Solar large-scale emitting chains: some CME-associated events

Transient large-scale emitting chains and threads, associated with several coronal mass ejections (CMEs), are analyzed by the SOHO/EIT, TRACE, Yohkoh/SXT, Nobeyama Radioheliograph, and some other imaging data. It is illustrated that a pronounced evolution of the chains and threads in the EUV, soft X-ray, microwave, and other ranges can occur many hours both before and after a CME on a considerable part of the solar visible disk, especially near the place of a CME eruption. Such relations between chains and CMEs seem to be plausible due to both phenomena being the consequences of the evolution of large-scale magnetic fields and have often a global character.     

The magnetic topology of a sigmoid

Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.     

Early life of coronal mass ejections

Coronal mass ejections (CMEs) are large-scale magnetized plasma structures ejected from closed magnetic field regions of the Sun. White light coronagraphic observations from ground and space have provided extensive information on CMEs in the outer corona. However, our understanding of the solar origin and early life of CMEs is still in an elementary stage because of lack of adequate observations. Recent space missions such as Yohkoh and Solar and Heliospheric Observatory (SOHO) and ground-based radioheliographs at Nobeyama and Nancay have accumulated a wealth of information on the manifestations of CMEs near the solar surface. We review some of these observations in an attempt to relate them to what we already know about CMEs. Our discussion relies heavily on non-coronagraphic data combined with coronagraphic data. Specifically, we discuss the following aspects of CMEs: (i) coronal dimming and global disk signatures, (ii) non-radial propagation during the early phase, (iii) Photospheric magnetic field changes during CMEs, and (iv) acceleration of fast CMEs. The relative positions and evolution of coronal dimming, arcade formation, prominence eruption will be discussed using specific events. The magnitude and spatial extent of CME acceleration may be an important parameter that distinguishes fast and slow CMEs.     

The relationship between prominence eruptions and coronal mass ejections

The SOHO observations with LASCO and EIT present an ideal opportunity to study the relationship between prominence eruptions and coronal mass ejections (CME). High-cadence measurements of prominence eruptions demonstrate that the prominence eruption is not generally the cause of the associated CME, but that it is more probable that the destabilisation of the CME in fact releases the constraints on the prominence, causing it to erupt. We report here selected observations of associated CMEs and prominence eruptions covering the period of SOHO operations from mid-January 1996 to October 1999. In addition to the causality, we find that in general the projected speed of the prominence eruption matches fairly closely the projected speed of the associated CME, but it is always lower. Furthermore, the prominence eruption is generally simply one facet of the coronal transient activity, of which there are often several other discrete parts. The prominence eruption is also generally offset in heliolatitude from the centre of the CME.     

Flares,ejections, proton events

Statistical analysis is performed for the relationship of coronal mass ejections (CMEs) and X-ray flares with the fluxes of solar protons with energies >10 and >100 MeV observed near the Earth. The basis for this analysis was the events that took place in 1976–2015, for which there are reliable observations of X-ray flares on GOES satellites and CME observations with SOHO/LASCO coronagraphs. A fairly good correlation has been revealed between the magnitude of proton enhancements and the power and duration of flares, as well as the initial CME speed. The statistics do not give a clear advantage either to CMEs or the flares concerning their relation with proton events, but the characteristics of the flares and ejections complement each other well and are reasonable to use together in the forecast models. Numerical dependences are obtained that allow estimation of the proton fluxes to the Earth expected from solar observations; possibilities for improving the model are discussed.     

Origin and structures of solar eruptions I: Magnetic flux rope

Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1–3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.     

Changes in the solar magnetic field preceding a coronal mass ejection

The combined observing power of the Yohkoh, SOHO and TRACE spacecraft, along with the continuing ground-based observations has proved invaluable for the detection of changes in the magnetic morphology preceding coronal mass ejections (CMEs). A wide range of activity from small scale dimmings to large scale eruptions covering half the solar disk have been observed. The relationship between flares and CMEs has also become clearer. Rather than one event causing the other it would seem that it is a global change in the magnetic field which causes both. Recently, there has been a lot of interest in the sigmoid (S-shaped) structures seen in soft X-rays. The likelihood of a CME occurring appears to increase if there is a sigmoidal structure observed. This has formed the basis of more extensive studies into predicting the time and location of a CME from the changes in behaviour of features on the solar disk.     

Method for determining the parameters of full halo coronal mass ejections

A method for determining the parameters of halo-type coronal mass ejections (full halo CMEs)—direction of motion, angular size, CME velocity along the Sun-Earth axis, etc.—has been proposed and tested. The method is based on the found empirical dependence between the angular sizes of CMEs located near the sky plane and angular sizes of associated eruptive prominences or post-eruptive arcades as well as on the relationships between the halo CME parameters derived in a simple geometrical CME model. Using this method and the SOHO/LASCO C3 and SOHO/EIT data, the parameters of 33 full halo CMEs have been determined. It is concluded that (1) the trajectories of all considered full halo CMEs deviate with recession of the CME front to R _F > (2–5)R ₀ toward the Sun-Earth axis; (2) the majority of full halo CMEs recorded by LASCO C3 coronagraphs have relatively large angular sizes, 2α > 60°.     

日冕物质抛射—空间天气的扰动源

日冕物质抛射是引起空间天气扰动的重要起因＿本文对日冕物质抛射的一般参量和形态、它与其它太阳活动现象的关系、它在行星际空间的表现以及它导致的地球空间环境扰动的研究进展作了介绍和讨论     

基于多视角观测的SEP事件与twin-CME关系研究

本文联合SOHO和STEREO-A/B（三视角）日冕观测和太阳高能粒子（SEP）观测,分析了2007—2014年间169个快速（速度>900 km·s^-1）、宽角度（>60°）日冕物质抛射（CME）及其先行CME和关联SEP事件.通过相关分析,给出了SOHO/EPHIN 25~53MeV及STEREO/HET 23.8~60 MeV能量范围的大SEP事件通量判断阈值,分别为0.01和0.014（cm²·s·sr·MeV）^-1.三视角CME观测能有效地避免投影效应产生的twin-CME事件误判,统计得到单一视角确定twin-CME事件的误判率一般低于10%,最高不超过15%.基于三视角判断的twin-CME事件及SEP事件峰值强度,得到判断twin-CME事件的时间阈值最短约为9 h（9~13 h）.single-CME产生的SEP事件强度与CME速度、动能的相关性明显高于twin-CME,并且三视角下的相关性结果与单视角类似.结果表明,一个主CME可能存在多个先行CME,依据单卫星观测判断先行CME时有一定的误判几率,但少数单个先行CME的误判并不影响基于单卫星的统计规律或统计结果.

     

Solar Flares in Extreme Ultraviolet and X-Ray Bands Based on SDO and TIMED Satellite Data

Geomagnetism and Aeronomy - Fluxes in flares are studied in soft X-ray (SXR) and extreme ultraviolet (EUV) bands based on data from GOES, the Solar Dynamics Observatory (SDO), and the Thermosphere,...     

对自相似扩展(SSE)模型的改进和研究

自相似扩展拟合法(Self-Similar Expansion,SSE)假设日冕物质抛射(CME)具有恒定角宽度、沿径向向外传播的、自相似扩展的球形前沿,由日心出发的、与这个球形前沿相切的圆锥的圆锥角就是通常所说的CME角宽度,半角宽度取值范围是[0°,90°],固定Φ角拟合法(Fixed-Φ,FΦ)和调和均值拟合法(Harmonic-mean,HM)分别对应SSE模型的半角宽度为0°和90°的特殊情况.本文中修改后的自相似扩展拟合法(MSSE)假设CME具有自相似扩展的半球形前沿,能够提取的CME参数包括由日心出发的、圆锥截面过球心的圆锥的半圆锥角和CME的主传播方向、传播速度,其中半圆锥角取值范围是[0°,90°],FΦ和HM分别对应MSSE模型的半圆锥角为0°和45°的特殊情况,半圆锥角为90°时,CME前沿是以日心为圆心的半圆.MSSE拟合法扩大了SSE模型对CME前沿形态的描述范围,将半圆锥角作为判断CME是否能够到达、何时到达某颗卫星的重要参数.基于单颗STEREO卫星日球层成像仪(Heliospheric Imager,HI)图像,结合STEREO和ACE卫星的太阳风实地观测数据,本文深入分析了2010年23个CME事件,结果发现:在用于预报CME事件是否能到达某颗卫星、何时到达时,MSSE拟合法相比FΦ、HM和SSE拟合法,不但可以更准确地拟合CME主传播方向和传播速度,也可以缩小预计到达时间和到达速度的误差.     

结合实地观测和STEREO/HI图像观测分析2010年CME事件

本文使用了基于单颗STEREO卫星日球层成像仪(Heliospheric Imager,HI)图像的固定Φ角拟合法(Fixed-Φ,FΦ)和调和均值拟合法(Harmonic-mean,HM),结合STEREO和ACE卫星的太阳风实地观测数据,深入分析了2010年15个日冕物质抛射(CME)事件,对比讨论了这两种方法在提取CME参数如太阳赤道平面的主传播方向、传播速度的效果,其中FΦ拟合法假设CME是固定方向传播的小质点,HM拟合法假设CME为具有球形前沿的通量绳结构,结果发现:(1)使用HM拟合法分析得到的CME主传播方向与太阳-实地观测点的夹角平均值是9.5°,小于FΦ拟合法的19.7°;(2)HM拟合法分析的预计到达时间与实测ICME起始时间的平均误差和最大误差分别为0.282天和0.805天,明显小于FΦ拟合法.本文也使用结合STEREO两颗卫星HI图像的直接三角法(Direct-triangulation,DT)和球面切线法(Tangent-to-a-sphere,TS),深入分析了5个朝向地球的CME事件,其中,DT和FΦ拟合法的假设相同,TS和HM拟合法的假设相同,结果发现:(1)这两种方法分析的CME主传播方向与日地连线的夹角最大值分别是13.2°和21.1°,明显小于单颗卫星观测的20.7°和27.5°;(2)其中4个CME事件使用方法得到的线性拟合加速度不超过0.4 m·s-2,这说明CME在主传播方向上的速度变化在1AU内不超过100 km·s-1;(3)使用TS方法得到的预计到达时间与实测ICME起始时间的绝对误差最小,平均值和最大值分别是2.3 h和5.8 h.可见,利用HI图像提取CME传播参数时,加入CME前沿结构假设和结合多角度观测都能够有效地减小拟合误差.     

GOES satellite and field observations of the 1998 eruption of Volcan Cerro Azul, Galápagos Islands

 The 1998 eruption of Volcan Cerro Azul, Isla Isabela, Galápagos Islands, was observed in near real-time by the Geostationary Operational Environmental Satellite-8 (GOES-8) weather satellite. Due to the remote location of the eruption site, 3.9-μm radiance values derived from GOES band 2 provide the best timing of the start and termination of the eruption, which occurred on 15 Sept. and 21 Oct., respectively. Throughout the 36-day long eruption, a total of 1335 thermal infrared images were collected, of which 851 were cloud-free and permitted the thermal anomaly to be detected. A detailed chronology including 77 separate events was assembled from the GOES data and field observations. Numerous attributes of the eruption were observed from the GOES data, including the sizes and dispersal of seven eruption plumes and the occurrence and timing of intra-caldera effusive activity. The growth of a lava flow on the SE flank, the formation of smoke and volcanic haze from the flank vent, and burning of vegetation caused by lava flows entering vegetated areas were monitored both on the ground and with the satellite data. In most cases GOES images were processed as they were received every 30 min and were then distributed over the Internet within minutes of reception. These data provided timely high-temporal information to field parties as well as enabled the documentation of the eruption. The GOES observations of Cerro Azul serve as a further example of the way in which the remote sensing community and field volcanologists can collaborate during future eruptions, and permit the temporal and spatial resolution requirements for future satellites systems to be better defined. Received: 27 April 1999 / Accepted: 21 February 2000     

Anatomy of 1986 Augustine volcano eruptions as recorded by multispectral image processing of digital AVHRR weather satellite data

Eighteen digital AVHRR (advanced very high resolution radiometer) data sets from NOAA-6 and NOAA-9 polar-orbiting satellites recorded between 27 March and 7 April 1986 depict the eruptive activity of Augustine volcano, located 280 km SW of Anchorage, Alaska. The synoptic view (resolution of either 1.1 or 4.4 km), frequent coverage (often twice a day), and multispectral coverage (five bands: 0.58–0.68; 0.72–1.1; 3.55–3.93; 10.5–11.3; and 11.5–12.5 m) makes the AVHRR broadly applicable to analyzing explosive eruption clouds. The small scale of the Augustine activity (column heights of 2–13 km and eruption rates of 2x10⁶–8x10⁷ metric tonnes/day) facilitated intensive multispectral study because the plumes generally covered areas within the 550x550 km area of one easily manipulated image field. Hourly ground weather data and twice-daily radiosonde measurements from stations surrounding the volcano plus numerous volcanological observations were made throughout the eruption, providing important ground truth with which to calibrate the satellite data. The total erupted volume is estimated to be at least 0.102 km³. The pattern of changing eruption rates determined by satellite observations generally correlate with more detailed estimates of explosion magnitudes. Multispectral processing techniques were used to distinguish eruption clouds from meteorological clouds. Variable weather during the Augustine eruption offered an opportunity to test various trial algorithms. A ratio between thermal IR channels four and five, served to delineate the ashbearing eruption plumes from ordinary clouds. Future work is needed to determine whether the successful multispectral discrimination is caused by wavelength-dependent variable emission of silicate ash or reflects a spectral role of sulfuric acid aerosol in the plume.     

The Dicke cycle: A ∼27-day solar oscillation

Measurements of solar EUV irradiance show, besides the ～11-year Schwabe cycle, a series of oscillations with a ～27-day period. They are generally explained by the passage of active regions across the solar surface resulting from the Sun's rotation, but the calculated amplitude underestimates the observed long-term variation in irradiance (Lean 1991). The variant of this model proposed here is modulation of EUV emission from the corona by rotation of the Sun's radiative zone. The response would be immediate, raising the prospect of short-term forecasting of EUV effects on space weather and on the Earth's atmospheric circulation.     

冕洞阻碍太阳高能粒子事件形成的典型事例分析

通过对比两次快速晕状日冕物质抛射(CME)事件,分析相应的日面和行星际的观测资料,发现源区距离冕洞较远的CME引起了极强的太阳高能粒子(Solar Energetic Particle,SEP)事件,而源区非常靠近冕洞的CME则没有引起大的SEP事件.该结果表明,冕洞可能对CME形成SEP事件有阻碍作用.继而分析1997～2003年所有爆发在冕洞边缘的快速晕状CME,发现源区离冕洞距离小于02Rs(太阳半径)的CME均没有引起大的SEP事件.从而进一步证实了冕洞可能对邻近CME形成大SEP事件有影响,它阻碍SEP事件的形成.最后讨论了冕洞阻碍CME形成大SEP事件的可能原因.     

太阳X-EUV成像望远镜

太阳X_EUV成像望远镜用来监测和预报影响空间天气变化的太阳活动,专门服务于空间天气预报研究. 望远镜工作在4～100?的X射线波段和195?极紫外谱段,视场角45′,角度分辨5″,提供全日面、高分辨的成像观测. 文中分析了太阳X、EUV波段的成像观测应用,介绍了X_EUV望远镜的基本设计,分析了望远镜对不同温度日冕等离子体的敏感性、对不同太阳活动现象的响应及反演日冕等离子体参数过滤片的组合利用. 太阳X_EUV成像望远镜集成了掠入射望远镜和正入射望远镜两套系统,扩展了单一X射线望远镜的成像功能,能够观测更多的太阳爆发先兆现象或者伴生现象,是目前国际上同类仪器中最新的太阳成像监测仪器.     

Regularities in the Formation of Coronal Mass Ejections Associated and not Associated with Type II Radio Bursts

Geomagnetism and Aeronomy - Regularities in the formation of coronal mass ejections (CMEs) associated (Radio Loud, RL CME) and not associated (Radio Quiet, RQ CME) with type II radio bursts (RBIIs)...