Haimin Wang

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.

Active-Region Monitoring and Flare Forecasting – I. Data Processing and First Results

This paper discusses a near real-time approach to solar active-region monitoring and flare prediction using the Big Bear Solar Observatory Active Region Monitor (ARM). Every hour, ARM reads, calibrates, and analyses a variety of data including: full-disk Hα images from the Global Hα Network; EUV, continuum, and magnetogram data from the Solar and Heliospheric Observatory (SOHO); and full-disk magnetograms from the Global Oscillation Network Group (GONG). For the first time, magnetic gradient maps derived from GONG longitudinal magnetograms are now available on-line and are found to be a useful diagnostic of flare activity. ARM also includes a variety of active-region properties from the National Oceanic and Atmospheric Administrations Space Environment Center, such as up-to-date active-region positions, GOES 5-min X-ray data, and flare-to-region identifications. Furthermore, we have developed a Flare Prediction System which estimates the probability for each region to produce C-, M-, or X-class flares based on nearly eight years of NOAA data from cyclexa022. This, in addition to BBSOs daily solar activity reports, has proven a useful resource for activity forecasting.

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.

Periodic Motion along a Solar Filament Initiated by a Subflare

A type of mass motion in solar filaments, not previously reported, is studied with high-cadence (1 minute) Hα observations made at the Big Bear Solar Observatory on 2001 October 24. This oscillatory motion is along the filament and extends over a long distance (~1.4 × 105 km) with a period of ~80 minutes and a very high velocity amplitude of ~92 km s-1. Another significant property of this oscillation is that it is triggered by a subflare that occurred near its footpoint. The oscillation completes three cycles before it damps out over a timescale of ~210 minutes. We mainly discuss whether this oscillation is an extreme form of the recently discovered counterstreaming flows in filaments or is a form of the large-amplitude filament oscillations (known as a winking filament) discovered a half-century ago.

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.

RELATIONSHIP BETWEEN CME KINEMATICS AND FLARE STRENGTH

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.

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.

Temperatures of Extreme-Ultraviolet-emitting Plasma Structures Observed by the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer has revealed, in unprecedented detail, various kinds of EUV-emitting plasma structures in the solar upper atmosphere. The filter ratio 195 A/171 A has been conventionally used to determine the plasma temperatures, but this method has a shortcoming in that it may not yield a unique temperature value for a given ratio. Therefore, we introduce a new method employing two filter ratios (195 A/171 A and 284 A/195 A). It is demonstrated that this color-color method is effective in determining a wide range of unambiguous plasma temperatures. We have obtained a temperature of 1 × 106 K for a loop that is bright in 171 A but hardly visible in 284 A, a higher temperature of 2 × 106 K for a loop that is clearly visible in 195 and 284 A but not in 171 A, and a transition-region temperature of 2.5 × 105 K for a low-lying loop that is clearly visible in all the EUV wavelengths. In addition, we have found that moss structures have temperatures of around 1 × 106 K and that EUV jets have temperatures of about 2.5 × 105 K.

Non-LTE Calculation of the Ni I 676.8 Nanometer Line in a Flaring Atmosphere

The Ni I 676.8 nm line is used by the Solar and Heliospheric Observatory Michelson Doppler Imager to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in an atmosphere that is bombarded by an energetic electron beam. This case is associated with the occurrence of solar flares. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni I line and an upward shift of its formation height, as well as an increased line source function. We find that the Ni I line may appear in emission in the presence of a fairly strong electron beam and preferentially in a cool atmosphere (i.e., sunspot umbrae/penumbrae). On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated to higher temperatures through other ways. This result implies that the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal but just an artifact associated with the production of an emission profile.