Yong-Jae Moon

A STATISTICAL STUDY OF TWO CLASSES OF CORONAL MASS EJECTIONS

A comprehensive statistical study is performed to address the question of whether two classes of coronal mass ejections (CMEs) exist. A total of 3217 CME events observed by SOHO/LASCO in 1996-2000 have been analyzed. We have examined the distributions of CMEs according to speed and acceleration, respectively, and investigated the correlation between speed and acceleration of CMEs. This statistical analysis is conducted for two subsets containing those CMEs that show a temporal and spatial association either with GOES X-ray solar flares or with eruptive filaments. We have found that CMEs associated with flares have a higher median speed than those associated with eruptive filaments and that the median speed of CMEs associated with strong flares is higher than that of weak-flare-associated CMEs. The distribution of CME acceleration shows a conspicuous peak near zero, not only for the whole data set, but also for the two subsets associated either with solar flares or with eruptive filaments. However, we have confirmed that the CMEs associated with major flares tend to be more decelerated than the CMEs related to eruptive filaments. The fraction of flare-associated CMEs has a tendency to increase with the CME speed, whereas the fraction of eruptive-filament-associated CMEs tends to decrease with the CME speed. This result supports the concept of two CME classes. We have found a possibility of two components in the CME speed distribution for both the CME data associated with flares larger than M1 class and the CME data related with limb flares. Our results suggest that the apparent single-peak distribution of CME speed can be attributed to the projection effect and possibly to abundance of small flares too. We also note that there is a possible correlation between the speed of CMEs and the time-integrated X-ray flux of the CME-associated limb flares.

RAPID CHANGES OF MAGNETIC FIELDS ASSOCIATED WITH SIX X-CLASS FLARES

In this paper, we present the results of the study of six X-class flares. We found significant changes in the photospheric magnetic fields associated with all of the events. For the five events in 2001, when coronagraph data were available, all were associated with halo coronal mass ejections. Based on the analyses of the line-of-sight magnetograms, all six events had an increase in the magnetic flux of the leading polarity of order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive because the changeover time, which we defined as the time to change from preflare to postflare state, ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear, and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops, (2) a change in the magnetic field direction, and (3) the expansion of the sunspot, which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference; i.e., the flux of leading polarity tends to increase while the flux of following polarity tends to decrease slightly.

Flare Activity and Magnetic Helicity Injection by Photospheric Horizontal Motions

We present observational evidence that the occurrence of homologous flares in an active region is physically related to the injection of magnetic helicity by horizontal photospheric motions. We have analyzed a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a period of 6.5 hours by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During this observing time span, seven homologous flares took place in the active region. We have computed the magnetic helicity injection rate into the solar atmosphere by photospheric shearing motions, and found that a signicant amount of magnetic helicity was injected during the observing period. In a strong M4.1 flare, the magnetic helicity injection rate impulsively increased and peaked at the same time as the X-ray flux did. The flare X-ray flux integrated over the Xray emission time strongly correlates with the magnetic helicity injected during the flaring interval. The integrated X-ray flux is found to be a logarithmically increasing function of the injected magnetic helicity. Our results suggest that injection of helicity and abrupt increase of helicity magnitude play a signicant role in flare triggering.

Impulsive Variations of the Magnetic Helicity Change Rate Associated with Eruptive Flares

In this paper, we investigate impulsive variations of the magnetic helicity change rate associated with eruptive solar flares (three X class flares and one M class flare) accompanying halo coronal mass ejections. By analyzing four sets of 1 minute cadence full-disk magnetograms taken by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, we have determined the rates of magnetic helicity transport due to horizontal photospheric motions. We have found that magnetic helicity of the order of 1041 Mx2 was impulsively injected into the corona around the flaring peak time of all the flares. We also found that there is a positive correlation between the impulsively injected magnetic helicity and the X-ray peak flux of the associated flare. The impulsive helicity variations are attributed to horizontal velocity kernels localized near the polarity inversion lines. Finally, we report that there is a close spatial proximity between the horizontal velocity kernels and Hα bright points.

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

Statistical Evidence for Sympathetic Flares

Sympathetic flares are a pair of flares that occur almost simultaneously in different active regions, not by chance, but because of some physical connection. In this paper statistical evidence for the existence of sympathetic flares is presented. From GOES X-ray flare data, we have collected 48 pairs of near simultaneous flares whose positional information and Yohkoh soft X-ray telescope images are available. To select the active regions that probably have sympathetic flares, we have estimated the ratio R of actual flaring overlap time to random-coincidence overlap time for 38 active region pairs. We have then compared the waiting-time distributions for the two different groups of active region pairs (R > 1 and R 1. This is the first time such strong statistical evidence has been found for the existence of sympathetic flares. To examine the role of interconnecting coronal loops, we have also conducted the same analysis for two subgroups of the R > 1 group: one with interconnecting X-ray loops and the other without. We do not find any statistical evidence that the subgroup with interconnecting coronal loops is more likely to produce sympathetic flares than the subgroup without. For the subgroup with loops, we find that sympathetic flares favor active region pairs with transequatorial loops.

FORCE-FREENESS OF SOLAR MAGNETIC FIELDS IN THE PHOTOSPHERE

It is widely believed that solar magnetic fields are force-free in the solar corona but not in the solar photosphere at all. In order to examine the force-freeness of active region magnetic fields at the photospheric level, we have calculated the integrated magnetic forces for 12 vector magnetograms of three flare-productive active regions. The magnetic field vectors are derived from simultaneous Stokes profiles of the Fe I doublet λλ6301.5 and 6302.5 obtained by the Haleakala Stokes Polarimeter of Mees Solar Observatory, with a nonlinear least-squares method adopted for field calibration. The resulting vertical Lorentz force normalized to the total magnetic pressure force |Fz/Fp| ranges from 0.06 to 0.32 with a median value of 0.13, which is smaller than the values (~0.4) obtained by Metcalf et al., who applied a weak field derivative method to the Stokes profiles of Na I λ5896. Our results indicate that the photospheric magnetic fields are not so far from force-free as conventionally regarded. As a good example of a linear force-free field, NOAA Active Region 5747 is examined. By applying three different methods (a most probable value method, a least-squares fitting method, and comparison with linear force-free solutions), we have derived relatively consistent linear force-free coefficients for NOAA AR 5747. It is found that the scaled downward Lorentz force (|Fz/Fp|) in the solar photosphere decreases with increasing |α|. Our results also show that the force-freeness of photospheric magnetic fields depends not only on the character of the active region but also on its evolutionary status.

Flux Rope Acceleration and Enhanced Magnetic Reconnection Rate

A physical mechanism of flares has emerged from our 2.5-dimensional resistive MHD simulations of the dynamical evolution of current sheet formation and magnetic reconnection and flux rope acceleration subject to the continuous, slow increase of magnetic shear in the arcade. With anomalous nonuniform resistivity in the current sheet the simulation results relate the flux ropes accelerated rising motion with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. The simulation results provide good quantitative agreement with observations of the acceleration of flux ropes, which are manifested in the form of ejected soft X-ray plasmas, erupting filaments, or CMEs. For the X-class flare events studied in this paper the peak reconnection electric field is ~O(103 V m-1) or larger, enough to accelerate electrons to over 100 keV in a field-aligned distance of 0.1 km and produce impulsive hard X-ray emission observed during the flare rise phase.

Observational Evidence of Magnetic Flux Submergence in Flux Cancellation Sites

Using high-resolution vector magnetograms of NOAA Active Region 10043, observed on 2002 July 26 with the Advanced Stokes Polarimeter and low-order adaptive optics system, we studied the magnetic field topology and line-of-sight velocities in two flux cancellation sites. We found that the magnetic field is near horizontal at the place where two opposite polarities cancel each other. In addition, we observed significant downflows of about 1 km s-1 near the polarity reversal line, where the field is horizontal. We interpret these observations as the direct evidence of the magnetic flux submergence of concave-down (Ω-shaped) magnetic loop at the flux cancellation sites.

Sympathetic Coronal Mass Ejections

We address the question whether there exist sympathetic coronal mass ejections (CMEs), which take place almost simultaneously in different locations with a certain physical connection. For this study, the following three investigations are performed. First, we have examined the waiting-time distribution of the CMEs that were observed by the SOHO Large Angle and Spectrometric Coronagraph (LASCO) from 1999 February to 2001 December. The observed waiting-time distribution is found to be well approximated by a time-dependent Poisson distribution without any noticeable overabundance at short waiting times. Second, we have investigated the angular difference distribution of successive CME pairs to examine their spatial correlations. A remarkable overabundance relative to background levels is found within 10° of the position angle difference, which supports the existence of quasi-homologous CMEs that occur sequentially in the same active region. Both of the above results indicate that sympathetic (interdependent) CMEs are far less frequent than independent CMEs. Third, we have examined the EUV Imaging Telescope running difference images and the LASCO images of quasi-simultaneous CME pairs and found a candidate sympathetic CME pair, of which the second CME may be initiated by the eruption of the first CME. Possible mechanisms of the sympathetic CME triggering are discussed.