Radio observations of the fine structure inside a post-CME current sheet

A solar radio burst was observed in a coronal mass ejection/flare event by the Solar Broadband Radio Spectrometer at the Huairou Solar Observing Station on2004 December 1. The data exhibited various patterns of plasma motions, suggestive of the interaction between sunward moving plasmoids and the flare loop system during the impulsive phase of the event. In addition to the radio data, the associated whitelight, Hα, extreme ultraviolet light, and soft and hard X-rays were also studied.     

Relationship between CME velocities and X-ray fluxes of associated flares

Coronal mass ejection (CME) velocities have been studied over recent decades. We present a statistical analysis of the relationship between CME velocities and X-ray fluxes of the associated flares. We study two types of CMEs. One is the FL type associ- ated only with flares, while the other is the intermediate type associated with both filament eruptions and flares. It is found that the velocities of the FL type CMEs are strongly cor- related with both the peak and the time-integrated X-ray fluxes of the associated flares. However, the correlations between the intermediate type CME velocities and the corre- sponding two parameters are poor. It is also found that the correlation between the CME velocities and the peak X-ray fluxes is stronger than that between the CME velocities and the time-integrated X-ray fluxes of the associated flares.     

Formation of the CME Leading Edge Observed in the 2003 February 18 Event

This work investigates a typical coronal mass ejection (CME) observed on 2003 February 18, by various space and ground instruments, in white light, Ha, EUV and X-ray. The Ha and EUV images indicate that the CME started with the eruption of a long filament located near the solar northwest limb. The white light coronal images show that the CME initiated with the rarefaction of a region above the solar limb and followed by the formation of a bright arcade at the boundary of the rarefying region at height 0.46 R(?) above the solar surface. The rarefying process synchronized with the slow rising phase of the eruptive filament, and the CME leading edge was observed to form as the latter started to accelerate. The lower part of the filament brightened in Ha as the filament rose to a certain height and parts of the filament was visible in the GOES X-ray images during the rise. These brightenings imply that the filament may be heated by the magnetic reconnection below the filament in the early stage of the eruption. We suggest that a possible mechanism which leads to the formation of the CME leading edge and cavity is the magnetic reconnection which takes place below the filament after the filament has reached a certain height.     

The Filament Eruption of 1999 March 21 and Its Associated Coronal Dimmings and CME

We report a filament eruption near the center of the solar disk on 1999 March 21, in multi-wavelength observations by the Yohkoh Soft X-Ray Telescope (SXT), the Extreme-ultraviolet Images Telescope (EIT) and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). The eruption involved in the disappearance of an Ha filament can be clearly identified in EIT 195 A difference images. Two flare-like EUV ribbons and two obvious coronal dimming regions were formed. The two dimming regions had a similar appearance in lines formed in temperature range 6×104 K to several 106 K. They were located in regions of opposite magnetic polarities near the two ends of the eruptive filament. No significant X-ray or Ha flare was recorded associated with the eruption and no obvious photospheric magnetic activity was detected around the eruptive region, and particularly below the coronal dimming regions. The above surface activities were closely associated with a partial halo-type coronal mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs (LASCO) on the SOHO. In terms of the magnetic flux rope model of CMEs, we explained these multiple observations as an integral process of large-scale rearrangement of coronal magnetic field initiated by the filament eruption, in which the dimming regions marked the evacuated feet of the flux rope.     

The Relationship between Magnetic Gradient and Magnetic Shear in Five Super Active Regions Producing Great Flares

We study the magnetic structure of five well-known active regions that produced great flares (X5 or larger). The six flares under investigation are the X12 flare on 1991 June 9 in AR 6659, the X5.7 flare on 2000 July 14 in AR 9077, the X5.6 flare on 2001 April 6 in AR 9415, the X5.3 flare on 2001 August 25 in AR 9591, the X17 flare on 2003 October 28 and the X10 flare on 2003 October 29, both in AR 10486. The last five events had corresponding LASCO observations and were all associated with Halo CMEs. We analyzed vector magne-tograms from Big Bear Solar Observatory, Huairou Solar Observing Station, Marshall Space Right Center and Mees Solar Observatory. In particular, we studied the magnetic gradient derived from line-of-sight magnetograms and magnetic shear derived from vector magne-tograms, and found an apparent correlation between these two parameters at a level of about 90%. We found that the magnetic gradient could be a better proxy than the shear for predicting where a major flare might occur: all six flares occurred in neutral lines with maximum gradient. The mean gradient of the flaring neutral lines ranges from 0.14 to 0.50 G km-1, 2.3 to 8 times the average value for all the neutral lines in the active regions. If we use magnetic shear as the proxy, the flaring neutral line in at least one, possibly two, of the six events would be mis-identified.     

Numerical simulations of fast and slow coronal mass ejections

Solar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux‐rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force‐free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetic properties of coronal mass ejections corrected for the projection effect in Cycle 23

Using Howard et al.'s method, we investigate, before and after the projection correction, the speed and acceleration distributions for 1747 coronal mass ejections (CMEs) associated solely with flares (FL CMEs) and 631 CMEs associated solely with filament eruptions (FE CMEs) observed by the Large Angle and Spectrometric Coronagraph on board the Solar and Heliographic Observatory ( SOHO /LASCO) from 1996 September to 2007 September, corresponding to almost an entire solar cycle. The results show the following. (1) Before the correction, the speed distributions for FL and FE CMEs are statistically different from each other; after the correction, the speed distributions for FL and FE CMEs should also be statistically different from each other. (2) Before the correction, the acceleration distributions for FL and FE CMEs are statistically different from each other. However, after the correction, FL and FE CMEs should have quite similar acceleration distributions.     

A Parametric Survey of the CME Triggering Process by Numerical Simulations

Observations indicate that solar coronal mass ejections (CMEs) are closely associated with reconnection-favored flux emergence, which was explained in the emerging flux trigger mechanism for CMEs by Chen & Shibata based on numerical simulations. We present a parametric survey of the triggering agent: its polarity orientation, position, and the amount of the unsigned flux. The results suggest that whether a CME can be triggered depends on both the amount and location of the emerging flux, in addition to its polarity orientation. A diagram is presented to show the eruption and non-eruption regimes in the parameter space. The work is aimed at providing useful information for the space weather forecast.     

Relationship between CME dynamics and solar flare plasma

The relationship between the velocity of CMEs and the plasma temperature of the associated X-ray solar flares is investigated.The velocity of CMEs increases with plasma temperature(R=0.82)and photon index below the break energy(R=0.60)of X-ray flares.The heating of the coronal plasma appears to be significant with respect to the kinetics of a CME from the reconnection region where the flare also occurs.We propose that the initiation and velocity of CMEs perhaps depend upon the dominant process of conversion of the magnetic field energy of the active region to heating/accelerating the coronal plasma in the reconnected loops.Results show that a flare and the associated CME are two components of one energy release system,perhaps,magnetic field free energy.     

Speed Distributions of CMEs in Cycle 23 at Low and High Latitudes

We analyzed the speed (v) distributions of 11584 coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment on board the Solar and Heliospheric Observatory (SOHO/LASCO) in cycle 23 from 1996 to 2006. We find that the speed distributions for high-latitude (HL) and low-latitude (LL) CME events are nearly identical and to a good approximation they can be fitted with a lognormal distribution. This finding implies that statistically the same driving mechanism of a nonlinear nature is acting in both HL and LL CME events, and CMEs are intrinsically associated with the source's magnetic structure on large spatial scales. Statistically, the HL CMEs are slightly slower than the LL CMEs. For HL and LL CME events respectively, the speed distributions for accelerating and decelerating events are nearly identical and also to a good approximation they can be both fitted with a lognormal distribution, thus supplementing the results obtained by Yurchyshyn et al.     

Evolution of morphological features of CMEs deduced from catastrophe model of solar eruptions

We describe the evolution of morphological features of the magnetic configuration of CME according to the catastrophe model developed previously. For the parameters chosen for the present work, roughly half of the total mass is nominally contained in the initial flux rope, while the remaining plasma is brought by magnetic reconnection from the corona into the current sheet and from there into the CME bubble. The physical attributes of the difference in the observable features between CME bubble and flare loop system were studied. We tentatively identified distinguishable evolutionary features like the outer shell, the expanding bubble and the flux rope with the leading edge, void and core of the 3-component CME structure. The role of magnetic reconnection is discussed as a possible mechanism for the heating of the prominence material during eruptions. Several aspects of this explanation that need improvement are outlined.     

Downflow-induced brightening following a filament eruption

The plasma from solar filament eruptions sometimes falls down to the lower solar atmosphere. These interesting events can help us to understand the properties of downflows, such as the temperature and the conversion between kinetic energy and thermal energy. We analyze the case of a filament eruption in active region NOAA 11283 and brightening caused by the return of filament material on September 7 and 8, 2011, observed by the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). Magnetic flux cancellation was observed as a result of the eruption after the eruptive filament started to ascend. Another filament near the eruptive filament was disturbed by an extreme ultraviolet (EUV) wave that was triggered by the eruptive filament, causing it to oscillate. Based on coronal seismology, the mean magnetic field strength in the oscillatory filament was estimated to be approximately 18 ± 2 G. Some plasma separated from the filament and fell down to the solar northwest surface after the filament eruption. The velocities of the downflows increased at accelerations lower than the gravitational acceleration. The main characteristic temperature of the downflows was about 5 × 10⁴ K. When the plasma blobs fell down to lower atmospheric heights, the high-speed downward-travelling plasma collided with plasma at lower atmospheric heights, causing the plasma to brighten. The brightening was observed in all 8 AIA channels, demonstrating that the temperature of the plasma in the brightening covered a wide range of values, from 10⁵ K to 10⁷ K. This brightening indicates the conversion between kinetic energy and thermal energy.     

ASO-S卫星工程LST爆发模式触发和终止方案

先进天基太阳天文台(ASO-S)是计划于2021年底或2022年上半年发射的中国首颗综合性太阳探测卫星,莱曼阿尔法太阳望远镜(LST)作为ASO-S的有效载荷之一,具体包括莱曼阿尔法全日面成像仪(SDI)、日冕仪(SCI)以及白光望远镜(WST) 3台科学仪器和2台导行镜(GT),其主要目标是在多个波段对太阳上的两类剧烈爆发现象(太阳耀斑和日冕物质抛射)进行连续不间断的高分辨率观测.为了实现这一观测目标, LST所有仪器的观测模式中均包含了一种针对爆发事件而设置的爆发模式.该模式下, SCI将以更高的频率进行图像采集, SDI和WST则以更高的频率对爆发所在区域进行图像采集.测试结果表明,观测图像经过中值滤波、像元合并处理后,可以通过监测图像各像元亮度的相对变化提取爆发事件的时间和位置信息.这些信息将为LST观测模式间的相互切换提供重要电子学输入.     

Association of CMEs with solar surface activity during the rise and maximum phases of solar cycles 23 and 24

The cyclical behaviors of sunspots,flares and coronal mass ejections(CMEs) for 54 months from 2008 November to 2013 April after the onset of Solar Cycle(SC) 24 are compared,for the first time,with those of SC 23 from 1996 November to 2001 April.The results are summarized below.(i) During the maximum phase,the number of sunspots in SC 24 is significantly smaller than that for SC 23 and the number of flares in SC 24 is comparable to that of SC 23.(ii) The number of CMEs in SC 24 is larger than that in SC 23 and the speed of CMEs in SC 24 is smaller than that of SC 23 during the maximum phase.We individually survey all the CMEs(1647 CMEs) from 2010 June to 2011 June.A total of 161 CMEs associated with solar surface activity events can be identified.About 45%of CMEs are associated with quiescent prominence eruptions,27%of CMEs only with solar flares,19%of CMEs with both active-region prominence eruptions and solar flares,and 9%of CMEs only with active-region prominence eruptions.Comparing the association of the CMEs and their source regions in SC 24 with that in SC 23,we notice that the characteristics of source regions for CMEs during SC 24 may be different from those of SC 23.     

Transequatorial Filament Eruption and Its Link to a Coronal Mass Ejection

We revisit the Bastille Day flare/CME Event of 2000 July 14, and demonstrate that this flare/CME event is not related to only one single active region (AR). Activation and eruption of a huge transequatorial filament are seen to precede the simultaneous filament eruption and flare in the source active region, NOAA AR 9077, and the full halo-CME in the high corona. Evidence of reconfiguration of large-scale magnetic structures related to the event is illustrated by SOHO EIT and Yohkoh SXT observations, as well as, the reconstructed 3D magnetic lines of force based on the force-free assumption. We suggest that the AR filament in AR9077 was connected to the transequatorial filament. The large-scale magnetic composition related to the transequatorial filament and its sheared magnetic arcade appears to be an essential part of the CME parent magnetic structure. Estimations show that the filament-arcade system has enough magnetic helicity to account for the helicity carried by the related CMEs. In addition, rather global magnetic connectivity, covering almost all the visible range in longitude and a huge span in latitude on the Sun, is implied by the Nancay Radioheliograph (NRH) observations. The analysis of the Bastille Day event suggests that although the triggering of a global CME might take place in an AR, a much larger scale magnetic composition seems to be the source of the ejected magnetic flux, helicity and plasma. The Bastille Day event is the first described example in the literature, in which a transequatorial filament activity appears to play a key role in a global CME. Many tens of halo-CME are found to be associated with transequatorial filaments and their magnetic environment.     

Formation of Transient Coronal Holes during the Eruption of a Quiescent Filament and its Overlying Sigmoid

By using Hα, He I 10830, EUV and soft X-ray (SXR) data, we examined a filament eruption that occurred on a quiet-sun region near the center of the solar disk on 2006 January 12, which disturbed a sigmoid overlying the filament channel observed by the GOES-12 SXR Imager (SXI), and led to the eruption of the sigmoid. The event was associated with a partial halo coronal mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs (LASCO) on board the Solar and Heliospheric Observatory (SOHO), and resulted in the formation of two flare-like ribbons, post-eruption coronal loops, and two transient coronal holes (TCHs), but there were no significantly recorded GOES or Hα flares corresponding to the eruption. The two TCHs were dominated by opposite magnetic polarities and were located on the two ends of the eruptive sigmoid. They showed similar locations and shapes in He Ⅰ 10830, EUV and SXR observations. During the early eruption phase, brightenings first appeared on the locations of the two subsequent TCHs, which could be clearly identified on He Ⅰ 10830, EUV and SXR images. This eruption could be explained by the magnetic flux rope model, and the two TCHs were likely to be the feet of the flux rope.