Solar Phys (2013) 283:489–504DOI 10.1007/s11207-013-0228-5

Alternating Twist Along an Erupting Prominence

W.T. Thompson

Received: 2 May 2012 / Accepted: 8 January 2013 / Published online: 31 January 2013© Springer Science+Business Media Dordrecht 2013

Abstract Triangulation measurements using observations from the two Solar TerrestrialRelations Observatory (STEREO) spacecraft, combined with observations from the SolarDynamics Observatory (SDO), are used to characterize the behavior of a prominence in-volved in two successive coronal mass ejections 6 – 7 December 2010. The STEREO sep-aration at the time was 171.6◦, which was functionally equivalent to a separation of 8.4◦,and thus very favorable for feature co-identification above the limb. The first eruption at≈ 14:16 UT on 6 December of the middle branch of the prominence starts off a series ofmagnetic reconfigurations in the right branch, which itself erupts at ≈ 2:06 UT the nextday, about 12 hours after the first eruption. The cool prominence material seen at 304 Ådrains back down to the surface, but a flux-rope-like magnetic structure is seen to erupt inboth 195 Å by the STEREO/Extreme Ultraviolet Imager (EUVI), and in white light by theSTEREO/COR1 inner coronagraph. In between the two eruptions, two different signs of he-licity are seen in the measured twist of the right branch. This is interpreted to be caused bythe overall prominence channel being composed of different segments with alternating he-licity signs. The erupting parts on 6 and 7 December both show positive twist, but negativetwist is seen in between these positive sections. Negative twist is consistent with the dextralchirality signs seen in the He II line at 304 Å prior to both eruptions. However, during theperiod between the eruptions, a region of positive twist grows and replaces the region ofnegative twist, and finally erupts. We interpret these observations in the light of models thatpredict that helicity cancellation can be an important factor in the triggering of flares andcoronal mass ejections.

Keywords Prominences, active · Prominences, magnetic field · Coronal mass ejections,initiation and propagation

Electronic supplementary material The online version of this article(doi:10.1007/s11207-013-0228-5) contains supplementary material, which is available to authorizedusers.

W.T. Thompson (�)Adnet Systems Inc., Lanham, MD, USAe-mail: William.T.Thompson@nasa.gov

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Figure 1 Images at 304 Å of the prominence at 0:06 UT on 6 December 2010, about 14 hours prior to the firsteruption, as seen from the EUVI telescope on STEREO-Behind, and the AIA telescope on SDO. The crossesare included simply to guide the eye to the path of the prominence, and mark the same three-dimensionalposition on both images. The letters L, M, and R mark respectively the left, middle, and right branches of theprominence, as discussed in the text.

1. Introduction

One of the most useful times for applying triangulation techniques to images from the SolarTerrestrial Relations Observatory mission (STEREO: Kaiser et al., 2008) was early in themission, when the separation angle between the two spacecraft was relatively small. Whenboth the STEREO-Ahead and -Behind spacecraft were observing from similar viewpoints,it was relatively easy to match the same features in both images, and thus derive the three-dimensional position. This became more difficult as the spacecraft continued to separate,although triangulation was still possible even at separation angles as high as 48◦ (Thompson,Kliem, and Török, 2012). It is also fairly simple to match features between images when thespacecraft are close to 180◦ apart, so long as the features are above the limb, and the materialis optically thin. In this situation, the images will appear very similar in both telescopes,except that one will be close to being the mirror image of the other.

We report here on the analysis of a pair of prominence eruptions that occurred 6 – 7 De-cember 2010, when the STEREO spacecraft separation was 171.6◦. For the purposes oftriangulation, this is functionally equivalent to a separation angle of 8.4◦, and thus veryfavorable for feature co-identification. These eruptions were well observed on the limb bythe Sun Earth Connection Coronal Heliospheric Investigation (SECCHI) telescopes on bothSTEREO spacecraft (Howard et al., 2008), and simultaneously face-on by the Solar Dynam-ics Observatory mission (SDO: Pesnell, Thompson, and Chamberlin, 2012) AtmosphericImaging Assembly telescopes (AIA: Lemen et al., 2012).

Figure 1 shows the configuration of the prominence (or filament depending on the viewangle) about 14 hours before the eruption, as seen in the He II emission line at 304 Å byAIA and the STEREO/Extreme Ultraviolet Imager (EUVI). The prominence subtends morethan 80◦ in heliographic longitude, with part of its length passing through the remnants ofNOAA Active Region 11121. The first eruption, starting around 14:16 UT on 6 December2010, effectively separates this prominence into three sections. For simplicity, we will refer

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Figure 2 Upper panels: Images from all three telescopes at 16:46 UT on 6 December 2010, midway throughthe first eruption. The middle branch of the prominence has separated from the left branch, and is pivotingwith one end tied to the active region, where it appears to connect with the right branch. Lower panels:Three-dimensional tracings of filamentary structures derived via triangulation from the STEREO- Ahead and-Behind images and then reprojected into the STEREO and SDO viewing angles. See Section 2 for details.

to these as the left, middle, and right branches (see Figure 1). (Later, we will show that theright branch can be further separated into two segments.) It is the middle branch that eruptsfirst, as demonstrated in Figure 2. The left-hand side of the middle branch breaks away fromthe left branch and erupts, pivoting with the right end tied to the remnant active region, andstill apparently connected to the right branch (i.e. both are rooted next to each other in thephotosphere) through most of the eruption. The eruption continues out through the fieldsof view of the SECCHI inner (COR1) and outer (COR2) coronagraphs, where it is seen asa classic three-part coronal mass ejection (CME). QuickTime™ movies of the eruption asseen from the three spacecraft at 304 Å, and by the EUVI and COR1 telescopes on STEREO-Behind are available as movie_with_aia.mov and prom_20101206b.mov respectively in theElectronic Supplementary Materials.

Both the left and right branches are perturbed by the eruption of the middle branch,but the right branch is most affected. It first starts to slowly rise in height from an initialvalue of about 0.1 solar radii, which was the same height that it had at the beginning ofthe day, to an eventual height of around 0.5 solar radii, as shown in Figure 3. Midwaythrough this process, there is a sudden temporary jump in maximum height between around18:36 – 19:46 UT, due solely to one small piece of the prominence, which is seen to beejected upward in the STEREO images (Figure 4) and westward in the AIA images. At thesame time, the basic appearance of the prominence also seems to change, quickly takingon a twisted shape. This twisted shape persists until the second eruption and is best seen

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Figure 3 Maximum height of the right branch of the prominence as a function of time, as measured throughtriangulation (see Section 2).

Figure 4 Helium II 304 Å imageof the prominence at 19:06 UT on6 December 2010 as seen fromSTEREO-Behind. The arrowpoints to the temporary surgefeature seen in the right branchbetween 18:36 – 19:46 UT. Thelate stage of the eruption of themiddle branch is also visible inthe lower part of the image.

in the later stages of the eruption (Figure 5). As with the middle branch, one end of theright branch, both before and while it is erupting, appears to be anchored to the remnantactive region – the bulk of the right branch lies just outside of the active region. Around2:06 UT the next day, about 12 hours after the first eruption, a second eruption occurs. Thecool 304 Å material appears to come close to erupting but fails, instead draining down to thesurface. However, material is seen being ejected in the hotter Fe XII 195 Å bandpass, and aslow (≈200 km s−1) CME is visible in the COR1 and COR2 coronagraph data with a clear

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Figure 5 Right branch of the prominence, as seen at 0:06 UT on 7 December 2010, about ten hours after thefirst eruption, and about two hours before the second eruption. Note the near mirror-image appearance of theprominence above the limb in the two STEREO telescopes.

Figure 6 Running-differenceimage of the second CME as seenby the COR1 telescope onSTEREO-Behind at 8:31 UT on7 December 2010. The whitecircle represents the position ofthe Sun behind the occulter.

flux-rope appearance (Figure 6). This second “failed” eruption is the primary focus of thisarticle.

2. Triangulation Measurements

The method used to derive the three-dimensional structure of prominences via triangula-tion has been described previously (Thompson, 2011). The prominence structure was de-

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termined from combined EUVI images from STEREO-Ahead and -Behind by using theroutine scc_measure from the SECCHI library to trace out individual threads at each ten-minute time step between 14:06 UT on 6 December 2010 to 02:56 UT the next day. Thelower panels of Figure 2 show the results for 16:46 UT. Only STEREO data were usedin this analysis; the good agreement with the AIA observations demonstrates the validityof the triangulation technique. The left branch of the prominence does not appear in thisreconstruction, because it is not visible from STEREO-Ahead.

Because the measurements are three dimensional, one is not restricted to viewing theresults in the observing plane of the telescope. The lower-middle panel in Figure 2 is a re-projection into the AIA observing plane, which is completely different from the STEREOfields of view used to make the measurements. Figure 7 shows another way to examine thedata. Plotted for several representative time steps are the heliographic coordinates along theleft, and the height profile plotted against longitude on the right. The time steps plotted wereselected from a complete sequence of measurements at ten-minute cadence made between14:06 UT on 6 December 2010 and 02:56 UT the next day. A QuickTime™ movie of thesedata for all time steps is available as hg_and_rad_euvi.mov in the Electronic Supplemen-tary Materials. Initially, only the right branch and a small part of the middle branch of theprominence are visible. The bulk of the middle branch is not visible from STEREO-Aheaduntil it erupts. The plots at 18:46 UT represent the approximate time period during whicha reconfiguration appears to take place in the structure of the right branch. By 00:06 UT,the middle branch of the prominence has completely erupted, leaving a highly elevated rightbranch behind.

The SECCHI routine scc_measure is not restricted to analyzing STEREO data. Triangu-lation can also be performed with a combination of data from one of the STEREO spacecraftand from an Earth-orbiting spacecraft such as SDO. However, the quite different appearanceof the prominence as seen by STEREO from the filament observed by SDO makes it verydifficult to match fine-scale features in the two views. Only the most basic structures, suchas the spine, can be confidently co-identified in both views. Figure 8 shows the results of ananalysis of EUVI-B and AIA images for the 16:46 UT time step, overplotted in red (dashed)on top of the EUVI-A/B results from Figure 7. The agreement is quite good. CombiningEUVI-B and AIA data also allows the basic structure of the left branch to be characterized.It is clear from Figure 8 that all of the erupting middle branch of the prominence is capturedby the STEREO observations, and that the middle branch separates completely from the leftbranch during the eruption. There is also some deflection southward of the left end of themiddle branch as it erupts, by as much as 10◦ in latitude.

Several articles have been published using triangulation of STEREO data to characterizethe writhe of prominences as they erupt. Some examples include Bi et al. (2011), Bemporad,Mierla, and Tripathi (2011), Thompson (2011), and Thompson, Kliem, and Török (2012).However, no such writhe is visible in either the eruption of the middle branch on 6 December2010, or the eruption of the second branch the next day. Instead, in the present work, weexplore the effects of helicity of the prominence as expressed through its twist.

3. Prominence Twist

Twist is present in the right branch of the prominence prior to its eruption, as evident fromFigure 5. The middle branch also shows a small amount of twist as it is erupting. This isbest observed with tools that allow one to interactively view the data in three dimensions,but a sense of the twist can be determined from the pair of two-dimensional views shown in

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Figure 7 Triangulated data at several representative time steps 6 – 7 December 2010, plotted in Stonyhurstheliographic coordinates along the left, and as Stonyhurst longitude versus height along the right.

Figure 9. Selected traces in the erupting part of the prominence are colorized to better bringout the structure. Note in particular the red trace in comparison with the orange and greentraces. At about − 20◦ longitude, the three traces are very close together, but as the red (shortdash) trace climbs, it veers to the South, while the orange (long dash) and green (dash–dot)traces climb less slowly, and to the North of the red trace. The relative positions of the tracesindicate a right-handed or positive twist. From a visual inspection using interactive 3D tools,the total amount of visible twist in the erupting middle branch for this time step is estimatedto be roughly 180◦, or one half rotation. Additional twist of an unknown amount is likely to

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Figure 8 Triangulated data at 16:46 UT plotted as in Figure 7, in Stonyhurst heliographic coordinates onthe top, and as Stonyhurst longitude versus height on the bottom, with data derived from a combination ofSTEREO-Behind and SDO overplotted in red (dashed).

be present in the part of the magnetic structure beyond that which is filled with observablematerial.

It is a bit easier to see the twist in the right branch just before it erupts, as shown inFigure 10. At around − 20◦ longitude, the blue (long dash) trace is close to passing underthe red (short dash) trace, while at around + 7◦ longitude the blue trace clearly passes overthe red trace. Again, this indicates positive twist, and the total amount of twist for this timestep is visually estimated to be just under 270◦, or three quarters of a rotation. Inspectionof the entire time series of the right branch suggests that there is an additional part of themagnetic structure which is not filled with material emitting at 304 Å in the 23:46 UT timestamp, and which anchors the prominence to the surface at around + 20◦ longitude. Thispart becomes visible when the material drains back down to the surface during the seconderuption, and the total estimated level of twist for this structure is ≈ 360◦, or one completerotation.

The situation is more complicated if one examines the twist of the right branch priorto the start of the surge around 18:36 UT. Figure 11 shows the configuration at 17:26 UT.Note how the red trace (short dash) passes beneath the green (long dash) and blue (dash-dot) traces going from left to right; this is a property of negative twist, in apparent conflictwith the positive twists seen at other times. In fact, a visual inspection of the data using3D interactive tools consistently shows negative twist in the right branch for all times upthrough 18:06 UT, while the estimates from 18:26 UT onward show positive twist.

It is worth noting what helicity is expected in the prominence prior to the second eruption.Figure 12a shows a close-up of the right branch of the filament as observed at 304 Å by SDO

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Figure 9 Heliographic and height plots showing the first eruption, that of the middle branch, at 19:06 UTon 6 December 2010. Different colors and line styles have been applied to selected traces in the erupting partof the prominence to highlight the presence of positive twist.

at 14:16 UT, prior to the eruption. The red circle highlights two fibril features on oppositesides of the filament. According to the classification of Martin (2003), this configurationof fibrils indicates dextral chirality. A clear right-bearing barb immediately to the right ofthe red circle (red arrow) also indicates dextral chirality, as do the two barb features justbelow the circle (orange arrows). Figure 12b shows the same image with contours at 25and 50 gauss of positive (green) and negative (red) line-of-sight magnetic field from theSDO/Helioseismic and Magnetic Imager (HMI: Scherrer et al., 2012). The barb features areassociated with small negative-field regions in the predominantly positive-field region Southof the neutral line, as expected. A QuickTimeTM movie of the second eruption as seen fromAIA at 304 Å is available as aia_movie_fullres.mov (or the smaller aia_movie_halfres.mov)in the Electronic Supplementary Materials.

Dextral filaments typically have left-skewed overlying arcades (Martin, 1998). If theprominence threads are twisted in the same direction as the skew of the overlying arcades,then filaments with dextral chirality should have left-handed, or negative, twist. This ex-pectation is supported by Chae (2000), who uses the crossing of bright and dark threads infilaments to derive the helicity, and compares it with the chirality derived from the filamentbarbs. He finds that the threads of dextral (sinistral) filaments exhibit thread crossings con-sistent with negative (positive) twist. The behavior of the right branch prior to 18:26 UT is

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Figure 10 Heliographic and height plots showing the elevated right branch at 23:46 UT on 6 December2010, just two hours prior to its eruption. Different colors and line styles have been applied to selected tracesto highlight the presence of positive twist.

consistent with this expectation. The data in Figure 11 can be directly related to the tech-nique used in Chae (2000), with the top panel showing the thread crossings, and the bottompanel showing which threads overlie others. Flux-rope models of filament channels predictthat dextral (sinistral) filaments reside within negative (positive)-helicity-flux ropes – see thereview by Mackay et al. (2010). Other researchers have since applied the technique used inChae (2000), and have confirmed the same helicity/chirality relationship (Liu et al., 2012;Bi et al., 2012). Romano et al. (2009) validated this technique against other helicity mea-sures, and came to the same conclusion.

A detailed examination of the temporal history of the right branch reveals that it is actu-ally made up of two separate components. One component, which we will label R1, initiallymakes up the left side of the right branch, and has negative twist. The other component, R2,makes up the right side of the right branch, and has positive twist. These two componentscan be seen in Figure 11. The colored lines all belong to R1, while the black lines on theright belong to R2. The location from which the R2 component emerges is indicated by thegreen circle in Figure 12a, although it is not clear that R2 had emerged as a separate entity asearly as 14:16 UT. Initially, R1 dominates the length, and thus the estimation of twist, in theright branch. That changes when section R2 starts to grow, and it dominates the twist deter-mination from 18:26 UT onward. Prior to 18:26 UT, the twist of component R2 is unknown,

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Figure 11 Heliographic and height plots showing the right branch at 17:26 UT on 6 December 2010, aboutan hour before it undergoes an apparent reconfiguration. Different colors and line styles have been applied toselected traces to highlight the presence of negative twist.

but is assumed to also be positive due to helicity conservation. The growth of section R2 isdemonstrated in Figure 13. In the initial time steps the R2 structure is almost entirely to theright of 0◦ longitude, but it clearly extends leftward to − 10◦ longitude by 18:46 UT, and to− 20◦ longitude by 20:06 UT.

Although it is not always easy to pick out the fibril features shown in Figure 12a, the tworight-bearing barbs marked by the orange arrows in that figure persist throughout and afterthe eruption. Therefore, we deduce that a filament channel with dextral chirality, and thusnegative helicity, reforms after the eruption.

4. Discussion

We interpret the sequence of events in this pair of eruptions as the following. First, the mid-dle branch of the prominence breaks off from the left branch near 14:16 UT on 6 December2010, and erupts in a whip-like fashion, pivoting around the point where it is attached tothe right branch within the remnants of NOAA active region 11121. At the same time, theright branch starts a slow rise in height, preparatory to its own eventual eruption. This slowrise is driven by the growth of the positive-helicity region R2, which eventually replaces

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Figure 12 (a) Close-up(600′′ × 420′′) of the right branchof the filament as observed at304 Å by SDO at 14:16 UT on6 December 2010, prior to theeruption. The red circlehighlights two fibril features onopposite sides of the filamentshowing dextral chirality, whilethe arrows point to right-bearingbarbs. The green dashed circlemarks the region where thepositive-helicity region R2 lateremerges, as explained in the text.(b) Same image with contours ofpositive (green) and negative(red) line-of-sight magnetic fieldoverlaid.

the negative-helicity region R1. Finally, the slowly rising flux rope becomes unstable at≈ 2:06 UT on 7 December 2010, about 12 hours after the first eruption started. The coolprominence material quickly drains down to the surface, but the magnetic-flux rope under-goes a slow eruption, visible in the STEREO/COR1 and COR2 coronagraphs.

Filament systems with mixed helicity have been previously observed. Martin, Bilimoria,and Tracadas (1994) found that filament channels with the same helicity sign could readilyconnect with each other to form a single channel, but if the filament channels had oppositehelicity signs, then no merging occurred between the channels, and they instead formed acusp where the ends of the two filaments meet. These results are supported by DeVore andAntiochos (2005), who studied filament interactions under a variety of magnetic configu-rations. The configuration that is most likely to be relevant in this case is that of oppositechiralities with opposed axial fields. They found that such filament channels did not recon-nect, and tended to form a cusp shape. Note that we found no evidence of a cusp shape inthe SDO/AIA He II 304 Å images.

On the other hand, not all filaments with mixed helicity show cusps. For example, Chan-dra et al. (2010) discuss an eruption associated with a filament exhibiting opposite chiralityregions. No cusp is evident between the sinistral segment F2a and the dextral segment F2b

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Figure 13 Triangulated data at several representative times on 6 December 2010, plotted as Stonyhurstheliographic longitude versus height as in the right panels of Figure 7, but restricted to the region between− 30◦ and + 20◦ Stonyhurst heliographic longitude to show the growth of section R2 in the right branch.

in their Figure 4. Their analysis showed that the overall helicity of the active region in whichthe filament was embedded was negative, but that a localized emergence of positive helicityalong one part of the overall neutral line resulted in the eruption of this part of the filamentchannel. Similarly, Romano et al. (2011) discuss an eruption of a filament of negative helic-ity embedded within an active region of positive helicity. They theorize that the flux systemsof opposite helicity merged via reconnection leading to magnetic-helicity cancellation andenergy release, as modeled by Kusano et al. (2004). In this latter case, the helicity cancel-lation occurs between the filament and the magnetic region of the active region in which it

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is embedded, rather than with another filament channel, so that the question of a cusp shapedoes not arise.

Linton, Dahlburg, and Antiochos (2001) studied the interaction of flux tubes of coalignedand opposing helicity as a function of contact angle. They found that the interaction offlux tubes of opposite helicity fell into two categories, where the tubes either reconnected(slingshot interaction), or repelled each other (bounce interaction). This may explain whycusp shapes are seen in some cases, and not in others. Martin, Bilimoria, and Tracadas(1994) were studying the long-term magnetic structure of filament channels, rather than thestructure during eruptions, and thus would tend to choose those cases where the channelsdid not reconnect and erupt. Such cases tend to form cusp shapes, as shown by DeVore andAntiochos (2005), but there is no reason to assume that this is true for eruptive cases.

The most likely explanation for the second eruption is that the prominence section R2emerged from below the photosphere with positive helicity within a region of negative helic-ity. The result was slingshot reconnection as described by Linton, Dahlburg, and Antiochos(2001), causing the positive-helicity prominence to erupt, and the pre-existing negative-helicity prominence to be disrupted. The draining of the prominence material down thelegs during the eruption is consistent with slingshot reconnection. After the positive helicityerupts, the original negative-helicity prominence reforms.

Mok, Mikic, and Linker (2001) studied the interaction of magnetic loops with a numberof different properties, and find a wide range of interactions from gradual rearrangement ofthe field lines to rapid release of magnetic energy. The eruption of 7 December 2010 appearsto fall somewhere in the middle between these two extremes. There is definitely an eruptionof the magnetic structure starting at ≈ 2:06 UT, but there is no GOES flare signature for thissecond event (although there is a long-duration B2 flare associated with the first eruption),and the speed of the resulting CME is only 200 km s−1.

Martin and McAllister (1997) presented a different model of the helicity in prominenceeruptions. In their model, the sign of magnetic helicity within the prominence structure isopposite from that of the overlying arcade. Thus, an erupting dextral filament should showright-handed twist. If we use this model to interpret the eruptions of 6 – 7 December 2010,then the positive twist seen in the two eruptions would be a pre-existing property of theprominence material separate from the overlying arcade, and no injection of helicity wouldbe required. In this scenario, the apparent negative twist seen in the pre-eruptive prominencemight be explained as an artifact due to warping of the prominence spine. However, thisinterpretation is not consistent with the findings of Chae (2000) and others (Romano et al.,2009; Liu et al., 2012; Bi et al., 2012) that the sign of the twist derived from the crossing offilament threads is consistently negative for dextral barbs, while the pre-eruption behaviorshown in Figure 11 is consistent with the Chae (2000) results.

5. Conclusions

We have demonstrated a novel approach for deriving the helicity of erupting prominencestructures, by using the multiple viewpoints of the two STEREO spacecraft to perform trian-gulation of small filamentary features. This technique is most effective when the separationangle between the two spacecraft is either a few degrees, or a few degrees away from 180◦,but in principle can be applied at other separation angles if the filamentary structure is clearenough. Cross comparison of the STEREO measurements with the in-between view fromSDO/AIA shows good agreement, validating the technique.

Different signs of helicity are seen at different times during the observations. The he-licity of the middle branch, which erupts first around 14:16 UT on 6 December 2010, can

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only be measured during the eruption, and is positive. However, the part of the right branchadjacent to the eruption has negative helicity at first. As time goes by, the negative-helicityregion is gradually replaced by a growing positive-helicity region to its right (and possiblyalso by the erupting positive-helicity region on the left). This growing positive-helicity re-gion on the right itself then erupts at around 2:06 UT the next day. Examination of HMImagnetograms shows the emergence of small magnetic features in the region of the growingpositive-helicity region in the time period leading up to the eruption.

A number of researchers have discussed the possibility of flares and coronal mass ejec-tions being triggered by the interaction between oppositely directed helicity regions (Linton,Dahlburg, and Antiochos, 2001; Mok, Mikic, and Linker, 2001; Kusano et al., 2004), as wellas some observations indicating that this is happening on the Sun in at least some eruptions(Chandra et al., 2010; Romano et al., 2011). We propose that this is the explanation for thediffering signs of helicity observed during the present work. The emergence of positive he-licity in an otherwise negative-helicity region causes successive eruptions from two differentparts of the overall neutral line on both 6 December and 7 December 2010, with positive he-licity being ejected in both cases. The better observed of the two eruptions is the second,at ≈ 2:06 UT on 7 December. In this latter eruption, the draining of the prominence mate-rial down the legs during the eruption matches the slingshot reconnection effect modeled ofLinton, Dahlburg, and Antiochos (2001).

Acknowledgements This work was funded under NASA Contract NNG06EB68C. We would like to ac-knowledge the helpful comments on an early draft of this work from H. Gilbert, J. Karpen, and T. Kucera,as well as from the anonymous reviewer. The STEREO/SECCHI data used here are produced by an in-ternational consortium of the Naval Research Laboratory (USA), Lockheed Martin Solar and AstrophysicsLab (USA), NASA Goddard Space Flight Center (USA), Rutherford Appleton Laboratory (UK), Univer-sity of Birmingham (UK), Max-Planck-Institut für Sonnensystemforschung (Germany), Centre Spatiale deLiège (Belgium), Institut d’Optique Théorique et Appliqueé (France), and Institut d’Astrophysique Spatiale(France). The SDO/AIA and HMI data are courtesy of NASA/SDO and the AIA, EVE, and HMI scienceteams.

References

Bemporad, A., Mierla, M., Tripathi, D.: 2011, Rotation of an erupting filament observed by the STEREOEUVI and COR1 instruments. Astron. Astrophys. 531, A147. ADS:2011A&A...531A.147B, doi:10.1051/0004-6361/201016297.

Bi, Y., Jiang, Y.C., Yang, L.H., Zheng, R.S.: 2011, Nonradial eruption of a kinking filament observed fromSTEREO. New Astron. 16, 276 – 283. ADS:2011NewA...16..276B, doi:10.1016/j.newast.2010.11.009.

Bi, Y., Jiang, Y., Li, H., Hong, J., Zheng, R.: 2012, Eruption of a solar filament consisting of two threads.Astrophys. J. 758, 42. ADS:2012ApJ...758...42B, doi:10.1088/0004-637X/758/1/42.

Chae, J.: 2000, The magnetic helicity sign of filament chirality. Astrophys. J. 540, 115 – 118. ADS:2000ApJ...540L.115C, doi:10.1086/312880.

Chandra, R., Pariat, E., Schmeider, B., Mandrini, C.H., Uddin, W.: 2010, How can a negative mag-netic helicity active region generate a positive helicity magnetic cloud? Solar Phys. 261, 127 – 148.ADS:2010SoPh..261..127C, doi:10.1007/s11207-009-9470-2.

DeVore, C.R., Antiochos, S.K.: 2005, Solar prominence interactions. Astrophys. J. 629, 1122 – 1134.ADS:2005ApJ...629.1122D, doi:10.1086/431721.

Howard, R.A., Moses, J.D., Vourlidas, A., Newmark, J.S., Socker, D.G., Plunkett, S.P., Korendyke, C.M.,Cook, J.W., Hurley, A., Davila, J.M., Thompson, W.T., St. Cyr, O.C., Mentzell, E., Mehalick, K., Lemen,J.R., Wuelser, J.P., Duncan, D.W., Tarbell, T.D., Wolfson, C.J., Moore, A., Harrison, R.A., Waltham,N.R., Lang, J., Davis, C.J., Eyles, C.J., Mapson-Menard, H., Simnett, G.M., Halain, J.P., Defise, J.M.,Mazy, E., Rochus, P., Mercier, R., Ravet, M.F., Delmotte, F., Auchere, F., Delaboudiniere, J.P., Bothmer,V., Deutsch, W., Wang, D., Rich, N., Cooper, S., Stephens, V., Maahs, G., Baugh, R., Mcmullin, D.:2008, Sun earth connection coronal and heliospheric investigation (SECCHI). Space Sci. Rev. 136, 67 –115. ADS:2008SSRv..136...67H, doi:10.1007/s11214-008-9341-4.

504 W.T. Thompson

Kaiser, M.L., Kucera, T.A., Davila, J.M., St. Cyr, O.C., Guhathakurta, M., Christian, E.: 2008, TheSTEREO mission: an introduction. Space Sci. Rev. 136, 5 – 16. ADS:2008SSRv..136....5K, doi:10.1007/s11214-007-9277-0.

Kusano, K., Maeshiro, T., Yokoyama, T., Sakurai, T.: 2004, The trigger mechanism of solar flares in a coronalarcade with reversed magnetic shear. Astrophys. J. 610, 537 – 549. ADS:2004ApJ...610..537K, doi:10.1086/421547.

Lemen, J.R., Title, A.M., Akin, D.J., Boerner, P.F., Chou, C., Drake, J.F., Duncan, D.W., Edwards, C.G.,Friedlaender, F.M., Heyman, G.F., Hurlburt, N.E., Katz, N.L., Kushner, G.D., Levay, M., Lindgren,R.W., Mathur, D.P., McFeaters, E.L., Mitchell, S., Rehse, R.A., Schrijver, C.J., Springer, L.A., Stern,R.A., Tarbell, T.D., Wuelser, J.-P., Wolfson, C.J., Yanari, C., Bookbinder, J.A., Cheimets, P.N., Caldwell,D., Deluca, E.E., Gates, R., Golub, L., Park, S., Podgorski, W.A., Gummin, M.A., Smith, P., Auker, G.,Jerram, P., Pool, P., Soufli, R., Windt, D.L., Beardsley, S., Clapp, M., Lang, J., Waltham, N.: 2012,The Atmospheric Imaging Assembly on the Solar Dynamics Observatory. Solar Phys. 275, 17 – 40.ADS:2012SoPh..275...17L, doi:10.1007/s11207-011-9776-8.

Linton, M.G., Dahlburg, R.B., Antiochos, S.K.: 2001, Reconnection of twisted flux ropes as a function ofcontact angle. Astrophys. J. 553, 905 – 921. ADS:2001ApJ...553..905L, doi:10.1086/320974.

Liu, R., Kliem, B., Török, T., Liu, C., Titov, V.S., Lionello, R., Linker, J.A., Wang, H.: 2012, Slow rise andpartial eruption of a double-decker filament. I. observations and interpretation. Astrophys. J. 756, 59.ADS:2012ApJ...756...59L, doi:10.1088/0004-637X/756/1/59.

Mackay, D.H., Karpen, J.T., Ballester, J.L., Schmieder, B., Aulanier, G.: 2010, Physics of solar prominences:II – Magnetic structure and dynamics. Space Sci. Rev. 151, 333 – 399. ADS:2010SSRv..151..333M,doi:10.1007/s11214-010-9628-0.

Martin, S.F.: 1998, Conditions for the formation and maintenance of filaments (Invited review). Solar Phys.182, 107 – 137. ADS:1998SoPh..182..107M, doi:10.1023/A:1005026814076.

Martin, S.F.: 2003, Signs of helicity in solar prominences and related features. Adv. Space Res. 32, 1883 –1893. ADS:2003AdSpR..32.1883M, doi:10.1016/S0273-1177(03)90622-3.

Martin, S.F., McAllister, A.H.: 1997, Predicting the sign of magnetic helicity in erupting filaments and coro-nal mass ejections. In: Crooker, N., Joselyn, J.A., Feynman, J. (eds.) Coronal Mass Ejections, AmericanGeophysical Union, Washington, 127 – 138.

Martin, S.F., Bilimoria, R., Tracadas, P.W.: 1994, Magnetic field configurations basic to filament channels andfilaments. In: Rutten, R.J., Schrijver, C.J. (eds.) Solar Surface Magnetism, Kluwer Academic, Dordrecht,303 – 338. ADS:1994ssm..work..303M.

Mok, Y., Mikic, Z., Linker, J.: 2001, Interaction of two magnetic loops in the solar corona. Astrophys. J. 555,440 – 447. ADS:2001ApJ...555..440M, doi:10.1086/321458.

Pesnell, W.D., Thompson, B.J., Chamberlin, P.C.: 2012, The Solar Dynamics Observatory (SDO). Solar Phys.275, 3 – 5. ADS:2012SoPh..275....3P, doi:10.1007/s11207-011-9841-3.

Romano, P., Zuccarello, F., Poedts, S., Soenen, A., Zuccarello, F.P.: 2009, Magnetic helicity and activefilament configuration. Astron. Astrophys. 506, 895 – 900. ADS:2009A&A...506..895R, doi:10.1051/0004-6361/200912425.

Romano, P., Pariat, E., Sicari, M., Zuccarello, F.: 2011, A solar eruption triggered by the interactionbetween two magnetic flux systems with opposite magnetic helicity. Astron. Astrophys. 525, A13.ADS:2011A&A...525A..13R, doi:10.1051/0004-6361/201014437.

Scherrer, P.H., Schou, J., Bush, R.I., Kosovichev, A.G., Bogart, R.S., Hoeksema, J.T., Liu, Y., Duvall, T.L.,Zhao, J., Title, A.M., Schrijver, C.J., Tarbell, T.D., Tomczyk, S.: 2012, The Helioseismic and MagneticImager (HMI) investigation for the Solar Dynamics Observatory (SDO). Solar Phys. 275, 207 – 227.ADS:2012SoPh..275..207S, doi:10.1007/s11207-011-9834-2.

Thompson, W.T., Kliem, B., Török, T.: 2012, 3D reconstruction of a rotating erupting prominence. SolarPhys. 276, 241 – 259. ADS:2012SoPh..276..241T, doi:10.1007/s11207-011-9868-5.

Thompson, W.T.: 2011, Strong rotation of an erupting quiescent polar crown prominence. J. Atmos. Solar-Terr. Phys. 73(10), 1138 – 1147. ADS:2011JASTP..73.1138T, doi:10.1016/j.jastp.2010.07.005.