On-Disc Observations of Flux Rope Formation Prior to Its Eruption

Coronal mass ejections (CMEs) are one of the primary manifestations of solar activity and can drive severe space weather effects. Therefore, it is vital to work towards being able to predict their occurrence. However, many aspects of CME formation and eruption remain unclear, including whether magnetic flux ropes are present before the onset of eruption and the key mechanisms that cause CMEs to occur. In this work, the pre-eruptive coronal configuration of an active region that produced an interplanetary CME with a clear magnetic flux rope structure at 1 AU is studied. A forward-S sigmoid appears in extreme-ultraviolet (EUV) data two hours before the onset of the eruption (SOL2012-06-14), which is interpreted as a signature of a right-handed flux rope that formed prior to the eruption. Flare ribbons and EUV dimmings are used to infer the locations of the flux rope footpoints. These locations, together with observations of the global magnetic flux distribution, indicate that an interaction between newly emerged magnetic flux and pre-existing sunspot field in the days prior to the eruption may have enabled the coronal flux rope to form via tether-cutting-like reconnection. Composition analysis suggests that the flux rope had a coronal plasma composition, supporting our interpretation that the flux rope formed via magnetic reconnection in the corona. Once formed, the flux rope remained stable for two hours before erupting as a CME.     

Comparison of Helicity Signs in Interplanetary CMEs and Their Solar Source Regions

If all coronal mass ejections (CMEs) have flux ropes, then the CMEs should keep their helicity signs from the Sun to the Earth according to the helicity conservation principle. This study presents an attempt to answer the question from the Coordinated Data Analysis Workshop (CDAW), “Do all CMEs have flux ropes?”, by using a qualitative helicity sign comparison between interplanetary CMEs (ICMEs) and their CME source regions. For this, we select 34 CME–ICME pairs whose source active regions (ARs) have continuous SOHO/MDI magnetogram data covering more than 24 hr without data gap during the passage of the ARs near the solar disk center. The helicity signs in the ARs are determined by estimation of cumulative magnetic helicity injected through the photosphere in the entire source ARs. The helicity signs in the ICMEs are estimated by applying the cylinder model developed by Marubashi (Adv. Space. Res., 26, 55, 2000) to 16 second resolution magnetic field data from the MAG instrument onboard the ACE spacecraft. It is found that 30 out of 34 events (88 %) are helicity sign-consistent events, while four events (12 %) are sign-inconsistent. Through a detailed investigation of the source ARs of the four sign-inconsistent events, we find that those events can be explained by the local helicity sign opposite to that of the entire AR helicity (28 July 2000 ICME), incorrectly reported solar source region in the CDAW list (20 May 2005 ICME), or the helicity sign of the pre-existing coronal magnetic field (13 October 2000 and 20 November 2003 ICMEs). We conclude that the helicity signs of the ICMEs are quite consistent with those of the injected helicities in the AR regions from where the CMEs erupted.     

Hα Dimming Associated With the Eruption of a Coronal Sigmoid in the Quiet Sun

We report on the occurrence of Hα dimming associated with a sigmoid eruption in a quiet-sun region on 14 August 2001. The coronal sigmoid in soft X-ray images from the Yohkoh Soft X-ray Telescope was located over an Hα filament channel. Its eruption was accompanied by a flare of GOES X-ray class C2.3 and possibly associated with a halo coronal mass ejection (CME) observed with the Large Angle and Spectroscopic Coronagraphs (LASCO) on board the Solar and Heliospheric Observatory (SOHO). During the eruption, coronal bipolar double dimming took place at the regions with opposite magnetic polarities around the two sigmoid ends, but the underlying chromospheric channel did not show observable changes corresponding to the coronal eruption. Different from the erupting coronal sigmoid itself, however, the coronal dimming had a detectable chromosphere counterpart, i.e., Hα dimming. By regarding the sigmoid as a coronal sign for a flux rope, these observations are explained in the framework of the flux rope model of CMEs. The flux rope is possibly deeply rooted in the chromosphere, and the coronal and Hα dimming regions mark its evacuated feet, through which the material is possibly fed to the halo CME.     

A Major Geoeffective CME from NOAA 12371: Initiation,CME–CME Interactions,and Interplanetary Consequences

In this article, we present a multi-wavelength and multi-instrument investigation of a halo coronal mass ejection (CME) from active region NOAA 12371 on 21 June 2015 that led to a major geomagnetic storm of minimum \(\mathrm{Dst} = -204\) nT. The observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory in the hot EUV channel of 94 Å confirm the CME to be associated with a coronal sigmoid that displayed an intense emission (\(T \sim6\) MK) from its core before the onset of the eruption. Multi-wavelength observations of the source active region suggest tether-cutting reconnection to be the primary triggering mechanism of the flux rope eruption. Interestingly, the flux rope eruption exhibited a two-phase evolution during which the “standard” large-scale flare reconnection process originated two composite M-class flares. The eruption of the flux rope is followed by the coronagraphic observation of a fast, halo CME with linear projected speed of 1366 km?s^?1. The dynamic radio spectrum in the decameter-hectometer frequency range reveals multiple continuum-like enhancements in type II radio emission which imply the interaction of the CME with other preceding slow speed CMEs in the corona within \(\approx10\)?–?\(90~\mbox{R} _{\odot}\). The scenario of CME–CME interaction in the corona and interplanetary medium is further confirmed by the height–time plots of the CMEs occurring during 19?–?21 June. In situ measurements of solar wind magnetic field and plasma parameters at 1 AU exhibit two distinct magnetic clouds, separated by a magnetic hole. Synthesis of near-Sun observations, interplanetary radio emissions, and in situ measurements at 1 AU reveal complex processes of CME–CME interactions right from the source active region to the corona and interplanetary medium that have played a crucial role towards the large enhancement of the geoeffectiveness of the halo CME on 21 June 2015.     

A Numerical Study of the Response of the Coronal Magnetic Field to Flux Emergence

Large-scale solar eruptions, known as coronal mass ejections (CMEs), are regarded as the main drivers of space weather. The exact trigger mechanism of these violent events is still not completely clear; however, the solar magnetic field indisputably plays a crucial role in the onset of CMEs. The strength and morphology of the solar magnetic field are expected to have a decisive effect on CME properties, such as size and speed. This study aims to investigate the evolution of a magnetic configuration when driven by the emergence of new magnetic flux in order to get a better insight into the onset of CMEs and their magnetic structure. The three-dimensional, time-dependent equations for ideal magnetohydrodynamics are numerically solved on a spherical mesh. New flux emergence in a bipolar active region causes destabilisation of the initial stationary structure, finally resulting in an eruption. The initial magnetic topology is suitable for the ??breakout?? CME scenario to work. Although no magnetic flux rope structure is present in the initial condition, highly twisted magnetic field lines are formed during the evolution of the system as a result of internal reconnection due to the interaction of the active region magnetic field with the ambient field. The magnetic energy built up in the system and the final speed of the CME depend on the strength of the overlying magnetic field, the flux emergence rate, and the total amount of emerged flux. The interaction with the global coronal field makes the eruption a large-scale event, involving distant parts of the solar surface.     

The Magnetic Helicity Budget of a cme-Prolific Active Region

Coronal mass ejections (CMEs) are thought to be the way by which the solar corona expels accumulated magnetic helicity which is injected into the corona via several methods. DeVore (2000) suggests that a significant quantity is injected by the action of differential rotation, however Démoulin et al. (2002b), based on the study of a simple bipolar active region, show that this may not be the case. This paper studies the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations. As a result of this changing orientation of the bipole, the helicity injection by differential rotation is not a monotonic function of time. Instead, it experiences a maximum and even a change of sign. In this particular active region, both differential rotation and localized shearing motions are actually depleting the coronal helicity instead of building it. During this period of five solar rotations, a high number of CMEs (35 observed, 65 estimated) erupted from the active region and the helicity carried away has been calculated, assuming that each can be modeled by a twisted flux rope. It is found that the helicity injected by differential rotation (–7×10⁴² Mx²) into the active region cannot provide the amount of helicity ejected via CMEs, which is a factor 5 to 46 larger and of the opposite sign. Instead, it is proposed that the ejected helicity is provided by the twist in the sub-photospheric part of the magnetic flux tube forming the active region.     

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.     

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)     

Relation of CME Speed and Magnetic Helicity in CME Source Regions on the Sun during the Early Phase of Solar Cycles 23 and 24

To investigate the relations between coronal mass ejection (CME) speed and magnetic field properties measured in the photospheric surface of CME source regions, we selected 22 disk CMEs in the rising and early maximum phases of the current Solar Cycle 24. For the CME speed, we used two-dimensional (2D) projected speed observed by the Large Angle and Spectroscopic Coronagraph onboard the Solar and Heliospheric Observatory (SOHO/LASCO), as well as a 3D speed calculated from the triangulation method using multi-point observations. Two magnetic parameters of CME source regions were considered: the average of magnetic helicity injection rate and the total unsigned magnetic flux. We then classified the selected CMEs into two groups, showing: i) a monotonically increasing pattern with one sign of helicity (group A: 16 CMEs) and ii) a pattern of significant helicity injection followed by its sign reversal (group B: 6 CMEs). We found that: 1) 3D speed generally shows better correlations with the magnetic parameters than the 2D speed for 22 CME events in Solar Cycle 24; 2) 2D speed and the magnetic parameters of 22 CME events in this solar cycle have lower values than those of 47 CME events in Solar Cycle 23; 3) all events of group B in Solar Cycle 24 occur only after the beginning of the maximum phase, a trend well consistent with that shown in Solar Cycle 23; 4) the 2D speed and the helicity parameter of group B events continue to increase in the declining phase of Solar Cycle 23, while those of group A events abruptly decrease in the same period. Our results indicate that the two CME groups have a different tendency in the solar cycle variations of CME speed and the helicity parameters. Active regions that show a complex helicity evolution pattern tend to appear in the maximum and declining phases, while active regions with a relatively simple helicity evolution pattern appear throughout the whole solar cycle.     

Two Sympathetic Homologous CMEs on 2002 May 22

Sympathetic coronal mass ejections (CMEs) usually occur in different active regions connected by interconnecting magnetic loops, while homologous CMEs occur within the same active region with an almost the same background magnetic field, and so are similar in shapes. Two sympathetic CMEs erupted within 3 hours on 2002 May 22, originating from the same active region, AR 9948. Their multi-wavelength data were collected and analyzed. It is suggested that emerging flux triggered the occurrence of the first CME and the corresponding flare, the reconnection inflow of which in turn triggered the eruption of the second CME. Based on the fact that the two sympathetic CMEs have many similarities, in their shapes, their low-lying dimming areas, etc., we tentatively propose, for the first time, the phenomenon of sympathetic homologous CMEs.     

The Interplanetary Responses to the Great Solar Activities in Late October 2003

Based on the observations of the Sun and the interplanetary medium, a series of solar activities in late October 2003 and their consequences are studied comprehensively. Thirteen X-ray flares with importance greater than M-class, six frontside halo coronal mass ejections (CMEs) with span angle larger than 100 and three associated eruptions of filament materials are identified by examining lots of solar observations from October 26 to 29. All these flares were associated with type III radio bursts, all the frontside halo CMEs were accompanied by type II or type II-like radio bursts. Particularly, among these activities, two major solar events caused two extraordinary enhancements (exceeding 1000 particles/(cm²s^–1sterMev^–1) of solar energetic particle (SEP) flux intensity near the Earth, two large ejecta with fast shocks preceding, and two great geomagnetic storms with Dst peak value of –363 and –401 nT, respectively. By using a cross correlation technique and a force-free cylindrical flux rope model, the October 29 magnetic cloud associated with the largest CME are analyzed, including its orientation and the sign of its helicity. It is found that the helicity of the cloud is negative, contrary to the regular statistical pattern that negative- and positive-helical interplanetary magnetic clouds would be expected to come from northern and southern solar hemisphere. Moreover, the relationship between the orientation of magnetic cloud and associated filament is discussed. In addition, some discussion concerning multiple-magnetic-cloud structures and SEP events is also given.     

The Role of Interchange Reconnection in Facilitating a Filament Eruption

We study the interaction between an erupting solar filament and a nearby coronal hole, based on multi-viewpoint observations from the Solar Dynamics Observatory and STEREO. During the early evolution of the filament eruption, it exhibits a clockwise rotation that brings its easternmost leg in contact with the oppositely aligned field at the coronal hole boundary. The interaction between the two magnetic-field systems is manifested as the development of a narrow contact layer in which we see enhanced EUV brightening and bi-directional flows, suggesting that the contact layer is a region of strong and ongoing magnetic reconnection. The coronal mass ejection (CME) resulting from this eruption is highly asymmetric, with its southern portion opening up to the upper corona, while the northern portion remains closed and connected to the Sun. We suggest that the erupting flux rope that made up the filament reconnected with both the open and closed fields at the coronal hole boundary via interchange reconnection and closed-field disconnection, respectively, which led to the observed CME configuration.     

The Global Context of Solar Activity During the Whole Heliosphere Interval Campaign

The Whole Heliosphere Interval (WHI) was an international observing and modeling effort to characterize the 3-D interconnected ??heliophysical?? system during this solar minimum, centered on Carrington Rotation 2068, March 20??C?April 16, 2008. During the latter half of the WHI period, the Sun presented a sunspot-free, deep solar minimum type face. But during the first half of CR 2068 three solar active regions flanked by two opposite-polarity, low-latitude coronal holes were present. These departures from the quiet Sun led to both eruptive activity and solar wind structure. Most of the eruptive activity, i.e., flares, filament eruptions and coronal mass ejections (CMEs), occurred during this first, active half of the interval. We determined the source locations of the CMEs and the type of associated region, such as active region, or quiet sun or active region prominence. To analyze the evolution of the events in the context of the global solar magnetic field and its evolution during the three rotations centered on CR 2068, we plotted the CME source locations onto synoptic maps of the photospheric magnetic field, of the magnetic and chromospheric structure, of the white light corona, and of helioseismological subsurface flows. Most of the CME sources were associated with the three dominant active regions on CR 2068, particularly AR 10989. Most of the other sources on all three CRs appear to have been associated with either isolated filaments or filaments in the north polar crown filament channel. Although calculations of the flux balance and helicity of the surface magnetic features did not clearly identify a dominance of one region over the others, helioseismological subsurface flows beneath these active regions did reveal a pronounced difference among them. These preliminary results suggest that the ??twistedness?? (i.e., vorticity and helicity) of subsurface flows and its temporal variation might be related to the CME productivity of active regions, similar to the relationship between flares and subsurface flows.     

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. I. Unusual History of an Eruptive Filament

This is the first of four companion papers, which comprehensively analyze a complex eruptive event of 18 November 2003 in active region (AR) 10501 and the causes of the largest Solar Cycle 23 geomagnetic storm on 20 November 2003. Analysis of a complete data set, not considered before, reveals a chain of eruptions to which hard X-ray and microwave bursts responded. A filament in AR 10501 was not a passive part of a larger flux rope, as usually considered. The filament erupted and gave origin to a coronal mass ejection (CME). The chain of events was as follows: i) a presumable eruption at 07:29 UT accompanied by a not reported M1.2 class flare probably associated with the onset of a first southeastern CME (CME1), which most likely is not responsible for the superstorm; ii) a confined eruption (without a CME) at 07:41 UT (M3.2 flare) that destabilized the large filament; iii) the filament acceleration around 07:56 UT; iv) the bifurcation of the eruptive filament that transformed into a large “cloud”; v) an M3.9 flare in AR 10501 associated to this transformation. The transformation of the filament could be due to the interaction of the eruptive filament with the magnetic field in the neighborhood of a null point, located at a height of about 100 Mm above the complex formed by ARs 10501, 10503, and their environment. The CORONAS-F/SPIRIT telescope observed the cloud in 304 Å as a large Y-shaped darkening, which moved from the bifurcation region across the solar disk to the limb. The masses and kinematics of the cloud and the filament were similar. Remnants of the filament were not clearly observed in the second southwestern CME (CME2), previously regarded as a source of the 20 November geomagnetic storm. These facts do not support a simple scenario, in which the interplanetary magnetic cloud is considered as a flux rope formed from a structure initially associated with the pre-eruption filament in AR 10501. Observations suggest a possible additional eruption above the bifurcation region close to solar disk center between 08:07 and 08:17 UT, which could be the source of the 20 November superstorm.     

A Filament—Associated Halo Coronal Mass Ejection

1 INTRODUCTIONCoronal majss ejections (CMEs) are often seen as spectacular eruptions of matter fromthe Sun which propagate outward through the heliosphere and often interact with the Earth'smagnetosphere (Hundhausen, 1997; Gosling, 1997; and references herein). It is well known thatthese interactions can have substalltial consequences on the geomagnetic environment of theEarth, sometimes resulting in damage to satellites (e.g., McAllister et al., 1996; Berdichevskyet al., 1998). CMEs…     

Magnetic source regions of coronal mass ejections

The majority of flare activity arises in active regions which contain sunspots, while Coronal Mass Ejection (CME) activity can also originate from decaying active regions and even so-called quiet solar regions which contain a filament. Two classes of CME, namely flare-related CME events and CMEs associated with filament eruption are well reflected in the evolution of active regions. The presence of significant magnetic stresses in the source region is a necessary condition for CME. In young active regions magnetic stresses are increased mainly by twisted magnetic flux emergence and the resulting magnetic footpoint motions. In old, decayed active regions twist can be redistributed through cancellation events. All the CMEs are, nevertheless, caused by loss of equilibrium of the magnetic structure. With observational examples we show that the association of CME, flare and filament eruption depends on the characteristics of the source regions:

?the strength of the magnetic field, the amount of possible free energy storage,

?the small- and large-scale magnetic topology of the source region as well as its evolution (new flux emergence, photospheric motions, cancelling flux), and

?the mass loading of the configuration (effect of gravity). These examples are discussed in the framework of theoretical models.

     

Simultaneous SOHO and Ground-Based Observations of a Large Eruptive Prominence and Coronal Mass Ejection

Coronal mass ejections (CMEs) are frequently associated with erupting prominences near the solar surface. A spectacular eruption of the southern polar crown prominence was observed on 2 June 1998, accompanied by a CME that was well-observed by the LASCO coronagraphs on SOHO. The prominence was observed in its quiescent state and was followed throughout its eruption by the SOHO EIT and later by LASCO as the bright, twisted core of the CME. Ground-based H observations of the prominence were obtained at the Ondejov Observatory in the Czech Republic. A great deal of fine structure was observed within the prominence as it erupted. The prominence motion was found to rotate about its axis as it moved outward. The CME contained a helical structure that is consistent with the ejection of a magnetic flux rope from the Sun. Similar structures have been observed by LASCO in many other CMEs. The relationship of the flux rope to other structures in the CME is often not clear. In this event, the prominence clearly lies near the trailing edge of the structure identified as a flux rope. This structure can be observed from the onset of the CME in the low corona all the way out to the edge of the LASCO field of view. The initiation and evolution of the CME are modeled using a fully self-consistent, 3D axisymmetric, MHD code.     

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.     

How Can a Negative Magnetic Helicity Active Region Generate a Positive Helicity Magnetic Cloud?

The geoeffective magnetic cloud (MC) of 20 November 2003 was associated with the 18 November 2003 solar active events in previous studies. In some of these, it was estimated that the magnetic helicity carried by the MC had a positive sign, as did its solar source, active region (AR) NOAA 10501. In this article we show that the large-scale magnetic field of AR 10501 has a negative helicity sign. Since coronal mass ejections (CMEs) are one of the means by which the Sun ejects magnetic helicity excess into interplanetary space, the signs of magnetic helicity in the AR and MC must agree. Therefore, this finding contradicts what is expected from magnetic helicity conservation. However, using, for the first time, correct helicity density maps to determine the spatial distribution of magnetic helicity injections, we show the existence of a localized flux of positive helicity in the southern part of AR 10501. We conclude that positive helicity was ejected from this portion of the AR leading to the observed positive helicity MC.