日冕物质抛射

评述了近几年来冕物抛射研究的新进展,包括Ｙｏｈｋｏｈ、ＳＯＨＯ、ＷＩＮＤ、Ｕｌｙｓｓｅｓ、Ｇｅｏｔａｉｌ和ＰＯＬＡＲ等飞船最近取得的观测结果,并由典型事件探计日冕物质抛射与太阳活动、行星际扰动和地球空间环境变化之间的关系。进而介绍日冕物质抛射形成机制和传播过程的主要理论模型和数值模拟结果,提出今后日冕物质抛射研究中一些值得注意的问题。     

Deceleration of Coronal Mass Ejections

Decelerated motion of 12 coronal eruptions is studied. It is found that the measured decelerations and deceleration rates depend on the events' plane-of-sky velocities and heights. The dependence of deceleration on the velocity is described better by a quadratic function then by linear fit. Results are interpreted in terms of a viscous drag. An empirical relation expressing the decrease of the drag effectiveness with the projected height is established. The interplay between the Lorentz force, viscous drag, and gravity is discussed. Several examples are considered to illustrate the relative contributions of these forces under various circumstances.     

日冕物质抛射基本物理参数的统计特征

日冕物质抛射(CME),是太阳大气中尺度最大,最为壮观的太阳活动现象.自1971年12月14日,人类第一次观测到CME以来,CME受到了越来越多的关注,许多空基日冕仪和地面设备对其观测得到了丰富的观测资料.但是,直到现在,CME的基本物理参数研究中还是存在一些不确定性,当然其中也受观测设备局限性的制约.该文综述了近年来CME基本物理参数的统计特征--速度、加速度、角宽度及纬度等--研究的新进展,指出了这些基本物理参数中存在的一些问题,并提出了今后日冕物质抛射研究中要加强的一些重大问题.     

Cyclical Behavior of Coronal Mass Ejections

With the use of coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) from January 1996 through December 2005, it is found that, for the cyclical activity of CMEs, there is surprisingly no equatorward drift at low latitudes (thus, no “butterfly diagram”) and no poleward drift at high latitudes, and no antiphase relationship between CME activity at low and high latitudes. The cyclical behaviors of CMEs differ in a significant way from that of the small-scale solar photospherical and chromospherical phenomena. Thus, our analysis leads to results that are inconsistent with a close, physical relationship with small-scale aspects of solar activity, and it is suggested that there is possibly a single so-called large-scale activity cycle in CMEs.     

Expansion Speed of Coronal Mass Ejections

A large set of limb coronal mass ejections (CMEs) are used to determine the accurate relationship between radial (V _rad) and expansion (V _exp) speeds of CMEs. It is demonstrated that this relation is exceptionally well described by the function f(w)=1/2(1+cot w), representing a full cone model for the CME with a half-width, w. We also demonstrate that for extremely fast CMEs (V _exp>3000 km s⁻¹), it is better to use the approximation V _rad≈V _LE. This implies that such CMEs expand spherically above the solar surface.     

Earth-Affecting Coronal Mass Ejections Without Obvious Low Coronal Signatures

We present a study of the origin of coronal mass ejections (CMEs) that were not accompanied by obvious low coronal signatures (LCSs) and yet were responsible for appreciable disturbances at 1 AU. These CMEs characteristically start slowly. In several examples, extreme ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory reveal coronal dimming and a post-eruption arcade when we make difference images with long enough temporal separations, which are commensurate with the slow initial development of the CME. Data from the EUV imager and COR coronagraphs of the Sun Earth Connection Coronal and Heliospheric Investigation onboard the Solar Terrestrial Relations Observatory, which provide limb views of Earth-bound CMEs, greatly help us limit the time interval in which the CME forms and undergoes initial acceleration. For other CMEs, we find similar dimming, although only with lower confidence as to its link to the CME. It is noted that even these unclear events result in unambiguous flux rope signatures in in situ data at 1 AU. There is a tendency that the CME source regions are located near coronal holes or open field regions. This may have implications for both the initiation of the stealthy CME in the corona and its outcome in the heliosphere.     

Origin of Coronal Shocks without Mass Ejections

We present an analysis of all the events (around 400) of coronal shocks for which the shock-associated metric type IIs were observed by many spectrographs during the period April 1997– December 2000. The main objective of this analysis is to give evidence for the type IIs related to only flare-blast waves, and thus to find out whether there are any type II-associated coronal shocks without mass ejections. By carefully analyzing the data from multi-wavelength observations (Radio, GOES X-ray, Hα, SOHO/LASCO and SOHO/EIT-EUV data), we have identified only 30 events for which there were actually no reports of CMEs. Then from the analysis of the LASCO and EIT running difference images, we found that there are some shocks (nearly 40%, 12/30) which might be associated with weak and narrow mass ejections. These weak and narrow ejections were not reported earlier. For the remaining 60% events (18/30), there are no mass ejections seen in SOHO/LASCO. But all of them are associated with flares and EIT brightenings. Pre-assuming that these type IIs are related to the flares, and from those flare locations of these 18 cases, 16 events are found to occur within the central region of the solar disk (longitude ≤45^∘). In this case, the weak CMEs originating from this region are unlikely to be detected by SOHO/LASCO due to low scattering. The remaining two events occurred beyond this longitudinal limit for which any mass ejections would have been detected if they were present. For both these events, though there are weak eruption features (EIT dimming and loop displacement) in the EIT images, no mass ejection was seen in LASCO for one event, and a CME appeared very late for the other event. While these two cases may imply that the coronal shocks can be produced without any mass ejections, we cannot deny the strong relationship between type IIs and CMEs.     

Distributions of Energy and Mass of Coronal Mass Ejections

The present study investigates the energy and mass distributions of all (11 322) coronal mass ejections (CMEs), 1406 CMEs associated solely with flares (FL CMEs), and 325 CMEs associated solely with filament eruptions (FE CMEs), all of which were observed by the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory (SOHO/LASCO) from January 1996 to December 2009. The results show the following. i) The mean energy of FL CMEs is significantly lower than that of all CMEs. The mean energy of FE CMEs is significantly higher than those of FL CMEs and all CMEs. ii) The mean mass of FL CMEs is slightly larger than that of all CMEs. The mean mass of FE CMEs is significantly larger than those of FL CMEs and all CMEs. Our results suggest that CMEs should shed excess helicity stored in the corona and that the magnetic complexity determines the likelihood of CMEs.     

Waiting Time Distribution of Coronal Mass Ejections

Inspired by the finding that the large waiting time of solar flares presents a power-law distribution, we investigate the waiting time distribution (WTD) of coronal mass ejections (CMEs). SOHO/LASCO CME observations from 1996 to 2003 are used in this study. It is shown that the observed CMEs have a similar power-law behavior to the flares, with an almost identical power-law index. This strongly supports the viewpoint that solar flares and CMEs are different manifestations of the same physical process. We have also investigated separately the WTDs of fast-type and slow-type CMEs and found that their indices are identical, which imply that both types of CME may originate from the same physical mechanism.     

Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter

The Coronal Multichannel Polarimeter (CoMP) measures not only the polarization of coronal emission, but also the full radiance profiles of coronal emission lines. For the first time, CoMP observations provide high-cadence image sequences of the coronal line intensity, Doppler shift, and line width simultaneously over a large field of view. By studying the Doppler shift and line width we may explore more of the physical processes of the initiation and propagation of coronal mass ejections (CMEs). Here we identify a list of CMEs observed by CoMP and present the first results of these observations. Our preliminary analysis shows that CMEs are usually associated with greatly increased Doppler shift and enhanced line width. These new observations provide not only valuable information to constrain CME models and probe various processes during the initial propagation of CMEs in the low corona, but also offer a possible cost-effective and low-risk means of space-weather monitoring.     

Coronal Mass Ejections and Non-recurrent Forbush Decreases

Coronal mass ejections (CMEs) and their interplanetary counterparts (interplanetary coronal mass ejections, ICMEs) are responsible for large solar energetic particle events and severe geomagnetic storms. They can modulate the intensity of Galactic cosmic rays, resulting in non-recurrent Forbush decreases (FDs). We investigate the connection between CME manifestations and FDs. We used specially processed data from the worldwide neutron monitor network to pinpoint the characteristics of the recorded FDs together with CME-related data from the detailed online catalog based upon the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) data. We report on the correlations of the FD magnitude to the CME initial speed, the ICME transit speed, and the maximum solar wind speed. Comparisons between the features of CMEs (mass, width, velocity) and the characteristics of FDs are also discussed. FD features for halo, partial halo, and non-halo CMEs are presented and discussed.     

Post-Eruption Arcades and Interplanetary Coronal Mass Ejections

We compare the temporal and spatial properties of posteruption arcades (PEAs) associated with coronal mass ejections (CMEs) at the Sun that end up as magnetic cloud (MC) and non-MC events in the solar wind. We investigate the length, width, area, tilt angle, and formation time of the PEAs associated with 22 MC and 29 non-MC events and we find no difference between the two populations. According to current ideas on the relation between flares and CMEs, the PEA is formed together with the CME flux-rope structure by magnetic reconnection. Our results indicate that at the Sun flux ropes form during CMEs in association with both MC and non-MC events; however, for non-MC events the flux-rope structure is not observed in the interplanetary space because of the geometry of the observation, i.e. the location of the spacecraft when the structure passes through it.     

Coronal Mass Ejections as Expanding Force-Free Structures

We model solar coronal mass ejections (CMEs) as expanding force-free magnetic structures and find the self-similar dynamics of configurations with spatially constant ??, where J=?? B, in spherical and cylindrical geometries, expanding spheromaks and Lundquist fields, respectively. The field structures remain force-free, under the conventional non-relativistic assumption that the dynamical effects of the inductive electric fields can be neglected. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, caused by inductive electric fields. The structure depends only on overall radius R(t) and rate of expansion $\dot{R}(t)$ measured at a given moment, and thus is applicable to arbitrary expansion laws. In case of cylindrical Lundquist fields, magnetic flux conservation requires that both axial and radial expansion proceed with equal rates. In accordance with observations, the model predicts that the maximum magnetic field is reached before the spacecraft reaches the geometric center of a CME.     

The ARTEMIS Catalog of LASCO Coronal Mass Ejections

The LASCO-C2 coronagraph aboard the SOHO solar observatory has been providing a continuous flow of coronal images since 1996. Synoptic maps for each Carrington rotation have been built from these images, and offer a global view of the temporal evolution of the solar corona, particularly the occurrence of transient events. Coronal Mass Ejections (CMEs) present distinct signatures thus offering a novel approach to the problem of their identification and characterization. We present in this article an automated method of detection based on their morphological appearance on synoptic maps. It is based on adaptive filtering and segmentation, followed by merging with high-level knowledge. The program builds a catalog which lists the CMEs detected for each Carrington Rotation, together with their main estimated parameters: time of appearance, position angle, angular extent, average velocity and intensity. Our final catalog LASCO-ARTEMIS (Automatic Recognition of Transient Events and Marseille Inventory from Synoptic maps) is compared with existing catalogs, CDAW, CACTUS and SEEDS. We find that, likewise the automated CACTUS and SEEDS catalogs, we detect many more events than the CDAW catalog which is based on visual detection. The total number of detected CMEs strongly depends upon the sensitivity to small, faint and numerous events.     

Automatic Detection and Classification of Coronal Mass Ejections

We present an automatic algorithm to detect, characterize, and classify coronal mass ejections (CMEs) in Large Angle Spectrometric Coronagraph (LASCO) C2 and C3 images. The algorithm includes three steps: (1) production running difference images of LASCO C2 and C3; (2) characterization of properties of CMEs such as intensity, height, angular width of span, and speed, and (3) classification of strong, median, and weak CMEs on the basis of CME characterization. In this work, image enhancement, segmentation, and morphological methods are used to detect and characterize CME regions. In addition, Support Vector Machine (SVM) classifiers are incorporated with the CME properties to distinguish strong CMEs from other weak CMEs. The real-time CME detection and classification results are recorded in a database to be available to the public. Comparing the two available CME catalogs, SOHO/LASCO and CACTus CME catalogs, we have achieved accurate and fast detection of strong CMEs and most of weak CMEs.     

Uniformity of Longitudinal Distribution of Coronal Mass Ejections

It was found that the heliographic longitudinal distribution of the occurrence of coronal mass ejections (CMEs) was not uniform around the solar minimum of cycle 21. This is confirmed by using the ² test for goodness of fit. Sunspot groups and flares show similar non-uniform distributions during the same period. This suggests that there was a longitudinal range where both large-scale and small-scale solar magnetic activities were very low around the solar minimum of cycle 21. The longitudinal range with low activity was more than 150 degrees and continued for approximately four rotations. As solar activity increased, CMEs occurred almost uniformly in heliographic longitude.     

Initiation of Coronal Mass Ejections by Sunspot Rotation

We study a filament eruption, two-ribbon flare, and coronal mass ejection (CME) that occurred in NOAA Active Region 10898 on 6 July 2006. The filament was located South of a strong sunspot that dominated the region. In the evolution leading up to the eruption, and for some time after it, a counter-clockwise rotation of the sunspot of about 30 degrees was observed. We suggest that the rotation triggered the eruption by progressively expanding the magnetic field above the filament. To test this scenario, we study the effect of twisting the initially potential field overlying a pre-existing flux-rope, using three-dimensional zero-β MHD simulations. We first consider a relatively simple and symmetric system, and then study a more complex and asymmetric magnetic configuration, whose photospheric-flux distribution and coronal structure are guided by the observations and a potential field extrapolation. In both cases, we find that the twisting leads to the expansion of the overlying field. As a consequence of the progressively reduced magnetic tension, the flux-rope quasi-statically adapts to the changed environmental field, rising slowly. Once the tension is sufficiently reduced, a distinct second phase of evolution occurs where the flux-rope enters an unstable regime characterised by a strong acceleration. Our simulations thus suggest a new mechanism for the triggering of eruptions in the vicinity of rotating sunspots.     

日冕物质抛射的低频射电特征

日冕物质抛射(ChIEs)经常被观测到和其他日面活动相伴生，太阳耀斑、日珥爆发、盔状冕流等许多太阳现象，都与它有直接或间接的关系。射电观测是研究CMEs的一种重要的补充工具。多频率的射电成像观测能很好地研究CMEs的初始阶段，而且可以得到关于CMEs触发机制特征的更多信息。概括了CMEs与低频射电的关系，介绍了低频射电的观测仪器，分析了CMEs低频射电所表现出来的特征，考虑了CMEs的初发机制，总结了尚待解决的问题，表明了CMEs研究是基于多类数据和全电磁辐射波段的。     

Advances in 3D Reconstruction of Coronal Mass Ejections

Coronal mass ejection (CME) is the large scale magnetized plasmoid ejected from the Sun, which brings huge amount of magnetic flux and plasma into interplanetary space. An earthward CME will interact with the magnetosphere of the Earth, and invokes the substorm and the other phenomena of the space weather as it approaches to the Earth. The 2-dimensional data provided by the current observational techniques cannot describe the true magnetic structure and the plasma distribution of CMEs comprehensively. We need to look into the 3-dimensional structure and the associated three components of CME speeds in order to predict the time when an ICME (Interplanetary CME) reaches the Earth, and the potential consequent impact on the Earth and the nearby environment. In this paper, 3D reconstruction methods of CME based on existing imaging observations are introduced, including two kinds of reconstruction methods based on coronagraph data and heliosphere imager data, and CME-driven shock wave 3D reconstruction methods with high correlation with CME imaging reconstruction. Each method shows apparent advantages in dealing with specific events, but its weakness and necessary constrains to its applications exist as well. Results obtained via various methods are compared in this work, and we found that CME velocities and moving directions deduced from these methods are fairly close to one another, which shows high reliability of these methods. Finally, the hot topics related to the 3-dimensional reconstruction of CME (ICME) and the relevant development in reconstructing methods are also discussed.