Kap-Sung Kim

A statistical comparison of interplanetary shock and CME propagation models

[1] We have compared the prediction capability of two types of Sun-Earth connection models: (1) ensemble of physics-based shock propagation models (STOA, STOA-2, ISPM, and HAFv.2) and (2) empirical CME propagation (CME-ICME and CME-IP shock) models. For this purpose, we have selected 38 near-simultaneous pairs of coronal mass ejections (CMEs) and metric type II radio bursts. By applying the adopted models to these events, we have estimated the time difference between predicted and observed arrivals of interplanetary (IP) shocks and ICMEs at the Earth or L1. The mean absolute error of the shock arrival time (SAT) within an adopted window of ±24 hours is 9.8 hours for the ensemble of shock propagation models, 9.2 hours for the CME-IP shock model, and 11.6 hours for the CME-ICME model. It is also found that the success rate for all models is about 80% for the same window. The results imply that the adopted models are comparable in their prediction of the arrival times of IP shocks and interplanetary CMEs (ICMEs). The usefulness of these models is also discussed in terms of real-time forecasts, underlying physics, and identification of IP shocks and ICMEs at the Earth. INDEX TERMS: 2722 Magnetospheric Physics: Forecasting; 7519 Solar Physics, Astrophysics, and Astronomy: Flares; 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections; 2139 Interplanetary Physics: Interplanetary shocks; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; KEYWORDS: space weather forecasting, solar flares, CMEs, interplanetary shocks

The Challenge of the Largest Structures in the Universe to Cosmology

Large galaxy redshift surveys have long been used to constrain cosmological models and structure formation scenarios. In particular, the largest structures discovered observationally are thought to carry critical information on the amplitude of large-scale density fluctuations or homogeneity of the universe, and have often challenged the standard cosmological framework. The Sloan Great Wall (SGW) recently found in the Sloan Digital Sky Survey (SDSS) region casts doubt on the concordance cosmological model with a cosmological constant (i.e., the flat ΛCDM model). Here we show that the existence of the SGW is perfectly consistent with the ΛCDM model, a result that only our very large cosmological N-body simulation (the Horizon Run 2, HR2) could supply. In addition, we report on the discovery of a void complex in the SDSS much larger than the SGW, and show that such size of the largest void is also predicted in the ΛCDM paradigm. Our results demonstrate that an initially homogeneous isotropic universe with primordial Gaussian random phase density fluctuations growing in accordance with the general relativity can explain the richness and size of the observed large-scale structures in the SDSS. Using the HR2 simulation we predict that a future galaxy redshift survey about four times deeper or with 3 mag fainter limit than the SDSS should reveal a largest structure of bright galaxies about twice as big as the SGW.

Three-Dimensional Structure and Energy Balance of a Coronal Mass Ejection

The Ultraviolet Coronagraph Spectrometer (UVCS) observed Doppler-shifted material of a partial halo coronal mass ejection (CME) on 2001 December 13. The observed ratio of [O V]/O V] is a reliable density diagnostic important for assessing the state of the plasma. Earlier UVCS observations of CMEs found evidence that the ejected plasma is heated long after the eruption. We have investigated the heating rates, which represent a significant fraction of the CME energy budget. The parameterized heating and radiative and adiabatic cooling have been used to evaluate the temperature evolution of the CME material with a time-dependent ionization state model. The functional form of a flux-rope model for interplanetary magnetic clouds was also used to parameterize the heating. We find that continuous heating is required to match the UVCS observations. To match the O VI bright knots, a higher heating rate is required such that the heating energy is greater than the kinetic energy. The temperatures for the bright knots in Lyα and C III emission indicate that smaller heating rates are required for those regions. In the context of the flux-rope model, about 75% of the magnetic energy must go into heat in order to match the O VI observations. We derive tighter constraints on the heating than earlier analyses, and we show that thermal conduction with the Spitzer conductivity is not sufficient to account for the heating at large heights.

A Study of Flare-associated X-Ray Plasma Ejections. I. Association with Coronal Mass Ejections

We have made a comprehensive statistical study of the relationship between flare-associated X-ray plasma ejections and coronal mass ejections (CMEs). For this we considered all flare-mode data in Yohkoh SXT observations from 1999 April to 2001 March and then selected 279 limb flares seen at longitudes greater than 60°. For these events, we identified whether there were associated X-ray plasma ejections or not. We found that about half (137/279) of the flares have X-ray plasma ejections, and we present a comprehensive list of these with their event times and speeds. We then determined whether there was an association between the flares with plasma ejections and CMEs detected by the Solar and Heliospheric Observatory LASCO instrument, on the basis of temporal and spatial proximity. It is found that about 69% (95/137) of the X-ray plasma ejections are associated with CMEs and that about 84% (119/142) of the events without plasma ejections do not have related CMEs. The associations are found to increase with flare strength and duration. We find that X-ray plasma ejections occur nearly simultaneously with the hard X-ray flare peak, supporting the idea that the X-ray plasma ejections are tightly associated with the flaring process. When the CMEs are extrapolated into the Yohkoh field of view for 43 selected, well-observed events, it is found that about 80% of the CMEs preceded X-ray plasma ejections, by approximately 20 minutes on average. Our results show that X-ray plasma ejections usually do not represent the early signature of a CMEs leading edge but are closely associated with CMEs.

Three-dimensional Structure of the 2002 April 21 Coronal Mass Ejection

A three-dimensional reconstruction of the 2002 April 21 partial halo Coronal Mass Ejection (CME) has been made based on the O VI 1032 A and [Fe XVIII] 974 A lines observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). We use the Doppler velocities to derive the CME structure along the line of sight. UVCS observed the O VI line profiles split into strongly Doppler red- and blueshifted components, and the region of split profiles grew rapidly along the long spectrograph slit. The more localized [Fe XVIII] bright emission starts at the same time as the maximum Doppler redshift of O VI, indicating that it is inside the CME. In the view from the solar west, the O VI looks like halo CMEs seen by the Large Angle and Spectrometric Coronagraph (LASCO). The [Fe XVIII] bright emission appears as a barlike structure seen nearly end-on from the Earth, and side-on from the solar north and from solar west. The reconstructed [Fe XVIII] emission allows two interpretations, as ejection of preexisting hot plasma or as a current sheet. The evidence favors the current sheet interpretation, although we cannot rule out the alternatives.

Multiband photometric re-classification of ROTSE-I δ Scuti type stars

We present multi-passband CCD photometry of 20 ROTSE-I Scuti type pulsating stars and 1 RR Lyrae star to re- classify their variable types using the comparison of amplitudes between V and I passbands. For the re-classification, we used a criterion that pulsating stars have larger amplitude dierences between passbands than eclipsing binaries because brightness changes of pulsating stars are mainly due to the temperature variations. As a result, only six stars were re-confirmed as Scuti variables and thirteen stars turned out to be W UMa type eclipsing binaries. The other two stars were identified as one cataclysmic variable and one non-variable, respectively. Our results suggest that a number of ROTSE-I Scuti type stars, which do not show typical pulsating light curves of high amplitude Scuti stars, are W UMa type eclipsing binaries.

NONLINEAR FORCE-FREE EXTRAPOLATION OF THE CORONAL MAGNETIC FIELD BASED ON THE MAGNETOHYDRODYNAMIC RELAXATION METHOD

We develop a nonlinear force-free field (NLFFF) extrapolation code based on the magnetohydrodynamic (MHD) relaxation method. We extend the classical MHD relaxation method in two important ways. First, we introduce an algorithm initially proposed by Dedner et al. to effectively clean the numerical errors associated with ? ? B . Second, the multigrid type method is implemented in our NLFFF to perform direct analysis of the high-resolution magnetogram data. As a result of these two implementations, we successfully extrapolated the high resolution force-free field introduced by Low & Lou with better accuracy in a drastically shorter time. We also applied our extrapolation method to the MHD solution obtained from the flux-emergence simulation by Magara. We found that NLFFF extrapolation may be less effective for reproducing areas higher than a half-domain, where some magnetic loops are found in a state of continuous upward expansion. However, an inverse S-shaped structure consisting of the sheared and twisted loops formed in the lower region can be captured well through our NLFFF extrapolation method. We further discuss how well these sheared and twisted fields are reconstructed by estimating the magnetic topology and twist quantitatively.

PREFLARE ERUPTION TRIGGERED BY A TETHER-CUTTING PROCESS

We have examined the preflare activity of an M1.2 flare that occurred in NOAA active region 8440 on 1999 January 16, using images from the Soft X-Ray Telescope (SXT) on board Yohkoh, 1600 A UV images from the Transition Region and Coronal Explorer (TRACE), X-ray flux data from the GOES satellite, and magnetograms from Big Bear Solar Observatory (BBSO). During the preflare phase, we note a weak GOES X-ray flux enhancement just 4 minutes before the main flare begins. The SXT images show that this enhancement occurs at one footpoint of a soft X-ray loop bundle, which exactly coincides with the kernel of the major flare. The series of TRACE images provides the following pieces of evidence for small-scale magnetic reconnections associated with the preflare activity. (1) A small-scale UV sigmoid is seen at the X-ray loop footpoint before the preflare activity, and it is located along the polarity inversion line. (2) The brightest among the UV brightenings is exactly coincident and cospatial with the soft X-ray brightening observed by the Yohkoh SXT and GOES. (3) There were several interactions and brightenings among small UV loops. After these brightenings, the connectivity of the UV loops was apparently changed. As a result, a large rising loop structure was formed, with a maximum rising speed of about 40 km s−1. (4) The main flare occurred in this structure. In the aspects of the overall configuration and morphological change of UV loops, the preflare activity is quite consistent with the tether-cutting model with a single-bipole magnetic explosion. We suggest that the preflare activity and the main flare in this event not only have similar physical mechanisms, but also have a causal relation.

COMPONENT-BASED DEVELOPMENT OF OBSERVATIONAL SOFTWARE FOR KASI SOLAR IMAGING SPECTROGRAPH

In this paper, we have made the component-based development of observational software for KASI solar imaging spectrograph (KSIS) that is able to obtain three-dimensional imaging spectrograms by using a scanning mirror in front of the spectrograph slit. Since 2002, the KASI solar spectrograph has been successfully operated to observe solar spectra for a given slit region as well as to inspect the response functions of narrow band filters. To improve its capability, we have developed the KSIS that can perform sequential observations of solar spectra by simultaneously controlling the scanning mirror and the CCD camera via Visual C++. Main task of this paper is to introduce the development of the component-based software for KSIS. Each component of the software is reusable on the level of executable file instead of source code because the software was developed by using CBD (component-based development) methodology. The main advantage of such a component-based software is that key components such as image processing component and display component can be applied to other similar observational software without any modifications. Using this software, we have successfully obtained solar imaging spectra of an active region (AR 10708) including a small sunspot. Finally, we present solar spectra () that were obtained at an active region and a quiet region in order to confirm the validity of the developed KSIS and its software.

Comparison of SOHO/UVCS and MLSO MK4 Coronameter Densities

We have compared the density distributions of solar corona obtained by SOHO Ultraviolet Coronagraph Spectrometer (UVCS) and Mauna Loa Solar Observatory (MLSO) MK4 coronameter. This is the first attempt to compare the coronal densities estimated by the two instruments. In the spectral data of UVCS, we have selected two emission lines (O vi 1032 A and 1037.6 A), which have both radiative and collisional components. The coronal number density is determined from the ratio of these two components. The MK4 coronameter has a field of view ranging from 1.08 to 2.85 solar radii. The coronal density can be determined by inverting MLSO MK4 polarization maps. We find that the mean electron number density in a helmet streamer observed by MK4 on 2003 April 28 is fairly consistent with that observed by UVCS. For a coronal hole and an active region observed on 1999 October 19 and 24, the MK4 coronal densities are close to those from the UVCS within a factor of two; the former values are twice the latter at 1.7 solar radii and closer to the latter at higher altitudes. Our results demonstrate that MK4 polarization data can provide us with a coronal density distribution in a large field of view with a time cadence of about three minutes. We suggest that the MK4 data can be used to derive 2-D density distributions of coronal structures and further to estimate the heights of CME-associated type II shocks.