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.     

First 4D Reconstruction of an Eruptive Prominence Using Three Simultaneous View Directions

Data from the STEREO (Solar Terrestrial Relations Observatory) mission are intensively used for 3D reconstruction of solar coronal structures. After the launch of the SDO (Solar Dynamic Observatory) satellite, its additional observations give the possibility to have a third eye for more accurate 3D reconstruction in the very low corona (<?1.5?R _??). With our reconstruction code MBSR (Multi-view B-spline Stereoscopic Reconstruction), we use three view directions (STEREO A, B, and SDO) to perform the 3D reconstruction and evolution of a prominence which triggered a CME on 1 August 2010. In the paper we present the reconstruction of this prominence from the moment it starts to erupt until it leaves the field of view of the coronagraph. We also determine the evolution of the leading edge of the CME. Based on the temporal evolution, we analyze some of its properties, such as velocity, acceleration, opening and rotation angles and evolution of the cavity.     

Comparisons of CME Morphological Characteristics Derived from Five 3D Reconstruction Methods

We compare different methods to reconstruct three-dimensional (3D) coronal mass ejection (CME) morphology. The explored methods include geometric localisation, mask fitting, forward modelling, polarisation ratio, and local correlation tracking plus triangulation. These five methods are applied to the same CME event that occurred on 7 August 2010. Their corresponding results are presented and compared, especially in their propagation direction and spatial extent in 3D. We find that the mask fitting and geometric localisation methods produce consistent results. Reconstructions including three-view observations are more precise than reconstructions done with only two views. Compared to the forward modelling method, in which an a priori shape of the CME geometry is assumed, the mask fitting has more flexibility. The polarisation ratio method makes use of the Thomson scattering geometry. We find that spatially the 3D CME derived from the mask fitting lies mostly in the overlap region obtained with the polarisation method using data from STEREO. In addition, the mask fitting can help resolve the front/back ambiguity inherent in the polarisation ratio method. However, the local correlation tracking plus triangulation did not show consistent results with the other four methods. This method performed poorly, primarily because the two STEREO spacecraft had a large angular separation. Under these circumstances, it is difficult to identify points taken from independent images that correspond to the same physical feature. Excluding the local correlation tracking method, the latitude of the CME??s centre of gravity derived from the other methods deviates within 1^°, and the longitude differs within 19^°.     

How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs

We intend to provide a comprehensive answer to the question on whether all Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we present a synthesis of the LASCO CME observations over the last 16 years, assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic observations from STEREO and SDO, and statistics from a revised LASCO CME database. We argue that the bright loop often seen as the CME leading edge is the result of pileup at the boundary of the erupting flux rope irrespective of whether a cavity or, more generally, a three-part CME can be identified. Based on our previous work on white light shock detection and supported by the MHD simulations, we identify a new type of morphology, the ‘two-front’ morphology. It consists of a faint front followed by diffuse emission and the bright loop-like CME leading edge. We show that the faint front is caused by density compression at a wave (or possibly shock) front driven by the CME. We also present highly detailed multi-wavelength EUV observations that clarify the relative positioning of the prominence at the bottom of a coronal cavity with a clear flux rope structure. Finally, we visually check the full LASCO CME database for flux rope structures. In the process, we classify the events into two clear flux rope classes (‘three-part’, and ‘Loop’), jets and outflows (no clear structure). We find that at least 40 % of the observed CMEs have clear flux rope structures and that ～?29 % of the database entries are either misidentifications or inadequately measured and should be discarded from statistical analyses. We propose a new definition for flux rope CMEs (FR-CMEs) as a coherent magnetic, twist-carrying coronal structure with angular width of at least 40^° and able to reach beyond 10 R_⊙ which erupts on a time scale of a few minutes to several hours. We conclude that flux ropes are a common occurrence in CMEs and pose a challenge for future studies to identify CMEs that are clearly not FR-CMEs.     

Reconstructing the 3D Morphology of the 17 May 2008 CME

We model the kinematics and three-dimensional distribution of mass in a coronal mass ejection (CME) observed on 17 May 2008, using a comprehensive analysis of STEREO images of the CME. The CME is a surprisingly fast one for solar minimum, reaching velocities of up to 1120 km?s^?1. It can be followed continuously from inception all the way out to 1 AU. We find that the appearance of the CME can be modeled reasonably well as a combination of two distinct fronts that expand outward in a self-similar fashion. The model implies that STEREO-B is struck by the weaker of these two fronts on 19 May, and the in situ instruments on STEREO-B do see a weak density and magnetic field enhancement at the expected time.     

Solar Wind Electron Strahls Associated with a High-Latitude CME: Ulysses Observations

Counterstreaming beams of electrons are ubiquitous in coronal mass ejections (CMEs) – although their existence is not unanimously accepted as a necessary and/or sufficient signature of these events. We continue the investigation of a high-latitude CME registered by the Ulysses spacecraft on 18?–?19 January 2002 (Dumitrache, Popescu, and Oncica, Solar Phys. 272, 137, 2011), by surveying the solar-wind electron distributions associated with this event. The temporal evolution of the pitch-angle distributions reveals populations of electrons that are distinguishable through their anisotropy, with clear signatures of i) electron strahls, ii) counter-streaming in the magnetic clouds and their precursors, and iii) unidirectionality in the fast wind preceding the CME. The analysis of the counter-streams inside the CME allows us to elucidate the complexity of the magnetic-cloud structures embedded in the CME and to refine the borders of the event. Identifying such strahls in CMEs, which preserve properties of the low β [<1] coronal plasma, gives more support to the hypothesis that these populations are remnants of the hot coronal electrons that escape from the electrostatic potential of the Sun into the heliosphere.     

Relation Between the 3D-Geometry of the Coronal Wave and Associated CME During the 26 April 2008 Event

We study the kinematical characteristics and 3D geometry of a large-scale coronal wave that occurred in association with the 26 April 2008 flare-CME event. The wave was observed with the EUVI instruments aboard both STEREO spacecraft (STEREO-A and STEREO-B) with a mean speed of ∼ 240 km s⁻¹. The wave is more pronounced in the eastern propagation direction, and is thus, better observable in STEREO-B images. From STEREO-B observations we derive two separate initiation centers for the wave, and their locations fit with the coronal dimming regions. Assuming a simple geometry of the wave we reconstruct its 3D nature from combined STEREO-A and STEREO-B observations. We find that the wave structure is asymmetric with an inclination toward East. The associated CME has a deprojected speed of ∼ 750±50 km s⁻¹, and it shows a non-radial outward motion toward the East with respect to the underlying source region location. Applying the forward fitting model developed by Thernisien, Howard, and Vourlidas (Astrophys. J. 652, 763, 2006), we derive the CME flux rope position on the solar surface to be close to the dimming regions. We conclude that the expanding flanks of the CME most likely drive and shape the coronal wave.     

Observations and Modeling of an Explosive Coronal Mass Ejection as Observed by LASCO

We present a qualitative and quantitative comparison of a single coronal mass ejection (CME) as observed by LASCO on 5 October 1996 with the results of a two-dimensional magnetohydrodynamic (MHD) model. This event was selected as a clear example of a CME that is not caused by the disruption of a helmet streamer. This CME occurs against the background of multiple bright streamers on the west limb. The CME is first seen as a brightening of the entire west limb. The CME has a bright, sharp front that moves outward with no significant change in shape. The CME moves outward with roughly constant velocity that is approximately twice as fast at high latitude as near the streamer. The measured CME mass is 1.2×10¹⁶ g. There are two parts to the MHD model. The pre-event corona was calculated using a 2-dimensional bi-modal model. The CME is simulated using a time dependent perturbation at the base of the corona. The model successfully reproduces the observed morphology, velocity profiles, and change in coronal mass. The observed velocity asymmetry is a natural consequence of the structure of the pre-event corona. Animations have been generated from both the data and model to illustrate the good agreement between the observations and simulation. These animations can be found on the CD-ROM which accompanies this volume. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005178630316     

Solar Sources of Interplanetary Coronal Mass Ejections During the Solar Cycle 23/24 Minimum

We examine solar sources for 20 interplanetary coronal mass ejections (ICMEs) observed in 2009 in the near-Earth solar wind. We performed a detailed analysis of coronagraph and extreme ultraviolet (EUV) observations from the Solar Terrestrial Relations Observatory (STEREO) and Solar and Heliospheric Observatory (SOHO). Our study shows that the coronagraph observations from viewpoints away from the Sun–Earth line are paramount to locate the solar sources of Earth-bound ICMEs during solar minimum. SOHO/LASCO detected only six CMEs in our sample, and only one of these CMEs was wider than 120^°. This demonstrates that observing a full or partial halo CME is not necessary to observe the ICME arrival. Although the two STEREO spacecraft had the best possible configuration for observing Earth-bound CMEs in 2009, we failed to find the associated CME for four ICMEs, and identifying the correct CME was not straightforward even for some clear ICMEs. Ten out of 16 (63 %) of the associated CMEs in our study were “stealth” CMEs, i.e. no obvious EUV on-disk activity was associated with them. Most of our stealth CMEs also lacked on-limb EUV signatures. We found that stealth CMEs generally lack the leading bright front in coronagraph images. This is in accordance with previous studies that argued that stealth CMEs form more slowly and at higher coronal altitudes than non-stealth CMEs. We suggest that at solar minimum the slow-rising CMEs do not draw enough coronal plasma around them. These CMEs are hence difficult to discern in the coronagraphic data, even when viewed close to the plane of the sky. The weak ICMEs in our study were related to both intrinsically narrow CMEs and the non-central encounters of larger CMEs. We also demonstrate that narrow CMEs (angular widths ≤?20^°) can arrive at Earth and that an unstructured CME may result in a flux rope-type ICME.     

From the Sun to the Earth: The 13 May 2005 Coronal Mass Ejection

We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time (Dst) index reaching ?263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation.     

Estimating Travel Times of Coronal Mass Ejections to 1 AU Using Multi-spacecraft Coronagraph Data

We study the relationship between the speeds of coronal mass ejections (CMEs) obtained close to the Sun and in the interplanetary medium during the low solar-activity period from 2008 to 2010. We use a multi-spacecraft forward-modeling technique to fit a flux-rope-like model to white-light coronagraph images from the STEREO and SOHO spacecraft to estimate the geometrical configuration, propagation in three-dimensions (3D), and the radial speeds of the observed CMEs. The 3D speeds obtained in this way are used in existing CME travel-time prediction models. The results are compared to the actual CME transit times from the Sun to STEREO, ACE, and Wind spacecraft as well as to the transit times calculated using projected CME speeds. CME 3D speeds give slightly better predictions than projected CME speeds, but a large scatter is observed between the predicted and observed travel times, even when 3D speeds are used. We estimate the possible sources of errors and find a weak tendency for large interplanetary CMEs (ICMEs) with high magnetic fields to arrive faster than predicted and small, low-magnetic-field ICMEs to arrive later than predicted. The observed CME transit times from the Sun to 1?AU show a particularly good correlation with the upstream solar-wind speed. Similar trends have not been observed in previous studies using data sets near solar maximum. We suggest that near solar minimum a relatively narrow range of CME initial speeds, sizes, and magnetic-field magnitudes led to a situation where aerodynamic drag between CMEs and ambient solar wind was the primary cause of variations in CME arrival times from the Sun to 1?AU.     

Heliospheric Observations of STEREO-Directed Coronal Mass Ejections in 2008?–?2010: Lessons for Future Observations of Earth-Directed CMEs

We present a study of coronal mass ejections (CMEs) which impacted one of the STEREO spacecraft between January 2008 and early 2010. We focus our study on 20 CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the other STEREO spacecraft up to large heliocentric distances. We compare the predictions of the Fixed-?? and Harmonic Mean (HM) fitting methods, which only differ by the assumed geometry of the CME. It is possible to use these techniques to determine from remote-sensing observations the CME direction of propagation, arrival time and final speed which are compared to in-situ measurements. We find evidence that for large viewing angles, the HM fitting method predicts the CME direction better. However, this may be due to the fact that only wide CMEs can be successfully observed when the CME propagates more than 100^° from the observing spacecraft. Overall eight CMEs, originating from behind the limb as seen by one of the STEREO spacecraft can be tracked and their arrival time at the other STEREO spacecraft can be successfully predicted. This includes CMEs, such as the events on 4 December 2009 and 9 April 2010, which were viewed 130^° away from their direction of propagation. Therefore, we predict that some Earth-directed CMEs will be observed by the HIs until early 2013, when the separation between Earth and one of the STEREO spacecraft will be similar to the separation of the two STEREO spacecraft in 2009??C?2010.     

The Interaction of Successive Coronal Mass Ejections: A Review

We present a review of the different aspects associated with the interaction of successive coronal mass ejections (CMEs) in the corona and inner heliosphere, focusing on the initiation of series of CMEs, their interaction in the heliosphere, the particle acceleration associated with successive CMEs, and the effect of compound events on Earth’s magnetosphere. The two main mechanisms resulting in the eruption of series of CMEs are sympathetic eruptions, when one eruption triggers another, and homologous eruptions, when a series of similar eruptions originates from one active region. CME?–?CME interaction may also be associated with two unrelated eruptions. The interaction of successive CMEs has been observed remotely in coronagraphs (with the Large Angle and Spectrometric Coronagraph Experiment – LASCO – since the early 2000s) and heliospheric imagers (since the late 2000s), and inferred from in situ measurements, starting with early measurements in the 1970s. The interaction of two or more CMEs is associated with complex phenomena, including magnetic reconnection, momentum exchange, the propagation of a fast magnetosonic shock through a magnetic ejecta, and changes in the CME expansion. The presence of a preceding CME a few hours before a fast eruption has been found to be connected with higher fluxes of solar energetic particles (SEPs), while CME?–?CME interaction occurring in the corona is often associated with unusual radio bursts, indicating electron acceleration. Higher suprathermal population, enhanced turbulence and wave activity, stronger shocks, and shock?–?shock or shock?–?CME interaction have been proposed as potential physical mechanisms to explain the observed associated SEP events. When measured in situ, CME?–?CME interaction may be associated with relatively well organized multiple-magnetic cloud events, instances of shocks propagating through a previous magnetic ejecta or more complex ejecta, when the characteristics of the individual eruptions cannot be easily distinguished. CME?–?CME interaction is associated with some of the most intense recorded geomagnetic storms. The compression of a CME by another and the propagation of a shock inside a magnetic ejecta can lead to extreme values of the southward magnetic field component, sometimes associated with high values of the dynamic pressure. This can result in intense geomagnetic storms, but can also trigger substorms and large earthward motions of the magnetopause, potentially associated with changes in the outer radiation belts. Future in situ measurements in the inner heliosphere by Solar Probe+ and Solar Orbiter may shed light on the evolution of CMEs as they interact, by providing opportunities for conjunction and evolutionary studies.     

The 2015 Summer Solstice Storm: One of the Major Geomagnetic Storms of Solar Cycle 24 Observed at Ground Level

We report on the 22?–?23 June 2015 geomagnetic storm that occurred at the summer solstice. There have been fewer intense geomagnetic storms during the current solar cycle, Solar Cycle 24, than in the previous cycle. This situation changed after mid-June 2015, when one of the largest solar active regions (AR 12371) of Solar Cycle 24 that was located close to the central meridian, produced several coronal mass ejections (CMEs) associated with M-class flares. The impact of these CMEs on the Earth’s magnetosphere resulted in a moderate to severe G4-class geomagnetic storm on 22?–?23 June 2015 and a G2 (moderate) geomagnetic storm on 24 June. The G4 solstice storm was the second largest (so far) geomagnetic storm of Cycle 24. We highlight the ground-level observations made with the New-Tupi, Muonca, and the CARPET El Leoncito cosmic-ray detectors that are located within the South Atlantic Anomaly (SAA) region. These observations are studied in correlation with data obtained by space-borne detectors (ACE, GOES, SDO, and SOHO) and other ground-based experiments. The CME designations are taken from the Computer Aided CME Tracking (CACTus) automated catalog. As expected, Forbush decreases (FD) associated with the passing CMEs were recorded by these detectors. We note a peculiar feature linked to a severe geomagnetic storm event. The 21 June 2015 CME 0091 (CACTus CME catalog number) was likely associated with the 22 June summer solstice FD event. The angular width of CME 0091 was very narrow and measured \({\sim}\, 56^{\circ }\) degrees seen from Earth. In most cases, only CME halos and partial halos lead to severe geomagnetic storms. We perform a cross-check analysis of the FD events detected during the rise phase of Solar Cycle 24, the geomagnetic parameters, and the CACTus CME catalog. Our study suggests that narrow angular-width CMEs that erupt in a westward direction from the Sun–Earth line can lead to moderate and severe geomagnetic storms. We also report on the strong solar proton radiation storm that began on 21 June. We did not find a signal from this SEP at ground level. The details of these observations are presented.     

Polarimetric Properties of Flux Ropes and Sheared Arcades in Coronal Prominence Cavities

The coronal magnetic field is the primary driver of solar dynamic events. Linear and circular polarization signals of certain infrared coronal emission lines contain information about the magnetic field, and to access this information either a forward or an inversion method must be used. We study three coronal magnetic configurations that are applicable to polar-crown filament cavities by doing forward calculations to produce synthetic polarization data. We analyze these forward data to determine the distinguishing characteristics of each model. We conclude that it is possible to distinguish between cylindrical flux ropes, spheromak flux ropes, and sheared arcades using coronal polarization measurements. If one of these models is found to be consistent with observational measurements, it will mean positive identification of the magnetic morphology that surrounds certain quiescent filaments, which will lead to a better understanding of how they form and why they erupt.     

On Flare-CME Characteristics from Sun to Earth Combining Remote-Sensing Image Data with <Emphasis Type="Italic">In Situ</Emphasis> Measurements Supported by Modeling

We analyze the well-observed flare and coronal mass ejection (CME) from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of effects – from Sun to Earth – to better understand the dynamic evolution of the CME and its embedded magnetic field. We study in detail the solar surface and atmosphere associated with the flare and CME using the Solar Dynamics Observatory (SDO) and ground-based instruments. We also track the CME signature off-limb with combined extreme ultraviolet (EUV) and white-light data from the Solar Terrestrial Relations Observatory (STEREO). By applying the graduated cylindrical shell (GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO (Solar and Heliospheric Observatory) coronagraph data, we track the temporal and spatial evolution of the CME in the interplanetary space and derive its geometry and 3D mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the magnetic cloud (MC) from in situ measurements from Wind. This is compared to nonlinear force-free (NLFF) model results, as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are lower in the interplanetary space by ～?50% and 75%, respectively, possibly indicating an erosion process. A CME mass increase of 10% is observed over a range of \({\sim}\,4\,\mbox{--}\,20~\mathrm{R}_{\odot }\). The temporal evolution of the CME-associated core-dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME.     

Propagation Characteristics of CMEs Associated with Magnetic Clouds and Ejecta

We have investigated the characteristics of magnetic cloud (MC) and ejecta (EJ) associated coronal mass ejections (CMEs) based on the assumption that all CMEs have a flux rope structure. For this, we used 54 CMEs and their interplanetary counterparts (interplanetary CMEs: ICMEs) that constitute the list of events used by the NASA/LWS Coordinated Data Analysis Workshop (CDAW) on CME flux ropes. We considered the location, angular width, and speed as well as the direction parameter, D. The direction parameter quantifies the degree of asymmetry of the CME shape in coronagraph images, and shows how closely the CME propagation is directed to Earth. For the 54 CDAW events, we found the following properties of the CMEs: i) the average value of D for the 23 MCs (0.62) is larger than that for the 31 EJs (0.49), which indicates that the MC-associated CMEs propagate more directly toward the Earth than the EJ-associated CMEs; ii) comparison between the direction parameter and the source location shows that the majority of the MC-associated CMEs are ejected along the radial direction, while many of the EJ-associated CMEs are ejected non-radially; iii) the mean speed of MC-associated CMEs (946 km?s^?1) is faster than that of EJ-associated CMEs (771 km?s^?1). For seven very fast CMEs (≥?1500 km?s^?1), all CMEs with large D (≥?0.4) are associated with MCs and the CMEs with small D are associated with EJs. From the statistical analysis of CME parameters, we found the superiority of the direction parameter. Based on these results, we suggest that the CME trajectory essentially determines the observed ICME structure.     

On the Coronal Mass Ejection onset and Coronal Dimming

A comprehensive case and statistical study of CME onsets has been conducted on the solar limb using the CDS, LASCO and EIT instruments aboard the SOHO spacecraft. This is the first dedicated campaign to establish firmly the EUV signatures of CME onsets and is based on a series of low-corona observing campaigns made in 2002. The event database consisted of 36 multiple emission line sequences observed with CDS and the study builds, in particular, on studies of EUV coronal dimming which have been associated with CME onsets. We witness a range of dimming events in EUV coronal emission line data. Shorter events, commonly of duration < 4 hours, we find are indirectly associated with CME onsets whereas longer-duration dimmings (> 4 hours) appear to be either due to coronal evolution or rotational effects. However, for some CME onsets, where the CDS pointing was appropriate, no dimming was observed. Dimming observed in EIT typically occurred immediately after the launch of a loop or prominence, and in 5 out of 9 events there is evidence of a matter buildup within the loop before launch. A total of 10 events occurred where CDS was used to directly observe the CME footprint, but no relationship between these events was found. The results suggest that the response of the corona to a CME launch differs between the low (1.0 R _⊙≤R≤1.2 R _⊙) and middle (1.2 R _⊙<R≤2.0 R _⊙) corona regions, hence implying a difference between dimming observations conducted with different instruments.     

An EUV Jet and Hα Filament Eruption Associated with Flux Cancelation in a Decaying Active Region

Using data from the Transition Region and Coronal Explorer (TRACE), Solar and Heliospheric Observatory (SOHO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Hida Observatory (HO), we present a detailed study of an EUV jet and the associated Hα filament eruption in a major flare in the active region NOAA 10044 on 29 July 2002. In the Hα line wings, a small filament was found to erupt out from the magnetic neutral line of the active region during the flare. Two bright EUV loops were observed rising and expanding with the filament eruption, and both hot and cool EUV plasma ejections were observed to form the EUV jet. The two thermal components spatially separated from each other and lasted for about 25 minutes. In the white-light corona data, a narrow coronal mass ejection (CME) was found to respond to this EUV jet. We cannot find obvious emerging flux in the photosphere accounting for the filament eruption and the EUV jet. However, significant sunspot decay and magnetic-flux cancelation owing to collision of opposite flux before the events were noticed. Based on the hard X-ray data from RHESSI, which showed evidence of magnetic reconnection along the main magnetic neutral line, we think that all of the observed dynamical phenomena, including the EUV jet, filament eruption, flare, and CME, should have a close relation to the flux cancelation in the low atmosphere.     

Bridging EUV and White-Light Observations to Inspect the Initiation Phase of a “Two-Stage” Solar Eruptive Event

The initiation phase of coronal mass ejections (CMEs) is a very important aspect of solar physics, as these phenomena ultimately drive space weather in the heliosphere. This phase is known to occur between the photosphere and low corona, where many models introduce an instability and/or magnetic reconnection that triggers a CME, often with associated flaring activity. To this end, it is important to obtain a variety of observations of the low corona to build as clear a picture as possible of the dynamics that occur therein. Here, we combine the EUV imagery of the Sun Watcher using Active Pixel System Detector and Image Processing (SWAP) instrument onboard the Project for Onboard Autonomy (PROBA2) with the white-light imagery of the ground-based Mark-IV K-coronameter (Mk4) at Mauna Loa Solar Observatory (MLSO) to bridge the observational gap that exists between the disk imagery of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the coronal imagery of the Large Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Methods of multiscale image analysis were applied to the observations to better reveal the coronal signal while suppressing noise and other features. This allowed an investigation into the initiation phase of a CME that was driven by a rising flux-rope structure from a “two-stage” flaring event underlying an extended helmet streamer. It was found that the initial outward motion of the erupting loop system in the EUV observations coincided with the first X-ray flare peak and led to a plasma pile-up of the white-light CME core material. The characterized CME core then underwent a strong jerk in its motion, as the early acceleration increased abruptly, simultaneously with the second X-ray flare peak. The overall system expanded into the helmet streamer to become the larger CME structure observed in the LASCO coronagraph images, which later became concave-outward in shape. Theoretical models for the event are discussed in light of these unique observations, and it is concluded that the formation of either a kink-unstable or torus-unstable flux rope may be the likeliest scenario.