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MAGNETIC HELICITY CHANGES OF SOLAR ACTIVE REGIONS  

Yong-Jae Moon   Submitted: 2003-04-18 11:52

In this paper, we review recent studies on the magnetic helicity changes of solar active regions by photospheric horizontal motions. Recently, Chae(2001) 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 17 imes 1040 mbox{ Mx}^2 mbox{ h}-1. 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.

Authors: Y.-J. Moon, Jongchul Chae, and Y. D. Park
Projects: Soho-MDI

Publication Status: JKAS, in press
Last Modified: 2003-04-18 11:52
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SYMPATHETIC CORONAL MASS EJECTIONS  

Yong-Jae Moon   Submitted: 2003-04-18 11:46

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 {it SOHO}/LASCO from February 1999 to December 2001. 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 10arcdeg of the position angle difference, which supports the existence of quasi-homologous CMEs that sequentially occur in the same active region. The above results both indicate that sympathetic (interdependent) CMEs are far less frequent than independent CMEs. Third, we have examined the EIT running difference images and the LASCO images of quasi-simultaneous CME pairs, and found a candidate of the sympathetic CME pair, of which the second CME may be initiated by eruption of the first CME. Possible mechanisms of the sympathetic CME triggering are discussed.

Authors: Y.-J. Moon, G. S. Choe, H. Wang, Y. D. Park
Projects: Soho-EIT

Publication Status: ApJ, 2003, May 10 issue
Last Modified: 2003-04-18 11:47
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Impulsive Variation of Magnetic Helicity Change Rate associated with Eruptive Flares  

Yong-Jae Moon   Submitted: 2002-08-07 21:56

In this paper, we investigate impulsive variations of magnetic helicity change rate associated with eruptive solar flares (three X-class flares and one M-Class flare) accompanying halo CMEs. By analyzing four sets of 1 minute cadence full-disk magnetograms taken by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO), 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 Mx{^2} 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.

Authors: Moon, Y.-J., Chae, J., Wang, H., Choe, G. S., Park, Y. D.
Projects:

Publication Status: ApJ, in press
Last Modified: 2002-08-07 21:56
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A Statistical Study of Two Classes of CMEs  

Yong-Jae Moon   Submitted: 2002-08-07 21:42

A comprehensive statistical study is performed to address the question whether two classes of coronal mass ejections (CMEs) exist. A total of 3217 CME events observed by SOHO/LASCO in 1996 to 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 which show a temporal and spatial association either with GOES X-ray solar flares or with eruptive filaments. %The number of CMEs in deceleration is comparable to the %number of CMEs in acceleration. 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.

Authors: Moon, Y.-J., Choe, G.S., Wang, H., Park, Y.D., Gopalswamy, N., Yang, G., Yashiro, S.
Projects:

Publication Status: ApJ, in press
Last Modified: 2002-08-07 21:49
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Flaring Time Interval Distribution and Spatial Correlation of Major X-ray Flares  

Yong-Jae Moon   Submitted: 2002-04-05 17:52

A statistical study is performed on X-ray flares stronger than C1 class that erupted during the solar maximum between 1989 and 1991. We have investigated the flaring time interval distribution (waiting-time distribution) and the spatial correlation of successive flare pairs. The observed waiting-time distribution for the whole data is found to be well represented by a nonstationary Poisson probability function with time varying mean flaring rates. The period most suitable for a constant mean flaring rate is determined to be 2-3 days by a Kolmogorov-Smirnov test. We have also found that the waiting-time distribution for flares in individual active regions follows a stationary Poisson probability function mexp(-mt) with a corresponding mean flaring rate. Therefore, the flaring probability within a given time is given by 1-exp(-mt), when the mean flaring rate m is properly estimated. It is also found that there is no systematic relationships between peak fluxes of flares and their waiting-time distributions. The above findings support the idea that the solar corona is in a self-organized critical state. A comparison of the angular distances of successively observed flare pairs with those of hypothetical flare pairs generated by random distribution shows a positive angular correlation within about 10 degrees (sim 180 arcsec in the observing field) of angular separation, which suggests that homologous flares occurring in the same active region should outnumber sympathetic flares.

Authors: Y.-J. Moon, G. S. Choe, H. S. Yun, Y. D. Park
Projects:

Publication Status: JGR-space physics, 2001, 106(A12), 29951-29961
Last Modified: 2002-04-05 17:52
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Force-Freeness of Solar Magnetic Fields in the Photosphere  

Yong-Jae Moon   Submitted: 2002-04-05 17:47

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 6301.5 and 6302.5 obtained by the Haleakala Stokes Polarimeter of Mees Solar Observatory, with a non-linear least square method adopted for field calibration. The resulting vertical Lorentz force normalized to the total magnetic pressure force |F_z/F_p| ranges from 0.06 to 0.36 with a median value of 0.13, which is smaller than the values (sim 0.4) obtained by Metcalf et al. who applied a weak field derivative method to the Stokes profiles of Na I 5896. 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, AR 5747 is examined. By applying three different methods (a most probable value method, a least square fitting method, and comparison with linear force-free solutions), we have derived relatively consistent linear force-free coefficients for AR 5747. It is found that the scaled downward Lorentz force (|F_z/F_p|) 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.

Authors: Y.-J. Moon, G. S. Choe, H. S. Yun, Y. D. Park, D. L. Mickey
Projects:

Publication Status: ApJ, 2002, 568, 422-431
Last Modified: 2002-04-05 17:55
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A Revised Shock Time of Arrival (STOA) Model for Interplanetary Shock Propagation: STOA-2  

Yong-Jae Moon   Submitted: 2002-04-05 17:40

We have examined a possibility for improvement of the STOA (Shock Time Of Arrival) model for interplanetary shock propagation. In the STOA model, the shock propagating velocity is given by V_s sim R-N with N=0.5, where R is the heliocentric distance. Noting observational and numerical findings that the radial dependence of shock wave velocity depends on initial shock wave velocity, we suggest a simple modified STOA model (STOA-2) which has a linear relationship between initial coronal shock wave velocity (Vis) and its deceleration exponent(N), N=0.05+4 imes10-4Vis, where Vis is a numeric value expressed in units of km s-1. Our results show that the STOA-2 model not only removes a systematic dependence of the transit time difference predicted by the previous STOA model on initial shock velocity, but also reduces the number of events with large transit time differences.

Authors: Y.-J. Moon, M. Dryer, Z. Smith, Y. D. Park, K. S. Cho
Projects:

Publication Status: Geophysical Research Letter, 2002, in press
Last Modified: 2002-04-06 00:45
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STATISTICAL EVIDENCE FOR SYMPATHETIC FLARES  

Yong-Jae Moon   Submitted: 2002-04-04 15:08

Sympathetic flares are a pair of flares which occur almost simultaneously in different active regions, not by chance, but due to some physical connections. 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/SXT images are available. To select the active regions which 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) with corresponding nonstationary Poisson distributions. As a result, we find a remarkable overabundance in short waiting times for the group with 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.

Authors: Y.-J. MOON, G. S. CHOE, Y. D. PARK, HAIMIN WANG, PETER T. GALLAGHER, JONGCHUL CHAE, H. S. YUN, AND PHILIP R. GOODE
Projects:

Publication Status: ApJ, 574, 434
Last Modified: 2002-08-15 16:53
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FLARE ACTIVITY AND MAGNETIC HELICITY INJECTION BY PHOTOSPHERIC HORIZONTAL MOTIONS  

Yong-Jae Moon   Submitted: 2002-04-04 14:55

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 significant 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 X-ray 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 significant role in flare triggering.

Authors: Y.-J. MOON, JONGCHUL CHAE, G. S. CHOE, HAIMIN WANG, Y. D. PARK, H. S. YUN, VASYL YURCHYSHYN, PHILIP R. GOODE
Projects:

Publication Status: ApJ, 574, 1066
Last Modified: 2002-08-15 16:55
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Abstracts by Author
MAGNETIC HELICITY CHANGES OF SOLAR ACTIVE REGIONS
SYMPATHETIC CORONAL MASS EJECTIONS
Impulsive Variation of Magnetic Helicity Change Rate associated with Eruptive Flares
A Statistical Study of Two Classes of CMEs
Flaring Time Interval Distribution and Spatial Correlation of Major X-ray Flares
Force-Freeness of Solar Magnetic Fields in the Photosphere
A Revised Shock Time of Arrival (STOA) Model for Interplanetary Shock Propagation: STOA-2
STATISTICAL EVIDENCE FOR SYMPATHETIC FLARES
FLARE ACTIVITY AND MAGNETIC HELICITY INJECTION BY PHOTOSPHERIC HORIZONTAL MOTIONS

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