Guo Yang

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.

On the Relation between Filament Eruptions, Flares, and Coronal Mass Ejections

We present a statistical study of 106 filament eruptions, which were automatically detected by a pattern recognition program implemented at Big Bear Solar Observatory using Hα full-disk data from 1999 to 2003. We compare these events with Geostationary Operational Environmental Satellite soft X-ray time profiles, solar-geophysical data (SGD) solar event reports, Michelson Doppler Imager magnetograms, and Large Angle and Spectrometric Coronagraph (LASCO) data to determine the relationship between filament eruptions and other phenomena of solar activity. (1) Excluding eight events with no corresponding LASCO data, 55% or 56% of 98 events were associated with coronal mass ejections (CMEs). (2) Active region filament eruptions have a considerably higher flare association rate of 95% compared to quiescent filament eruptions with 27%, but a comparable CME association rate, namely, 43% for active region filament eruptions and 54% for quiescent filament eruptions. (3) 54% or 68% of 80 disk events were associated with new flux emergence. In addition, we derived the sign of magnetic helicity and the orientation of the magnetic field associated with seven halo CMEs and demonstrated that the geoeffectiveness of a halo CME can be predicted by these two parameters.

High-Resolution Observations of Multiwavelength Emissions during Two X-Class White-Light Flares

We observed two X-class white-light flares (WLFs) on 2003 October 29 (~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope (DST) and its High-Order Adaptive Optics (HOAO) system in several wavelengths. The spatial resolution was close to the diffraction limit of DSTs 76 cm aperture, and the cadence was as high as 2 s. This is the first time that WLFs have been observed in the near-infrared (NIR) wavelength region. We present a detailed study in this paper comparing photospheric continuum observations during the two events with corresponding line-of-sight magnetograms from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR) data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We also discuss several models that provide possible mechanisms to explain these continuum enhancements, especially in the NIR.

Near-Infrared Observations at 1.56 Microns of the 2003 October 29 X10 White-Light Flare

We present high-resolution observations of an X10 white-light flare in solar NOAA Active Region 10486 obtained with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak on 2003 October 29. Our investigation focuses on flare dynamics observed in the near-infrared (NIR) continuum at 1.56 μm. This is the first report of a white-light flare observed at the opacity minimum. The spatial resolution was close to the diffraction limit of the 76 cm aperture DST. The data benefited from a newly developed high-order adaptive optics system and a state-of-the-art NIR complex metal oxide semiconductor focal plane array. In addition, we compared hard X-ray (HXR) data of RHESSI and magnetograms of the Michelson Doppler Imager on board SOHO with the NIR continuum images. Although it is still possible that some high-energy electrons penetrate deep to this layer, a more likely explanation of the observed emission is back-warming. During the impulsive phase of the flare, two major flare ribbons moved apart, which were both temporally and spatially correlated with RHESSI HXR ribbons. The maximum intensity enhancement of the two flare ribbons is 18% and 25%, respectively, compared to the quiet-Sun NIR continuum. The separation speed of the ribbons is about 38 km s-1 in regions with weak magnetic fields and decreases to about 19 km s-1, where stronger magnetic fields are encountered. The derived electric field in reconnection current sheet Ec is of the order of 45 V cm-1 during the flare maximum.

Inter-Active Region Connection of Sympathetic Flaring on 2000 February 17

We have analyzed high-resolution Ha full disk data from Big Bear Solar Observatory (BBSO); magnetograph and EUV data from the Michelson Doppler Imager, Large Angle and Spectrometric Coronagraph, and Extreme Ultraviolet Imaging Telescope on board SOHO; and Yohkoh soft X-ray data of 2000 February 17. Two sympathetic M-class solar —ares erupted in succession in NOAA Active Region 8869 and 8872, respectively. The eruption from AR 8872 was followed by an extremely symmetric halo coronal mass ejection (CME). We demonstrate the loop activation, which appears to be the consequence of the —rst —are in AR 8869 and the cause of the second —are in AR 8872. The activation started in the form of a surge just after a —lament eruption and its associated —are in AR 8869. The surge quickly turned into a set of disturbances that propagated at a speed of about 80 km s~1 toward the other active region AR 8872. The second —are followed in less than an hour after the arrival of the disturbances at AR 8872. The moving disturbances appeared in absorption in both Ha and EUV 195 The Ae images. disturbances may represent mass transfer, which had a signi—cant velocity component perpendicular to the —eld lines and, hence, caused the transport of —eld lines. In this case, the disturbances may be considered to be a special kind of surge, which we may call a ii sweeping closed-loop surge.ˇˇ We also demonstrated large area dimmings associated with the CME in three active regions. The dimming started from AR 8869 and AR 8872 and was extended to AR 8870, which was on the opposite side of the solar equator. We believe that both the activation of inter-active region loops and the large-scale dimming are the signatures of large-scale restructuring associated with the CME.

Multiwavelength Study of Flow Fields in Flaring Super Active Region NOAA 10486

We present high-resolution observations of horizontal flow fields measured by local correlation tracking from intensity images in three wavelengths, i.e., G band (GB), white light (WL), and near-infrared (NIR). The observations were obtained on 2003 October 29 within the flaring super active region NOAA 10486, which was the source of several X-class flares, including an X10 flare that occurred near the end of the observing run. The data were obtained at National Solar Observatory/Sacramento Peak (NSO/SP) using the newly developed high-order adaptive optics (AO) system. We also use Dopplergrams and magnetograms from MDI on board SOHO to study the line-of-sight flow and magnetic field. We observe persistent and long-lived (at least 5 hr) strong horizontal and vertical shear flows (both in the order of 1 km s-1) along the magnetic neutral line (NL) until the X10 flare occurred. From lower photospheric level (NIR), the direction of the flows does not change up to the upper photosphere (GB), while the flow speeds in the shear motion regions decrease and, on the contrary, those in regions without shear motions increase with increasing altitude. Right after the X10 flare, the magnetic gradient decreased, while both horizontal and vertical shear flows dramatically enhanced near the flaring NL. Our results suggest that photospheric shear flows and local magnetic shear near the NL can increase after the flare, which may be the result of shear release in the overlying large-scale magnetic system or the reflection of a twisted or sheared flux emergence carrying enough energy from the subphotosphere.

Comparison of the 1998 April 29 M6.8 and 1998 November 5 M8.4 Flares

We combined, and analyzed in detail, the Hα and magnetograph data from Big Bear Solar Observatory (BBSO), full-disk magnetograms from the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO), coronagraph data from the Large Angle Spectrometric Coronagraph (LASCO) of SOHO, Fe XII 195 A data from the Extreme ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray telescope (SXT) data of the M6.8 flare of 1998 April 29 in National Oceanic and Atmospheric Administration (NOAA) region 8375 and the M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares have remarkable similarities: 1. Partial halo coronal mass ejections (CMEs) were observed for both events. For the 1998 April 29 event, even though the flare occurred in the southeast of the disk center, the ejected material moved predominantly across the equator, and the central part of the CME occurred in the northeast limb. The direction in which the cusp points in the postflare SXT images determines the dominant direction of the CMEs. 2. Coronal dimming was clearly observed in EIT Fe XII 195 A for both but was not observed in Yohkoh SXT for either event. Dimming started 2 hr before the onset of the flares, indicating large-scale coronal restructuring before both flares. 3. No global or local photospheric magnetic field change was detected from either event; in particular, no magnetic field change was found in the dimming areas. 4. Both events lasted several hours and, thus, could be classified as long duration events (LDEs). However, they are different in the following important aspects. For the 1998 April 29 event, the flare and the CME are associated with an erupting filament in which the two initial ribbons were well connected and then gradually separated. SXT preflare images show the classical S-shape sheared configuration (sigmoid structure). For the 1998 November 5 event, two initial ribbons were well separated, and the SXT preflare image shows the interaction of at least two loops. In addition, no filament eruption was observed. We conclude that even though these two events resulted in similar coronal consequences, they are due to two distinct physical processes: eruption of sheared loops and interaction of two loops.

High Spatial Resolution Observations of Pores and the Formation of a Rudimentary Penumbra

We present high spatial resolution observation of small-scale magnetic activity in solar active region NOAA 9539. The observations were obtained on 2001 July 15 using the 65 cm vacuum reflector and 25 cm refractor of the Big Bear Solar Observatory (BBSO). The data sets include time series of speckle reconstructed continuum images at 5200 ?, H? filtergrams (blue line wing, line center, and red line wing), and line-of-sight magnetograms. Two pores, separated by a light bridge, were located in the central part of NOAA 9539. The formation of penumbral filaments near the light bridge indicated a sudden change of the local magnetic field topology from almost vertical to strongly inclined magnetic fields, which allowed cool material previously suspended in a filament to stream downward. During the downward motion of the cool material, H? Dopplergrams revealed twisted streamlines along the filament. Finally, there are several well-defined H? brightenings, Ellerman bombs (EBs), occurred near the region where the downflow of materials fell in. The EBs reside near a magnetic inversion line and are stationary, as opposed to EBs associated with moving magnetic features. We also found that the horizontal flow field of the white-light images derived from local correlation tracking is different from the previous observations. The horizontal movements in the superpenumbrae of leading sunspot and the following sunspots are opposite.

Core and Large-Scale Structure of the 2000 November 24 X-Class Flare and Coronal Mass Ejection

In this paper, we present three important aspects of the X1.8 flare and the associated coronal mass ejection (CME) that occurred on 2000 November 24: (1) The source of the flare is clearly associated with a magnetic channel structure, as was noted in a study by Zirin & Wang , which is due to a combination of flux emergence inside the leading edge of the penumbra of the major leading sunspot and proper motion of the sunspot group. The channel structure provides evidence for twisted flux ropes that can erupt, forming the core of a CME, and may be a common property of several superactive regions that have produced multiple X-class flares in the past. (2) There are actually three flare ribbons visible. The first can be seen moving away from the flare site, while the second and third make up a stationary ribbon near the leader spot. The moving ribbons could be due to a shock associated with the erupting flux rope or due to the interaction of erupting rope and the surrounding magnetic fields. In either case, the ribbon motion does not fit the classical Kopp-Pneuman model, in which the separation of ribbons is due to magnetic reconnection at successively higher and higher coronal altitudes. (3) From the coronal dimming observed with the EUV Imaging Telescope (EIT), the CME involved a much larger region than the initial X-class flare. By comparing high-resolution full-disk Hα and EIT observations, we found that a remote dimming area is cospatial with the enhanced Hα emission. This result is consistent with the recent model of Yokoyama & Shibata that some dimming areas near footpoints may be due to chromospheric evaporation.

The Variation of Relative Magnetic Helicity around Major Flares

We have investigated the variation of magnetic helicity over a span of several days around the times of 11 X-class flares which occurred in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696, and 10720) using the magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). As a major result we found that each of these major flares was preceded by a significant helicity accumulation, (1.8–16) × 1042 Mx2 over a long period (0.5 to a few days). Another finding is that the helicity accumulates at a nearly constant rate, (4.5–48) × 1040 Mx2 hr−1, and then becomes nearly constant before the flares. This led us to distinguish the helicity variation into two phases: a phase of monotonically increasing helicity and the following phase of relatively constant helicity. As expected, the amount of helicity accumulated shows a modest correlation with time-integrated soft X-ray flux during flares. However, the average helicity change rate in the first phase shows even stronger correlation with the time-integrated soft X-ray flux. We discuss the physical implications of this result and the possibility that this characteristic helicity variation pattern can be used as an early warning sign for solar eruptions.