Stereoscopic Analysis of the 31 August 2007 Prominence Eruption and Coronal Mass Ejection
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- Liewer, P.C., Panasenco, O. & Hall, J.R. Sol Phys (2013) 282: 201. doi:10.1007/s11207-012-0145-z
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The spectacular prominence eruption and CME of 31 August 2007 are analyzed stereoscopically using data from NASA’s twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. The technique of tie pointing and triangulation (T&T) is used to reconstruct the prominence (or filament when seen on the disk) before and during the eruption. For the first time, a filament barb is reconstructed in three-dimensions, confirming that the barb connects the filament spine to the solar surface. The chirality of the filament system is determined from the barb and magnetogram and confirmed by the skew of the loops of the post-eruptive arcade relative to the polarity reversal boundary below. The T&T analysis shows that the filament rotates as it erupts in the direction expected for a filament system of the given chirality. While the prominence begins to rotate in the slow-rise phase, most of the rotation occurs during the fast-rise phase, after formation of the CME begins. The stereoscopic analysis also allows us to analyze the spatial relationships among various features of the eruption including the pre-eruptive filament, the flare ribbons, the erupting prominence, and the cavity of the coronal mass ejection (CME). We find that erupting prominence strands and the CME have different (non-radial) trajectories; we relate the trajectories to the structure of the coronal magnetic fields. The possible cause of the eruption is also discussed.
KeywordsCoronaCoronal mass ejectionFilament eruptionProminence
The study of prominence eruptions is receiving increased attention because of their close association with coronal mass ejections (CMEs; Gopalswamy et al.2003). It is now recognized that the filament (called prominence when at the limb), the filament channel encompassing the polarity reversal boundary, the overlying arcade and the CME itself are all part of one linked magnetic system (Martin et al.2008; Pevtsov, Panasenco, and Martin 2012). The filament eruption does not cause the CME nor visa versa; rather, they are two manifestations of the same underlying magnetic phenomenon. Thus, by studying filament eruptions, we can better understand the cause of CMEs and improve our ability to predict these space-weather drivers. A CME may occur at the site of an empty filament channel (Pevtsov, Panasenco, and Martin 2012), but when the filament is visible, it can provide important information on the chirality and helicity of the magnetic system, as well as its evolution, prior to CME initiation.
The launch of the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft in October 2006 has provided the opportunity to view filament and CME eruptions from two viewpoints, giving new insights into the three-dimensional geometry and the relationships between various features associated with these eruptions. Each spacecraft of the STEREO mission carries four remote sensing and in situ instrument suites (Kaiser 2005). The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) imaging package on each spacecraft includes five telescopes: an Extreme Ultra Violet Imager (EUVI), inner (COR1) and outer (COR2) coronagraphs, and inner (HI1) and outer (HI2) heliospheric imagers (Howard et al., 2008). The EUVI telescope has four channels similar to those of the Extreme ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). The wavelengths and coronal temperature of peak response are 304 Å (6 – 8 × 104 K, primarily the He ii line), 171 Å (106 K, primarily Fe ix/x), 195 Å (1.4 × 106 K, primarily the Fe xii line) and 284 Å (2.2 × 106 K) (Wuelser et al.2004). The standard SECCHI synoptic program running during the eruption provided simultaneous A – B pairs at a ten min cadence for 304 Å and 195 Å and a 2.5 min cadence for 171 Å. SECCHI data are available through the STEREO Science Center (http://stereo-ssc.nascom.nasa.gov).
Since the launch of STEREO, there have been several studies of prominences that use the two viewpoints to reveal new features of prominences and filaments and their relationship to solar processes (Liewer et al.2009; Gosain et al.2009; Bemporad, 2009; Li et al.2010; Xu, Jing, and Wang 2010; Li et al.2010, 2011; Thompson 2011; Bemporad, Mierla, and Tripathi 2011; Panasenco et al.2011). In several of these, the stereoscopic images from the twin spacecraft were used to reconstruct prominence in three-dimensions (3D) before and/or during the eruption to study various aspects of the eruption trajectory such as the true time-height plot (Liewer et al.2009; Bemporad, 2009; Li et al.2010, 2011) and the amount of rotation (Thompson 2011; Bemporad, Mierla, and Tripathi 2011).
Here, we use the stereoscopic technique of tie-point and triangulation to analyze the prominence eruption and CME of 31 August 2007 using data from the twin STEREO spacecraft. The filament was stable for its entire passage across the solar disk until it erupted spectacularly as it reached the east limb. We were able to reconstruct in 3D the filament before and during the eruption using images in both extreme ultraviolet and white light (SECCHI EUVI and COR1). Prior to the eruption, we were able to reconstruct in 3D, for the first time, a barb on the filament connecting the spine to the solar surface. The prominence rotates clockwise through about 90∘ as it erupts. The rotation is in the direction expected for a filament system of the observed chirality (determined from the barb and the magnetogram and confirmed by other signatures as discussed below). As the filament erupts, the endpoints where it connects to the surface are observed to change; fan-like EUV brightenings, as discussed by Wang, Muglach, and Kliem (2009), are seen at the location of the new endpoints, which are also sites of strong downflows. These appear near the beginning of the fast-rise phase of the eruption as the CME begins to form. The 3D reconstructions allow us to study the spatial relationships among various features of the eruption including the pre-eruptive prominence, flare ribbons, the erupting prominence, the EUV brightenings and the CME cavity. We are also able to compare non-radial trajectories of erupting filament strands and the CME and relate these to the coronal magnetic fields as computed from a Potential Field Source Surface model.
This paper is organized as follows. In Section 2, the stereoscopic analysis technique used to analyze the eruption is described. Section 3 presents the results of the analysis of the pre-eruptive prominence and the determination of the chirality of the system. Section 4 presents the results of the analysis of the prominence eruption, rotation, rolling and trajectory, and the relation to observed EUV brightenings and appearance of flare ribbons. Section 5 presents results from stereoscopic analysis of the spatial relationship between the prominence and CME during the eruption and relates these to the structure of the coronal magnetic fields. The possible cause of the eruption is also discussed. Section 6 contains a summary and discussion.
2 Stereoscopic Analysis Technique
Figure 2 shows the 3D reconstruction of the erupting filament loop from the tie-points in Figure 1 (green loop) as seen from two vantage points: approximately along the direction of the eruption (left) and edge-on (right). Also shown are the 3D reconstructions of the pre-eruption U-shaped filament (orange) on 23 August 2007 (reconstructed from a 304 Å stereoscopic pair on 23 August at 01:46:14 UT) and the post-eruption flare ribbons (reconstructed from a 304 Å pair on 31 August at 20:58:45 UT). This illustrates that by using this stereoscopic analysis technique, we can determine the spatial relations between the various features of this prominence eruption. For example, Figure 2 shows that the endpoints of the erupting prominence at 31 August 2007 10:21:15 UT (green loop) are located at some distance from the pre-eruptive filament (orange loop) and the flare ribbons (blue segments). This spatial relationship as well as others, such as the spatial relationship between the erupting prominences and the CME cavity, will be discussed in the sections to follow.
3 Stereoscopic Analysis of Pre-eruptive Filament
3.1 Solar Context for the Eruption
A large coronal hole can be seen to the east of the filament channel in the 195 Å image (top panel of Figure 3). Near the top of the left side of the U is a new active region AR 10969; near the top of the right side of the U is an older decaying active region. There is considerable magnetic evolution occurring in the magnetic fields surrounding the filament throughout its disk passage, including eruptions in the decaying active region on 23 August (with a visible prominence eruption) and 24 August that left the U-shaped filament generally unaffected. From the bottom panel, it can be seen that the U-shaped filament spine lies above the polarity reversal zone separating regions of predominantly positive (white – outside the U) and predominantly negative (black – inside the U) photospheric magnetic fields. This information is needed to determine the chirality of the filament system. The new active region to the NE and the old decaying active region to the NW are also evident in this magnetogram.
3.2 Reconstruction of Pre-eruptive Prominence: Barb and Chirality
Barbs are important because, given the magnetogram and a barb orientation relative to the filament spine, the chirality, handedness, and helicity of a filament system, the skew of the overlying magnetic arcade and the minority polarity associated with the barb can be determined. A left-bearing barb, as seen here, implies a positive polarity (minor polarity) at its base, a sinistral filament that has a “right-skewed” overlying arcade; the filament-arcade system is right handed with positive helicity (Martin 1998, 2003; Martin, Lin, and Engvold, 2008). For the eastern segment of the U-shaped spine, since the magnetic field is positive outside the U, the magnetic field of the filament is directed northward along this segment (indicated by the black arrow on the middle panel), and the overlying arcade should be skewed somewhat clockwise relative the filament spine (Martin 2003). This implies that during the eruption, the filament should rotate clockwise to bring the magnetic field lines of the filament more in alignment with the overlying arcade, reducing the magnetic stress.
4 Stereoscopic Analysis of Erupting Prominence Trajectory and Rotation
4.1 Prominence Trajectory and Rotation
The filament could be analyzed stereoscopically for many days before and during the eruption using both the EUVI 304 Å images and the COR1 white light images. Two 3D filament reconstructions have been shown in Figures 2 and 4. The eruption could be followed seamlessly from the EUVI to the COR1 fields of view. At the times when the filament was visible to both spacecraft in both instrument’s fields of view, the 3D reconstructions from the EUVI and COR1 stereo pairs coincided exactly, as they should if the spacecraft pointing information and our stereoscopy tools are working correctly.
If a filament erupts non-radially, as in this case and many others, the top of its spine first bends to one side and the filament develops a sideways rolling motion called the roll effect (Martin 2003; Panasenco and Martin 2008; Panasenco et al., 2011, 2012). These four studies found that the roll effect appears during the beginning phase of the prominence eruption, before CME formation begins; it is caused by asymmetries in the arcade overlying the filament. These studies also showed that the overlying arcade, filament cavity, filament and CME rise together during the CME eruptions, but the roll effect occurs earlier and shows itself as a smaller scale motion, lower down in the corona. Any non-radial effects on the larger scale eruption of the whole filament system and CME are caused by the larger scale magnetic structure of the corona (Panasenco et al., 2011, 2012; Pevtsov, Panasenco, and Martin 2012).
From Figure 8 it can be seen that, in this case as well, the sideways rolling motion of the prominence is very strong for the first seven reconstructions in Figures 7 and 8. Inspection of the reconstructions show a lateral rolling motion of the prominence of about 8∘ in latitude as it rises to 1.46 Rsun (20:36 UT). After this time, the motion becomes more rotational and less lateral (last six reconstructions in Figures 7 and 8). This change is due to the influence of the overlying filament channel magnetic field, which has a right-handed skew similar to the arcade and forces the sinistral filament to rotate clock-wise during its eruption. The relation between the prominence and CME non-radial motions and the relation to the coronal magnetic fields are discussed further in Section 5.
4.2 Formation of New Filament Endpoints and EUV Brightenings
The observations that i) the EUV brightenings and new endpoints are co-located and ii) that they appear at some distance from the center of the filament channel are consistent with the analysis of such brightenings by Wang, Muglach, and Kliem (2009). In that paper, they find that the spike-like or fan-shaped brightenings occur at the outer boundaries of the filament channel and that they are most intense at the time of maximum CME acceleration. They interpret the brightenings as the footpoints of the current sheets formed at the leading edge of the erupting filament magnetic fields as these fields reconnect with the overlying arcade. Our analysis supports this interpretation. The brightenings appear near the start of the fast rise phase, which is the time during which the filament (still rotating) and newly formed CME are pushing out through, and presumably reconnecting with, the overlying coronal fields. Strong downflows are observed in these new filament endpoints (see movie 20070831_EUVI_B_304.mov in the Electronic Supplementary Material at 21:00 UT), possibly causing the brightenings, consistent with interpretation as the footpoints of the current sheets. Wang, Muglach, and Kliem (2009) also find that the brightening lie near the outer edges of the transient coronal hole created by the CME; unfortunately, any transient coronal holes were not visible for this CME because its location at the west limb. We speculate that these new endpoints are related to the “spurs” identified in Thompson (2011), which were also related to sites of strong downflows.
5 Stereoscopic Analysis of the CME and Relation to Prominence
Figure 17 shows the much larger deflection in latitude from the source region of the primary segment of the prominence compared to that of the CME. The CME, however, has a larger deflection in longitude. Non-radial effects on the CME are caused by the larger scale magnetic structure of the corona, while the non-radial motion of the filament is at first governed by asymmetries in the lower lying arcade fields (Panasenco et al., 2011, 2012; Pevtsov, Panasenco, and Martin 2012). Insight into the differences in the non-radial motion of the main prominence segment and the CME can be gained by examining the structure of the corona as computed from a potential-field source-surface (PFSS) model; here we use the model of Schrijver and De Rosa (2003).
What is the cause of this filament/CME eruption? The filament retained its basic U shape as it crossed the disk. No particular activations or other activity seemed to affect the stability of the filament; it survived major eruptions nearby on 23 and 24 August. The most notable effect observed was the continual evolution of the magnetic fields in the surrounding region. Specifically, an analysis of the magnetograms showed that the negative polarity flux of the new active region to the NE of the U-shape spine spread into and canceled with the positive flux to the east of the filament leg which erupted. Over the course of the seven days leading up to the eruption, the positive radial magnetic flux of the overlying arcade field to the east of the U decreased by about a factor of 3 (from ≈ 4×1020 Mx (maxwell) to ≈ 1.5×1020 Mx), with most of this decrease in the last two days. This would cause a weakening of the arcade fields over the left segment of the filament (a.k.a. “tether cutting”), which we interpret as leading to the observed slow rise (cf. Figure 6). At some point, consistent with the concept of Démoulin and Aulanier (2010), the filament must have reached the critical height at which the overlying arcade could no longer balance the upward magnetic stresses of the sheared magnetic field lines of the filament and cavity, leading to a loss of equilibrium (fast-rise phase) and the CME eruption.
6 Summary and Discussion
In this paper, the stereoscopic technique of tie-point and triangulation has been used to analyze the prominence and CME eruptions of 31 August 2007 using data from the STEREO spacecraft. This technique has allowed us to reconstruct in 3D many features associated with these eruptions in a heliocentric coordinate system. We were able to study the spatial relationships among various features of the eruption including the pre-eruptive prominence, flare ribbons, the erupting prominence, new filament endpoints formed during the eruption, the EUV brightenings, and the CME cavity. The stereoscopic analysis also allows us to compare trajectories of erupting filament strands and the CME and to establish the temporal relationship between the filament rotation and CME formation.
The U-shaped filament was stable for its entire passage across the solar disk until the eastern segment (only) erupted spectacularly as it reached the east limb. The western segment was still visible as it rotated over the west limb. We were able to reconstruct the prominence before and during the eruption using images in both extreme ultraviolet and white light. Prior to the eruption, we were able to reconstruct in 3D, for the first time, a filament barb connecting the spine to the solar surface. A barb is a bundle of field-aligned threads of a filament, which terminate in an opposite polarity inclusion within the dominant polarity region on one side of the filament channel, as described by Martin (1998). Using the bearing of the barb (left relative to the spine) and the magnetogram, the filament was determined to be sinistral and the helicity of the system positive. From 3D reconstructions using data from both the EUVI instrument and COR1, we determined that the prominence rotates clockwise through about 90∘ as it erupts, as had been reported previously by Bemporad, Mierla, and Tripathi (2011). The rotation is in the direction expected for such a right-handed system and brings the expanding magnetic field lines of the prominence into closer alignment with the magnetic field of the overlying loop system.
A clear transition from slow to fast rise phase of the erupting filament is observed in the “true” (3D) height-time plot; while rotation begins in the slow rise phase, most of the rotation occurs in the fast rise phase, after CME formation has begun. As the filament erupts, the endpoints where it connects to the surface are observed to change and move away from the filament channel; Fan-shaped EUV brightenings are observed at the locations of the new endpoints. These observations are consistent with the analysis of filament eruptions and EUV brightenings reported by Wang, Muglach, and Kliem (2009). The new endpoints, which appear near the beginning of the fast-rise phase, are also sites of strong downflows. They are consistent with the interpretation by Wang, Muglach, and Kliem (2009) that the brightenings are the footpoints of current sheets formed where the prominence magnetic fields are reconnecting with the overlying arcade field during the eruption. Wang, Muglach, and Kliem (2009) also find that these brightenings mark the outer edge of transient coronal holes formed as the CME erupts and thus they may mark the outer edge of the CME legs.
Only with the stereoscopic viewpoints and analysis can we measure deflections in longitude as well as latitude from the source region. Here, both the CME and prominence showed considerable deflections in both. The filament described was typical in that it exhibited larger non-radial motions than its associated CME. The stereoscopic analysis also showed that the primary segment of the prominence exhibited the roll effect prior to CME formation, as observed in many non-radial prominence eruptions (Martin 2003; Panasenco and Martin 2008; Panasenco et al., 2011, 2012). We attribute the prominence’s gradual deviation from radial motion and its roll to asymmetry in the arcade overlying the filament in the low corona prior to CME formation. The CME, responding to magnetic fields higher up in the corona, gradually changes in trajectory in both longitude and latitude as it is guided towards the null line of the coronal streamer overlying the entire U-shaped filament channel and the neighboring active regions.
From the quite different directions of motion of the filament from the overlying coronal fields forming the CME, we infer that the filament, although constrained by the surrounding coronal field, had its own magnetic fields, separate from the overlying coronal field. We have shown here that the filament barb extends from the spine down to the solar surface, which is inconsistent with the concept of a pre-existing flux rope supporting and isolating the filament from the surface. Assuming the EUV brightenings mark the legs of the CME, we have also shown that the CME footpoints are at a different location than the original filament footpoints. These observations are consistent with the concept that the filament magnetic fields configuration is constrained by the surrounding coronal magnetic field, but not supported by the surrounding coronal loop system either before or during their eruption. The observations are also consistent with the concept of Démoulin and Aulanier (2010) that a CME results when the filament reaches a critical height at which the overlying arcade can no longer balance the upward magnetic stresses of the sheared magnetic field lines of the filament and cavity, leading to a loss of equilibrium (fast-rise phase) and the CME eruption.
We would like to thank Sara Martin, Pascal Démoulin, Eric De Jong, Marco Velli and Bill Thompson for useful discussions on various aspects of this research. The work of PCL and JRH and was conducted at the Jet Propulsion Laboratory, California Institute of Technology under a contract from NASA. The work of OP was supported by the NASA grant NNX09AG27G and NSF SHINE grant 0852249. The STEREO/SECCHI data used here are produced by an international consortium of the Naval Research Laboratory (USA), Lockheed Martin Solar and Astrophysics Lab (USA), NASA Goddard Space Flight Center (USA) Rutherford Appleton Laboratory (UK), University of Birmingham (UK), Max-Planck-Institut für Sonnensystemforschung (Germany), Centre Spatiale de Liège (Belgium), Institut d’Optique Théorique et Appliquée (France), Institut d’Astrophysique Spatiale (France).