock speeds evolve qualitatively similarly when the results are presented in terms of diffusion length and time scales. Therefore, the time asymptotic value for the fraction of shock kinetic energy transferred to CRs is mainly controlled by shock Mach number rather than shock speed. Although the CR acceleration efficiency depends weakly on a well-constrained injection parameter, <TEX>$\epsilon$</TEX>, and on shock speed for low shock Mach numbers, the dependence disappears for high shock Mach numbers. We present the 'CR energy ratio', <TEX>${\phi}(M_s)$</TEX>, for a wide range of shock parameters and for <TEX>$\epsilon$</TEX> = 0.2 - 0.3 at terminal time of our simulations. We suggest that these values can be considered as time-asymptotic values for the CR acceleration efficiency, since the time-dependent evolution of CR modified shocks has become approximately self-similar before the terminal time.
618
- ZHANG XIAOLEI
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.223-239
- 2003
- 원문 바로보기
It is now a well established fact that galaxies undergo significant morphological transformation during their lifetimes, manifesting as an evolution along the Hubble sequence from the late to the early Hubble types. The physical processes commonly believed to be responsible for this observed evolution trend, i.e. the major and minor mergers, as well as gas accretion under a barred potential, though demonstrated applicability to selected types of galaxies, on the whole have failed to reproduce the most important statistical and internal properties of galaxies. The secular evolution mechanism reviewed in this paper has the potential to overcome most of the known difficulties of the existing theories to provide a natural and coherent explanation of the properties of present day as well as high-redshift galaxies.
619
- LEE SANGWOO
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.21-27
- 2003
- 원문 바로보기
In the present study, we have investigated morphology and evolution of small-scale Ha dynamic features on the quiet sun by analyzing video magnetograms and high resolution Ha images simultaneously taken for 5 hours at Big Bear Solar Observatory on April 18, 1997. From comparisons between time sequential longitudinal magnetograms and H<TEX>$\alpha$</TEX> images covering <TEX>$150' {\times} 150'$</TEX>, several small-scale H<TEX>$\alpha$</TEX> dynamic features have been observed at a site of magnetic flux cancellation. A close relationship between such features and cancelling magnetic fluxes has been revealed temporarily and spatially. Our results support that material injection by chromospheric magnetic reconnect ion may be essential in supporting numerous small-scale H<TEX>$\alpha$</TEX> dynamical absorption features, being in line with recent observational studies showing that material injection by chromospheric magnetic reconnect ion is essential for the formation of solar filaments.
620
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.2
- pp.61-66
- 2003
- 원문 바로보기
We have examined the relationship between the speeds of coronal mass ejections (CMEs) and the GOES X-ray peak fluxes of associated flares. Noting that previous studies were possibly affected by projection effects and random association effects, we have considered two sets of carefully selected CME-flare events: four homologous events and four well-observed limb events. In the respective samples, good correlations are found between the CME speeds and the GOES X-ray peak fluxes of the associated flares. A similarly good correlation is found for all eight events of both samples when the CME speeds of the homologous events are corrected for projection effect. Our results suggest that a close relationship possibly exists between CME kinematics and flaring processes.