• Journal of Astronomy and Space Sciences
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• v.24 no.3
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• pp.227-234
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• 2007

A comet that passes extremely close to the Sun at perihelion is called a sungrazing comet (of sungrazer). Around 1270 sungrazing comets have been discovered on coronagraph images from the SOHO (SOlar and Heliospheric Observatory) spacecraft since 1996 up to April, 2007. The major groups orbiting around the Sun in similar orbits are named as Kreutz, Meyer, Marsden, and Kracht 1&2 families. About 85% of SOHO comets belong to the Kreutz family with perihelion on distances of less than about $2{\sim}3R_{\bigodot}$. The distributions of perihelia and the orbital elements of SOHO sungrazing comets are analyzed. We investigated closely spaced pairs and clusters in the orbital element space. Here, we suggest three prospective groups of the sungrazing cornets: [(C/2000 Y6, C/2000 Y7), (C/2000 V4, C/2001 T5), (C/2003 H6, C/2003 H7)].

• Journal of The Korean Astronomical Society
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• v.39 no.4
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• pp.139-145
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• 2006

We have made a comprehensive statistical study on the coronal mass ejections(CMEs) associated with helmet streamers. A total number of 3810 CMEs observed by SOHO/LASCO coronagraph from 1996 to 2000 have been visually inspected. By comparing their LASCO images and running difference images, we picked out streamer-associated CMEs, which are classified into two sub-groups: Class-A events whose morphological shape seen in the LASCO running difference image is quite similar to that of the pre-existing streamer, and Class-B events whose ejections occurred in a part of the streamer. The former type of CME may be caused by the destabilization of the helmet streamer and the latter type of CME may be related to the eruption of a filament underlying the helmet streamer or narrow CMEs such as streamer puffs. We have examined the distributions of CME speed and acceleration for both classes as well as the correlation between their speed and acceleration. The major results from these investigations are as follows. First, about a quarter of all CMEs are streamer-associated CMEs. Second, their mean speed is 413 km $s^{-1}$ for Class-A events and 371 km $s^{-1}$ for Class-B events. And the fraction of the streamer-associated CMEs decreases with speed. Third, the speed-acceleration diagrams show that there are no correlations between two quantities for both classes and the accelerations are nearly symmetric with respect to zero acceleration line. Fourth, their mean angular width are about $60^{\circ}$, which is similar to that of normal CMEs. Fifth, the fraction of streamer-associated CMEs during the solar minimum is a little larger than that during the solar maximum. Our results show that the kinematic characteristics of streamer-associated CMEs, especially Class-A events, are quite similar to those of quiescent filament-associated CMEs.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.120.1-120.1
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• 2012

The Lunar Reconnaissance Orbiter (LRO) launched on June 16, 2009 has six experiments including of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) onboard. The CRaTER instrument characterizes the radiation environment to be experienced by humans during future lunar missions. The CRaTER instrument measures the effects of ionizing energy loss in matter specifically in silicon solid-state detectors due to penetrating solar energetic protons (SEP) and galactic cosmic rays (GCRs) after interactions with tissue-equivalent plastic (TEP), a synthetic analog of human tissue. The CRaTER instrument houses a compact and highly precise microdosimeter. It measures dose rates below one micro-Rad/sec in silicon in lunar radiation environment. Forbush decrease (FD) event is the sudden decrease of GCR flux. We use the data of cosmic ray and dose rates observed by the CRaTER instrument. We also use the CME list of STEREO SECCHI inner, outer coronagraph and the interplanetary CME data of the ACE/MAG instrument.We examine the origins and the characteristics of the FD-like events in lunar radiation environment. We also compare these events with the FD events on the Earth. We find that whenever the FD events are recorded at ground Neutron Monitor stations, the FD-like events also occur on the lunar environments. The flux variation amplitude of FD-like events on the Moon is approximately two times larger than that of FD events on the Earth. We compare time profiles of GCR flux with of the dose rate of FD-like events in the lunar environment. We figure out that the distinct FD-like events correspond to dose rate events in the CRaTER on lunar environment during the event period.