Stealth Coronal Mass Ejections from Active Regions

Draft version July 31, 2019 Typeset using LATEX twocolumn style in AASTeX62

Jennifer O’Kane,1 Lucie Green,1 David M. Long,1 and Hamish Reid2 —

1Mullard Space Science Laboratory, UCL, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK 2SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK

ABSTRACT Stealth coronal mass ejections (CMEs) are eruptions from the Sun that have no obvious low coronal signature. These CMEs are characteristically slower events, but can still be geoeffective and affect space weather at Earth. Therefore, understanding the science underpinning these eruptions will greatly improve our ability to detect and, eventually, forecast them. We present a study of two stealth CMEs analysed using advanced image processing techniques that reveal their faint signatures in observations from the extreme ultraviolet (EUV) imagers onboard the Solar and Heliospheric Observatory (SOHO), Solar Dynamics Observatory (SDO), and Solar Terrestrial Relations Observatory (STEREO) spacecraft. The different viewpoints given by these spacecraft provide the opportunity to study each eruption from above and the side contemporaneously. For each event, EUV and magnetogram observations were combined to reveal the coronal structure that erupted. For one event, the observations indicate the presence of a magnetic flux rope before the CME’s fast rise phase. We found that both events originated in active regions and are likely to be sympathetic CMEs triggered by a nearby eruption. We discuss the physical processes that occurred in the time leading up to the onset of each stealth CME and conclude that these eruptions are part of the low-energy and velocity tail of a distribution of CME events, and are not a distinct phenomenon.

Keywords: Sun: coronal mass ejections (CMEs) — Sun: activity — Sun: corona — Sun: magnetic ﬁelds

1. INTRODUCTION Stealth CMEs typically have plane-of-sky speeds less −1 Coronal mass ejections (CMEs) are large eruptions of than 500 km s (D’Huys et al. 2014) and are frequently solar plasma, embedded with the solar magnetic field. found to originate from quiet Sun regions (Ma et al. Upon occurrence, typically one or more signatures are 2010) and regions close to open magnetic field (Nitta and observed in the lower solar atmosphere, such as filament Mulligan 2017). They are fairly common; for example, eruptions, solar flares, post-eruptive arcades, EUV dim- Ma et al.(2010) found that 1/3 of Earth-sided CMEs at mings and EUV waves that enable the CME source re- solar minimum had no distinct signatures, while the sta- gion to be identified (see Webb and Howard 2012, for tistical study of Wang et al.(2011) during the solar min- an overview). Observations from the Earth’s viewpoint imum of 1997-1998 found that 16% of front sided CMEs means that a white light CME with no observable low showed no signatures of the eruption on disk. Addition- coronal signatures is normally assumed to be a back- ally, Kilpua et al.(2014), in a study of 16 interplane- arXiv:1907.12820v1 [astro-ph.SR] 30 Jul 2019 sided event. However, the launch of the twin Solar tary CMEs (ICMEs) from 2009, found that 10 ICMEs Terrestrial Relations Observatory (STEREO) spacecraft were stealth events. Despite their slow speeds, stealth (Kaiser et al. 2008) enabled the CME propagation direc- CMEs can be the source of geomagnetic activity. Zhang tion to be determined, and therefore the identification et al.(2007) studied 77 geomagnetic storms associated of which side of the Sun the CME originated from, us- with ICMEs that occurred in solar cycle 23. Of these ing geometric triangulation techniques. Robbrecht et al. events, nine could not be associated with phenomena oc- (2009) named those eruptions that are seen in corona- curring on the solar disk. A more recent study by Nitta graph data but which leave no observable signatures in and Mulligan(2017) focused on a set of stealth events the low corona as “stealth” CMEs. that caused disturbances at 1AU, three of which produced Dst (Disturbance storm time) values greater than -100nT, indicative of a moderate geomagnetic storm. 2 O’Kane et al.

The geomagnetic impact and frequency of stealth the CME models discussed above are operating. A va- CMEs means that interest in this type of eruption is riety of observational signatures can be utilised to do growing. There are many open issues, including the fun- this. For example, photospheric magnetic field can be damental question of whether or not stealth CMEs are used to identify both sustained shear flows along po- different from non-stealth events. A review by Howard larity inversion lines, and flux emergence. The config- and Harrison(2013) led the authors to propose that uration and evolution of the coronal field can be stud- stealth CMEs sit at the lower energy end of a contin- ied using emission structures observed in EUV or X-ray uous spectrum of events, and originate from streamer data. This may include for example the identification of blowouts. The authors suggest that the lack of observ- a pre-existing flux rope in the corona before CME onset able signatures is likely due to instrumentation limita- through EUV or soft X-ray sigmoidal structures (Green tions and that the classification of this type of event is and Kliem 2009; Green et al. 2011; James et al. 2017), a purely observational one. If this is the case, then the and hot flux ropes (Cheng et al. 2011; Zhang et al. 2012; trigger and driver mechanisms that are currently pro- Patsourakos et al. 2013; Nindos et al. 2015). In all these posed for CMEs should be relevant and, once the erup- cases, the observed flux rope forms via reconnection ei- tion is underway, observational signatures of the CSHKP ther at a low altitude as in the van Ballegooijen and standard flare model (Carmichael 1964; Sturrock 1966; Martens(1989) model or higher up in the corona (James Hirayama 1974; Kopp and Pneuman 1976) should be et al. 2017; James et al. 2018) with corresponding ob- sought with appropriate instrumentation or image pro- servational signatures. However, such investigations are cessing techniques. predicated on the identification of the correct source re- Theories and models of CMEs focus on key aspects gion for the stealth CME. such as the specific magnetic configuration of the non- Whilst there are many ways in which a CME can be potential pre-eruptive field (that stores the free magnetic formed, once the eruption is underway there is consen- energy used to power the eruption), its evolution due to sus within the community as to how the magnetic field photospheric flows and/or flux emergence (the energy evolves. The erupting structure proceeds through a se- storage phase), and whether ideal or non-ideal processes quence of distinct evolutionary phases in its kinematics. are able to affect the stability of the field and bring it to First, a slow rise of around 10 kms−1, possibly where the point of eruption (the energy release phase). Mod- the stability of the field is lost, second, a rapid accel- els include the breakout model (Antiochos et al. 1999) eration up to velocities of 100s km−1 to 1000s km−1 in which strong shear is invoked within the central ar- when the main energy release (and magnetic reconnec- cade of a multipolar system. The shear leads to infla- tion) occurs, and third, propagation into the heliosphere tion of the core field followed by reconnection with the (Zhang et al. 2001; Zhang and Dere 2006; Vrˇsnak 2008). overlying arcade. This removes overlying field and al- Any soft X-ray flare emission rises sharply during phase lows the sheared arcade to erupt if a second phase of 2, indicating the close coupling between the flare recon- reconnection occurs within the sheared arcade, trans- nection and CME acceleration. Indeed, phase two can forming it into a flux rope. The tether cutting model exhibit a variety of observational signatures, which are also involves a sheared arcade in which runaway recon- collectively described by the CSHKP standard model. nection both builds and ejects the flux rope (Moore et al. These signatures include flare reconnection under the 2001). On the other hand, some models require the pre- erupting structure that produces a post-eruption (flare) eruption field to be that of a magnetic flux rope. The arcade and, as the core field expands, the reduction of flux rope can become unstable due to the torus insta- plasma density in the lower corona (which produces dim- bility if the gradient of the field overlying the curved ming regions). Once the eruption is underway, so by flux rope falls sufficiently rapidly with height (Kliem and phase two, all CME models discussed above find that Török 2006). Removal of overlying field, resulting from the magnetic configuration is that of a flux rope regard- a nearby CME as in the sympathetic eruption model less of the pre-CME field details. Observations of CMEs (Töröket al. 2011), may create this condition, as could studied using coronagraph data (Vourlidas et al. 2013) an increase in flux of the rope which would raise the and in situ (Burlaga et al. 1981) indeed find flux ropes structure. A comprehensive review of CME models and in many cases. If stealth CMEs do not differ from other their observational indicators may be found in Table 1 CMEs, it can be expected that they would follow such in Green et al.(2018). evolutionary stages, but without obvious flare emission The challenge then for stealth CME studies is to try due to the low energy release, and exhibit a flux rope and determine whether existing data can be used to in- configuration as they leave the Sun. vestigate the processes involved, and whether aspects of Stealth CMEs from active regions 3

Table 1. Details of the instruments used within this study. SE1 = Stealth event 1, 27-Oct-2009. SE2 = Stealth event 2, 03-Mar-2011.

Instrument/Spacecraft Type Used for event Wavelengths Resolution FOV

(A)˚ (arcsec) (R ) EIT/SOHO EUV imager SE1 195 5.2 0-1.5 MDI/SOHO Magnetogram SE1 N/A 4 0-1 LASCO C2/SOHO WL coronagraph SE2 N/A 47 1.5-6 EUVI/STEREO EUV imager SE1, SE2 195, 304 3.2 0-1.7 COR1/STEREO WL coronagraph SE1, SE2 N/A 15 1.5-4 AIA/SDO EUV imager SE2 94, 131, 171, 193, 211, 304 1.2 0-1.5 HMI/SDO Magnetogram SE2 N/A 1 0-1 Nancay Radioheliograph Radio Interferometer SE2 150 MHz 200 0-2

There is yet to be a clear definition of stealth CMEs and these are discussed in Section4. Conclusions are with some works stating that a stealth CME is one with presented in Section5. no low coronal signatures whilst other works define a stealth CME as one with no obvious, or very weak, low coronal signatures. Although the differences between 2. OBSERVATIONS AND METHODS these two classifications may at first seem trivial, stating This work uses data from the the Atmospheric Imag- that there are no signatures at all suggests that the sig- ing Assembly (AIA; Lemen et al. 2012) onboard the So- natures simply do not exist. On the other hand, if they lar Dynamics Observatory (SDO; Pesnell et al. 2011), are events with very weak and/or no obvious signatures, the Extreme Ultraviolet Imager (EUVI; Howard et al. the events may not necessarily be fundamentally differ- 2008) onboard the Solar Terrestrial Relations Obser- ent to other CMEs, and work towards producing tools vatory (STEREO; Kaiser et al. 2008) and the Ex- and techniques that reveal these weaker signatures can treme ultraviolet Imaging Telescope (EIT; Delabou- progress. A comprehensive study by Alzate and Mor- diniere et al. 1995) onboard the Solar and Heliospheric gan(2017) showed,using advanced imaging processing Observatory (SOHO; Domingo et al. 1995). Photo- techniques applied to coronal observations, that all 40 spheric line of sight magnetograms are obtained from the stealth CMEs in a catalogue developed by D’Huys et al. Heliseismic and Magnetic Imager (HMI; Scherrer et al. (2014) did indeed manifest themselves with one or more 2012) and the Michelson Doppler Imager (MDI; Scher- lower coronal signatures. This suggests that the source rer et al. 1995) onboard SDO and SOHO respectively. regions of stealth CMEs can be found and studied. The CMEs are identified using data from the white- The aim of this study is to combine knowledge of the light coronagraphs, The Large Angle and Spectromet- observational signatures of CMEs related to the forma- ric Coronagraph (LASCO; Brueckner et al. 1995) and tion and eruption of non-potential fields with the lat- COR1/COR2 (part of the SECCHI instrument suite; est image processing techniques to extend the study of Howard et al. 2008) onboard SOHO and STEREO re- Alzate and Morgan(2017) for two stealth CME events. spectively. The details of instruments used in each Stealth event 1 occurred on 27 October 2009 and stealth stealth CME event are outlined in Table1. event 2 occurred on 03 March 2011. Both events have The graduated cylindrical shell model and stack plots had their approximate source region determined using of EUVI and COR1 were used to trace the CME back triangulation by Kilpua et al.(2014) and Pevtsov et al. to the start of the eruption. In order to increase the (2011), respectively. We aim to investigate whether likelihood of being able to identify lower coronal signa- these stealth CMEs show signatures of the CSHKP stan- tures of the stealth CMEs in EUV data, the Multi-scale dard model, albeit faint, in order to identify the ex- Gaussian Normalization technique (MGN; Morgan and act source region. We then look for signatures of the Druckmüller 2014) was applied and running difference processes that could account for the formation and de- images were created for both events and examined for stabilization of the eruptive structures. In Section2 we dimming regions. The radio data was imaged using the describe data used and our analysis techniques. Section Nan¸cay Radioheliograph (NRH) (Kerdraon and Delouis 3 displays our findings for the two stealth CME events, 1997a). 4 O’Kane et al.

2.1. Graduated Cylindrical Shell (GCS) Model gave the best result for our work. The running differ- The graduated cylindrical shell (GCS) model is an ence subtracts a following image from a leading image, empirical model developed by Thernisien et al.(2006). whilst the running ratio divides a leading image from a It is used to study the 3-D morphology, position, and following image. Temporal separations varied between kinematics of flux-rope CMEs. Flux-rope CMEs typi- 2 minutes and 3 hours. Thirty minutes proved to pro- cally have a 3-part structure; a bright leading front, a vide a clearer image, for capturing the dynamic motions, dark cavity, and a bright core, where the dark cavity is without having too much effect on the ambient back- representive of a flux rope within the CME structure. ground, whilst 3 hours proved best for capturing faint The geometry of the model includes cone shaped legs, a EUV dimmings. The first event studied here occurred pseudo-circular front, and a circular cross section, fitted on October 27th 2009, and was observed by SOHO and by eye using at least two different vantage points and six both STEREO spacecraft. The separation angle be- ◦ parameters; longitude, latitude, height, tilt angle, half tween STEREO-A and -B was 123 , with STEREO-B ◦ ◦ angle, and ratio. The resulting GCS model is a shape 60 behind Earth and STEREO-A 63 ahead of Earth similar to that of a hollow croissant, which expands in a (left panel of Figure1). For this event, one of the EIT self-similar way. The model requires at least two corona- wavebands was used, and two of the EUVI wavebands graph images from two different spacecraft (e.g COR2-A were used (see Table1). For EIT we used a 12 minute and COR2-B) taken at the same time. temporal separation, and for each of the EUVI wave- The GCS model used COR1 and COR2 data to find lengths we used 5 minute, 10 minute and 30 minute approximate source region for each stealth CME. A ra- temporal separations. The second event studied here dial path away from the Sun is assumed, however it is occurred on March 3rd 2011, and was observed on-disk noted that many CMEs in these lower regions are non- by SDO and at the limb by both STEREO spacecraft. radial with respect to their source regions (Cremades At this time the separation angle between STEREO-A ◦ ◦ and Bothmer 2004; Cremades et al. 2006). and -B was 178 , with STEREO-B 95 behind Earth ◦ Typically when fitting the GCS model to the coro- and STEREO-A 87 ahead of Earth (right panel of Fig- nagraph images, one would also ensure that the flux ure1). Running time difference images were created rope footpoints from the model match up with those for six of the AIA wavebands, and for two of the EUVI determined observationally in the EUV images. Due to wavebands, as outlined in Table1. For each of the AIA stealth CMEs having no obvious low coronal signatures, passbands we used 2 minute, 5 minute and 10 minute it is an opportunity to obtain an approximation of the temporal separations. For each of the EUVI passbands location of the CME footpoints, and thus the source re- we used 5 minute, 10 minute and 30 minute temporal gion of the CME. However due to both limitations with separations. Longer temporal separations were neces- the model and possible alterations in CME direction af- sary in order to observe dynamic structure of the stealth ter eruption, the source region obtained from the GCS CMEs, which evolve at a relatively slow rate. Temporal model may not be exactly correct. Therefore it is nec- separations were chosen based on the cadence of each essary to also search for observational signatures of the instrument. The temporal evolution of the CME as it eruption. propagates outward from the Sun was tracked using a stack plot which combined the fields of view of both 2.2. Observations EUVI and COR1 from the STEREO spacecraft. The MGN technique was applied to each of the EUV Stack plots of COR1 and EUVI were created for both passbands listed in Table1. This technique reveals events. The slices generated from the stack plots were faint structure in the low corona that is usually hid- radially outwards from the Sun. In each case, stack plots den as a result of bright regions that dominate over re- were created for all angles that crossed the CME struc- gions of the Sun with lower EUV emission. The abil- ture at a variety of angular widths, as observed in COR1 ity to observe this fine structure is obtained by nor- field of views. All stack plots were examined for poten- malizing images at multiple spatial scales, using the tial activity. The stack plots presented in this paper, local mean and standard deviation, and producing a are the radial slices that intersect through the centre of weighted combination of the normalised components. the concave-up structure, assumed to be the flux-rope The method produces detailed images similar to wavelet cavity, as this showed the most clear propagating CME based techniques (Stenborg and Cobelli 2003; Stenborg structure. et al. 2008), and the noise adaptive fuzzy equaliza- For each instrument, a variety of difference imaging tion technique (Druckmüller 2013), however MGN is techniques were employed at various temporal separa- much more computationally efficient, faster by at least tions. The running difference and running ratio images Stealth CMEs from active regions 5

Figure 1. Positions of STEREO-A and -B during on 27-Oct-2009 (left) and 03-Mar-2011 (right) an order of magnitude, and does not require a high- CME had a plane-of-sky speed of 208 kms−1 as seen by performance computer. The detailed images produced STEREO-A1. Kilpua et al.(2014) performed a multi- by this technique can reveal fine structural changes in spacecraft forward-modelling analysis using the GCS the low corona that are related to the formation and model and applied a triangulation technique, approxi- later eruption of the stealth CME. mating the source region of the event to be located at Running ratio images with 3 hour temporal separa- N03W06, and N03W10 respectively from the two meth- tions were used, in order to identify, track, and enhance ods. Although eruptive signatures were observed on the any EUV dimmings associated with the stealth CMEs. solar disk, the authors concluded that this activity was Edges of dimmings were manually selected every half not co-spatial with the approximated source region of hour, and the contours of the dimming regions at each the stealth CME. Instead, the estimated source region time are plotted. is approximately half way between two active regions, MDI and HMI were used to observe the evolution of one in its emergence phase (AR 11029) and one in its the photsopheric magnetic field for the October 27th decay phase with no NOAA active region number as- 2009 and March 3rd 2011 stealth CMEs respectively. signed. We searched for changes in the magnetic field such as 3.1.1. Analysis flux emergence, flux cancellation, and shearing motions that may play an important roll in the formation and Stack plots were created to track the CME through initiation of CMEs, as well as observing the configura- the coronagraph and EUV data, to the surface of the tion of the magnetic field at the time of eruption. Sun. Figure2 shows the stack plot created using a slice ◦ The NRH was used to analyse the radio emission that of the data at an angle of 80 from solar north in the arose during the March 3rd 2011 stealth event. The clockwise direction. The propagation of the concave- radio images were made using the NRH clean algorithm up section of the CME is clearly visible in the COR1-A from data with a 1 second cadence. The radio flux was stack plot created from a time series of these data slices, calculated from the images using a box of length 600 where the CME appears to exhibit a slow rise phase arcsecs. that is in progress by 23:00 UT 26-Oct-2009, followed by a phase of rapid acceleration ∼13:00 UT 27-Oct- 3. RESULTS 2009. However, the EUVI-A stack plot does not show 3.1. Stealth event 1 : 27-October-2009 any clear upward propagating structure. The result that it was unable to be picked up in the field of view cap- On 27 October 2009 the STEREO coronagraphs ob- tured by EUVI-A, combined with the observation of the served a CME that was found by Kilpua et al.(2014) slow rise phase in the COR1 field of view, suggests that to be Earth directed, but without any low coronal sig- the eruption was initiated from higher altitudes. The natures. The authors reported that the CME first appeared in STEREO-B (STEREO-A) COR1 at 10:30 UT (15:30 UT) on 27 October 2009, giving an estimated 1 http://solar.jhuapl.edu/Data-Products/COR-CME- eruption onset time of ∼06:00 UT 27-Oct-2009. The Catalog.php 6 O’Kane et al.

Figure 2. (Left) EUVI/COR1 image indicating the slice used to create the stack plot (solid line). (Right) EUVI-B 195A˚ (lower) and COR1-B (upper) stack plots. The height-time profile of the stealth CME observed on 27 October 2009 can be determined from the COR1-B data which show the underside (concave-up structure) of the stealth CME. Exponential (red line) and quadratic (blue line) fits have been applied to the COR1-B data. EUVI stack plot does however show a large brightening However, the stealth CME is seen more clearly, and ear- around the time of the rapid acceleration phase begin- lier, in COR1-B, which could suggest that the eruption is ning, likely to be post-eruption loops associated with the closer to the limb of STEREO-B than STEREO-A. The stealth CME. We then fitted exponential and quadratic estimated source region from triangulation techniques is curves to the CME position in COR1 field of view. The of similar distance to the nearest limbs in both space- curves can give indications of what mechanisms are driv- craft, whilst the decayed active region is directly on the ing the eruption (D’Huys et al. 2014; Schrijver et al. limb in STEREO-A and on disk in STEREO-B, and ac- 2008). Numerical simulations matched with observa- tive region 11029 is directly on the limb of STEREO-B tions have demonstrated that an exponential rise profile and on disk in STEREO-A. occurs when an instability is dominating the eruption, Using COR1 and COR2 from the twin STEREO such as the torus or kink instability, whilst a quadratic spacecraft, the wireframe of the GCS model (represent- rise profile occurs during a breakout model scenario. For ing the flux rope), was fitted to the concave-up cavity this event the exponential curve produced the best fit, structure in the coronagraph observations (Figure4). suggesting an instability dominating the eruption. Both The footpoints of the erupting structure were found to the torus and kink instability require a flux rope prior to be N03E05 and N03W18 from the GCS model, illus- eruption, and given that the concave-up structure could trated by the blue X’s in Figure3, centered around the be tracked for a period of hours during the slow rise region approximated by Kilpua et al.(2014) (red X in phase, it is likely that in addition to the structure be- Figure3) using the triangulation and the GCS model. ing initiated at a high altitude, the flux rope was also This approximated source region is located in a quiet formed at a high altitude in the corona. Sun region in the northern hemisphere. On the east The stealth CME is not observed in the COR1-A im- and west sides of the approximated source region were ages until ∼15:30UT on 27-Oct-2009, several hours after a decayed active region and NOAA active region 11029, the eruption onset time as determined by Kilpua et al. respectively (Figure3). (2014). The CME is also very faint and barely visible We then applied the MGN technique to the EUV data, in the images, making it difficult to pinpoint the exact and additionally produced running difference images us- time that it enters the field of view. It also emerges from ing the EUV data, enabling a variety of dynamical struc- a bright streamer that acts to mask the CME structure. tures to be observed in the lead up to, and following the Stealth CMEs from active regions 7

Figure 3. Top panel: MDI magnetograph image. Bottom panel: MGN processed EIT 195A˚ image. The triangulated source region determined by Kilpua et al.(2014) is illustrated by the red X. The footpoints of the eruption approximated by the GCS model are illustrated by the blue X’s. The estimated source region is located in a region of highly dispersed magnetic field, with no polarity inversion line, suggesting that an eruption is unlikely to have occurred here. A small decayed active region is located north-east, and a newly emerged active region (NOAA 11029) is located north-west of the estimated source region. stealth CME. The activity in the decayed active region be shown to be associated with the stealth CME and can be summarised as: a large and clear eruption be- therefore the decayed active region is not deemed to be ginning ∼11:25 UT 26-Oct-2009 as seen in STEREO-A its source region. The activity in active region 11029 EUVI data with an associated white light CME observed in the ∼ 1.5 days before the stealth CME was first ob- in COR1-A ∼12:00 UT (Figure4, first panel); a very served in coronagraph data can be summarised as: small faint structure that moves outwards through the field burst of bright plasma in the north of the active region of view of STEREO-A COR1 ∼21:00 UT 26-Oct-2009, ∼21:45 UT 26-Oct-2009; expanding loops begin forming this may be a part of the previous event or a separate early on 27-Oct-2009 to ∼05:10 UT; multiple C-class eruption that closely follows the former; an extremely flares between 18:38 UT on 26-Oct-2009, and 11:07 UT faint rising loop ∼07:10 UT that cannot be followed to on 27-Oct-2009 (Figure4, second panel). During this the edge of the field of view of EUVI, since no associ- time, active region 11029 continues to brighten and dis- ated post-eruption loops were observed, it is deemed to plays an ongoing reconfiguration of the loops, which may be a failed eruption. None of these activity events can be associated with the ongoing flux emergence in the re- 8 O’Kane et al.

Figure 4. Top : (Left) EUVI-A 195A˚ running difference image with a 5 minute temporal separation, showing an eruption from the decayed active region. The eruption occurs several hours before the eruption of the stealth CME. (Center) MGN processed EUVI-A 195A˚ showing a small eruptive burst of plasma from the newly emerged active region, believed to be the source of the stealth CME. (Right) Combined EUVI-B 195A˚ and COR1-B image. The CME is indicated by the arrow and has a dark cavity, indicative of a flux rope present in the CME. Bottom : The wire frame fitted to the CME in COR2-A (Left) and COR2- B (Right). gion. There is a weak dimming region to the north of Finally, we looked into the evolution of the photo- the active region and on its western side seen in EIT sphere. The red cross in Figure3 indicates the source of data that begins ∼06:30 UT 27-Oct-2009, however nei- the stealth CME as estimated by Kilpua et al.(2014), ther running difference nor running ratio images could whilst the blue crosses indicate the approximate foot- enhance this to a trackable feature. By ∼12:55 UT ac- points of the CME found from the GCS model. The tive region 11029 continues to brighten with new loops approximated source location is an area of weak dis- forming. From the location of active region 11029 along persed field with no clear polarity inversion line, rein- with the observed dimming and reconfigured field we forcing the conclusion that the stealth CME originated conclude that this is the most likely source region of the from a nearby active region. Active region 11029 first stealth CME on 27 October 2009. starts to emerge early on 22 October 2009 in the eastern hemisphere, into a region of weak mixed polarity field. Stealth CMEs from active regions 9

The active region builds up as the result of the emer- 11165) and a filament channel as can be seen in Fig- gence of several bipoles that coalesce. A second bipole ure6. Pevtsov et al.(2011) concluded that the filament begins to emerge ∼03:00 UT 24-Oct-2009 on the north- channel was the CME source region. Nitta and Mul- western side of the first bipole. A third bipole begins its ligan(2017) find two EUV dimmings centered around emergence ∼10:00 UT 26-Oct-2009 at the polarity in- S20, the region in which the active region was present. version line of the second bipole. Flux emergence is still The source region was on disk from the SDO perspec- underway at the time of the stealth CME as determined tive and at the solar limb from the perspective of both using the EUV data. At the time of the eruption the STEREO spacecraft. This means that the combined active region has a beta-gamma configuration accord- AIA and EUVI data allow the approximated source re- ing to the Hale classification scheme (Hale et al. 1919), gion to be studied when viewed at the limb as well as meaning that the region was bipolar overall but that from above. However, as detailed below, the STEREO- no continuous line could be drawn separating spots of A EUVI data show more clearly the evolution and erup- opposite polarities. tion of the source region and are focused on in this study.

3.1.2. Overall Remarks 3.2.1. Analysis The combined analysis of the photospheric magnetic The stack plot shown in Figure5 used a slice of the field, activity in the lower corona and CME propaga- ◦ tion as seen in coronagraph data together suggest that EUVI data and COR1 located at an angle of 240 clock- the stealth CME of 27 October 2009 originated in ac- wise from solar north (indicated by the black line in the tive region 11029. The CME is observed to be in its left panel). The propagation of the concave-up section slow rise phase as observed by STEREO-B COR1 data of the CME is clearly visible in the COR1-A stack plot by 23:00 UT 26-Oct-2009. The fast rise phase is ob- (top panel of Figure5) but shows that the slow rise to served to start around 13:00 UT 27-Oct-2009, similar fast rise transition was not captured. The slow rise to in time to the formation of new loops in the active re- fast rise transition of the underside of the CME was not gion, which are deemed to be post-eruption loops. The visible in the EUVI data, presumably because of insuffi- time of the fast rise phase onset is ∼7 hours later than cient plasma emission. This, combined with little to no the estimated time of eruption determined by (Kilpua structure observed in the EUVI, may be the result of the et al. 2014). This discrepancy may be partly due to the CME being initiated from a high altitude structure with stealth CME originating from a high altitude, as deter- weak plasma emission. Although the CME propagation mined from the absence of any signature of the erupting profile was fitted with an exponential and a quadratic CME in EUV data, and only the underside of the CME curve, neither curve fits the CME curve better than the being observable in COR1 data. other, and therefore no conclusions on the most likely initiation mechanism can be made from this. 3.2. Stealth event 2: 03-March-2011 The GCS model was applied in the same way as discussed previously (Figure7). The footpoints of the erup- On March 3rd 2011 the STEREO coronagraphs ob- tion from the GCS model were found to be S18E04 and served an Earth directed CME with a faint leading edge S16W18, illustrated by the blue X’s in Figure6. This that appeared to be slowly rising in STEREO-A COR1 was to the East and West of the small active region data from around 00:00 UT. Despite the faint leading (NOAA 11165), and further North than the region tri- edge, the CME is seen to have a clear circular shape, angulated by Pevtsov et al.(2011). The footpoints are the concave-up section of which was first observed in of a similar location to the dimmings found by (Nitta STEREO-A COR1 at approximately 03:00 UT on 03- and Mulligan 2017) centered around S20. Mar-2011. The CME had a plane-of-sky speed of 409 The MGN-processed AIA 211 A˚ images show dynamic kms−1 as seen by STEREO-A2. The LASCO corona- plasma emission structures that occur around active re- graphs observed this CME as a faint partial halo that gion 11165. Likewise, MGN-processed images and run- propagated to the south. This stealth CME was previ- ning difference images with a 30 minute temporal sepa- ously studied by Pevtsov et al.(2011) who found, using ration from EUVI-A 195 A˚ show a number of dynamic a triangulation approach, the CME source region to be structures located off-limb and out to the edge of the S35W10. In the vicinity of the approximated source field of view above active region 11165. Comparing region was a small active region (NOAA active region 195 A˚ EUVI-A and AIA 211 A˚ observations, we have been able to identify various dynamic structures that 2 http://solar.jhuapl.edu/Data-Products/COR-CME- temporally and spatially correlate between the data sets Catalog.php and which together enable an investigation of the corona 10 O’Kane et al.

Figure 5. (Left) EUVI/COR1 image indicating the slice used to create the stack plot. (Right) EUVI-A 195A(lower)˚ and COR1-A (upper) stack plots. The height-time profile of the stealth CME observed on 3rd March 2011 can be determined from the COR1-A data which show the cavity underside (concave-up structure) of the stealth CME. Exponential (red line) and quadratic (blue line) fits have been applied to the COR1-A data. in the lead up to the time of the eruption. Around 17:00 the flare ribbons form at ∼09:00 UT on 02-Mar-2011, UT on 01-Mar-2011, a filament is observed to begin to with a second phase of brightening and expansion away rise and gradually erupt over approximately a 6-hour pe- from the centre of the active region at ∼21:40 UT on riod. It is noted that the CME produced by this filament 02-Mar-2011. This location and evolution indicates the eruption is observed in the COR1-A field of view approx- occurrence of magnetic reconnection in a region above imately 23 hours before the stealth CME. The filament the active region loops. A flow of plasma is seen moving eruption creates new loops that connect the eastern side out from the south of active region 11165 from ∼08:35 of the filament channel with the west side of active re- UT on 02-Mar-2011, and a second flow follows at ∼13:55 gion 11165. The AIA data show that from ∼05:00 UT UT on 02-Mar-2011. The visible end of the second flow on 02-Mar-2011, a number of loops at the periphery of appears to be immediately followed by a structure that is the active region 11165 begin to reconfigure. On the east again almost parallel to the limb, stretching across from side of the active region, a loop is observed to have been the north to the south, and expanding outwards from disconnected from an area in the northern part of the ac- ∼17:50 UT on 02-Mar-2011 (Figure7, middle panel). tive region. It then swings up and over the active region Fainter flows consequently move outwards for a short pe- in an anti-clockwise direction at ∼05:15 on UT 02-Mar- riod from ∼22:10 UT on 02-Mar-2011. New loops slowly 2011. This structure is seen in EUVI data to be almost begin to form in the early hours of 03-Mar-2011 and, parallel to the solar limb and in motion at ∼06:10 UT given their close association with the CME observed in on 02-Mar-2011. The loop expands with a north-south STEREO-A COR1 data and EUV dimmings, they are motion and is shortly followed by the creation of a new, termed post-eruption loops. These post-eruption loops larger-scale loop system in the north of the active re- grow larger over a 6-hour period. Combined EUVI-A gion. These new loops are observed in both the AIA and 195 A˚ images and COR1-A images at different times EUVI-A data (Figure7, left-hand panel). The activity show that a bulb-shaped structure with a roughly circu- observed in active region 11165 using EUV data also in- lar centre is observed to move out of the EUVI-A images cludes the formation of a pair of faint flare ribbons and into COR1-A images where it becomes the stealth CME their associated loops, that are located at the edge of the studied here (Figure7, right-hand panel). The clear cir- magnetic bipole away from the internal polarity inver- cular cavity present in the center of the CME seen in sion line. AIA 211 A˚ and AIA 304 A˚ data indicate that Stealth CMEs from active regions 11

Figure 6. Top panel: HMI Magnetograph. A small active region (NOAA 11165) is located in the center of the image, with a polarity inversion line in a north-south direction. A second polarity inversion line runs in a east-west direction, where a filament channel is present. Bottom panel: MGN-processed SDO/AIA 211A˚ image. Active region NOAA 11165 is located in the center of the image. The filament channel runs in a east-west direction, south-east of the active region. A filament lies to the west of the active region. The red X represents the triangulated region (Pevtsov et al. 2011). The two blue X’s represent the footpoints of the structure as derived from the GCS model the COR1-A is suggestive of a flux rope configuration 11165 (Figure8a), which both match with those found at this time. by Nitta and Mulligan(2017), and are not too far from Running difference images and running ratio images where the footpoints estimated from the GCS model with 30 minute temporal separation revealed two faint were found to be. We can therefore conclude that these dimming regions located either side of active region two dimmings are representative of the footpoints of the 12 O’Kane et al.

MGN EUVI−A 195Å Running difference EUVI−A 195Å EUVI−A 195Å 02−Mar−11 18:00:30 02−Mar−11 11:25:30UT 02−Mar−11 17:45:30UT COR1−A 03−Mar−11 05:00:00UT 1000 0 0

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−1000 −1000 −3000 −1400 −1200 −1000 −800 −600 −1400 −1200 −1000 −800 −600 −400 −3000 −2000 −1000 0 X (arcsec) X (arcsec) X (arcsec) COR2−A GCS model 03−Mar−11 07:54:00UT COR2−B GCS model 03−Mar−11 07:54:29UT

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Figure 7. Top : (Left) MGN processed EUVI-A 195A˚ image. An expansion of bright loops were observed to the north of the active region, indicated by the arrow. (Center) Time difference EUVI-A 195A˚ image, with a 30-minute temporal separation. A structure was observed stretching from the north to the south, and traveling outwards from the solar disk, indicated by the red arrows. (Right) Combined EUVI-A 195A˚ and COR1-A image. The combination illustrates how the bulb-structure in the EUVI images prior to the eruption expands into the CME structure observed in the COR1 images, outlined by the two dotted lines. Bottom : The wire frame fitted to the CME in COR2-A (Left) and COR2- B (Right). erupting stealth CME. As can be seen from Figure8b, pulled and twisted out at ∼05:15 UT on 02-Mar-2011, the dimming region to the north-west of the active re- indicating a connection between the structure involved gion both grew in size and moved away from the active in the dynamics prior to the CME and the erupting field. region between 00:00 UT and 09:30 UT 03-Mar-2011, Looking at the photospheric evolution of the region, whilst the dimming region to the east of the active re- NOAA active region 11165 began to emerge on the Sun gion grew in size initially, and then shrunk back down on 25 February 2011 in the eastern hemisphere and into in size. There is an indication that the dimmings under- the magnetic field of a previously decayed active region. went a slight clockwise rotation between 00:00 UT on Active region 11165 emerged at the polarity inversion 03-Mar-2011 and 09:30 UT on 3-Mar-2011. It is notable line of the decayed pre-existing region and with the same that the eastern dimming region is located in the same field orientation (positive leading magnetic field). At area as the footpoint of the dynamical structure that the time of the stealth CME on 3 March 2011, the ac- Stealth CMEs from active regions 13

Running Ratio; 03−Mar−11 09:30:07 UT Plain; 03−Mar−11 09:30:07 UT 100 09:30:07

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Figure 8. (Left) AIA 211A˚ Running ratio image at the ﬁnal time of EUV dimming tracking. (Right) AIA 211A˚ plain image with inverted colour table showing the evolution of the dimming regions north and east of the active region, outlined with contours. The dimming regions evolved over a 9 hour period. Both images are de-rotated to the start time; 00:00 UT 03-Mar-2011

Figure 9. Left : Image at 13:55 UT showing the 50, 70 and 90 percent 150 MHz radio contours superimposed on the AIA 211 A˚ image. Right : The flux profile at 150 MHz tive region had a bipolar configuration and very dis- above which the CME originated (as determined from persed magnetic field having been acted on by super- dimmings and post-eruption loops) was oriented in a granulation. Two episodes of flux emergence occurred in north-south direction, indicating that differential rota- active region 11165 between its first appearance on disk tion had not yet had a significant effect on the active and the time of the stealth CME, at 22:40 UT on 25- region’s configuration (Figure6). To the south of the Feb-2011 and 06:30 UT on 28-Feb-2011. The evolution active region lies a polarity inversion line that is asso- of the photospheric field is dominated by flux emergence ciated with an (empty (Pevtsov et al. 2011)) filament rather than flux cancellation. The polarity inversion line channel. This inversion line was initially thought to be 14 O’Kane et al. the location of the origin of the stealth using triangula- ering the duration of the stealth CME lift-off, with no tion (Pevtsov et al. 2011). low-frequency emission observed using the full-disc in- Lastly, we looked at the radio emission of the region. tegrated radio spectrometers. A future imaging spec- The second flow observed in EUV data at ∼13:55 UT on troscopy task for the new, high sensitivity radio inter- 02-Mar-2011 coincides with a brightening in radio fre- ferometers like the Low Frequency Array (LOFAR) (van quencies around 150 MHz. The brightening was imaged Haarlem et al. 2013) and the upcoming Square Kilome- by the Nan¸cay Radioheliograph (Kerdraon and Delouis tre Array (SKA). 1997b) most prominently at 150 MHz between 13:52 and 3.2.2. Overall Remarks 14:02 UT (Figure9). The emission arises from a source that appears to the south of active region NOAA 11165, The combined analysis of the activity in the lower as viewed in the plane of the sky. This spatially and corona as observed on disk and at the limb, and the temporally corresponds to the second flow observed in CME propagation as seen in coronagraph data together EUV data. The impulsive nature of the radio emis- indicate that the stealth CME of 3 March 2011 origi- sion implies that particle acceleration occurs in conjunc- nated in active region 11165. The MGN technique en- tion with this second flow of plasma that is seen around hanced subtle changes in the evolution of plasma emis- 13:55 UT. Assuming second harmonic plasma emission, sions structures consistent with changes in the magnetic as the polarisation is less than 10%, the 150 MHz emis- field configuration of the active region. EUV dimmings sion corresponds to an altitude of 0.34 solar radii (238 at the periphery of the active region, the lack of open- Mm) using the Newkirk coronal density model (Newkirk ing of the active region arcade field and the side-on 1961). The emission is not observed above 173 MHz view afforded by the STEREO spacecraft reveal that by the Nan¸cay Radioheliograph, restricting it to these the erupting structure originated at a relatively high al- higher coronal altitudes. This coincides with the lack titude above the core active region loops that dominated of structure observed in the EUVI stack plots being a the EUV emission. The lack of significant flux cancella- result of a high altitude structure, supporting a hypoth- tion in the active region also suggests that the scenario esis that the stealth CME was ultimately the result of of low altitude flux rope formation and eruption of van a reconfiguration of high-altitude magnetic field, above Ballegooijen and Martens(1989) does not occur here. the active region core, that may have involved magnetic In the hours leading up to the stealth CME, active reconnection. The faint radio emission is not visible at region 11165 and the surrounding corona undergo some lower wavelengths below 150 MHz, detected by full-sun notable activity. First is the eruption of a filament im- spectrometers, so we cannot confirm whether the emis- mediately adjacent to the active region on its western sion is a type III burst, caused by propagating electron side. The filament eruption reconfigures the coronal beams (e.g. Reid and Ratcliffe(2014)), or localised elec- field and produces new connections between the filament tron acceleration more in-line with a type I burst (Kai channel and active region 11165. Such an eruption may et al. 1985). have altered the corona above the active region. In ad- Normal CMEs can have a multitude of accompany- dition, prior to the stealth CME, activity was also in ing radio emission, particularly from the upper solar active region 11165 itself. The formation of new EUV corona. The stealth CMEs do not have any accompa- loops from ∼05:00 to ∼06:00 UT on 02-Mar-2011, with- nying type II radio emission. The slow speed of stealth out any observed flux emergence at this time, are sug- CMEs means that we do not expect it to drive a shock, gestive of coronal reconnection, which may have played where shock-driven acceleration can generate type II ra- a role in creating the pre-eruptive field. dio emission. Faster, more intense CMEs can also dis- 4. DISCUSSION play moving type IV emission (James et al. 2017), gen- Both events were determined to be Earth directed erated via gyrosynchrotron emission by trapped, high- using triangulation and graduated cylindrical shell energy particles within the CME. Normal CMEs that modelling. MGN image processing was applied to have associated flares are frequently accompanied by SOHO/EIT, SDO/AIA, and STEREO/EUVI data. As type III bursts, signatures of accelerated electron beams shown by Alzate and Morgan(2017), such an approach escaping the Sun. Given the apparent high altitude of revealed subtle changes in the coronal emission struc- the stealth CMEs, if any electron beams are accelerated tures that were not identifiable in the unprocessed data. during the magnetic instability that initiates the CME For example, within the EUVI-A processed images, sev- liftoff, we might expect to detect faint, lower frequency eral structures were seen moving outwards from the solar (<100 MHz) type III emission. The emission that we limb, the timings of which correlated to dynamic struc- observe on March 3rd is very faint, and brief consid- tures observed in the sharpened AIA images as seen on Stealth CMEs from active regions 15

Table 2. Table summarising the timeline of the activity evolution related to the two stealth CME events.

Event 1: 27 October 2009 Event 2: 3 March 2011 Previous adjacent eruption 11:25 UT 26 October 2009 17:00 UT 1 March 2011 Flares and/or ribbons 09:00 UT 2 March 2011 21:40 UT 2 March 2011 Slow rise phase ∼23:00 UT 26 October 2009 Flares and/or ribbons ∼04:49 UT 27 October 2009 09:00 UT 2 March 2011 ∼07:05 UT 27 October 2009 21:40 UT 2 March 2011 Fast rise phase onset 13:00 UT 27 October 2009 – Dimming onset – 00:00 UT 3 March 2011 Post-eruption loop formation onset 13:00 UT 27 October 2009 00:00 UT 3 March 2011

disk and the white light CMEs seen in the coronagraph that extended polarity inversions lines should be present data. Indeed, the enhanced EUV images were able to in stealth CME regions. We note that we do not see such reveal several observational phenomena that are in line extended size scales in either of our stealth CMEs. In with the CSHKP standard model of an eruption, includ- light of this we will go on to analyse further the source ing dimming regions and post-eruption arcades. Such region characteristics and discuss them in the context of lower coronal signatures were observed in both stealth CME models in general. CME events and indicate that they originated in NOAA Even though both stealth CMEs originated in active active regions 11029 and 11165, respectively, contrary to regions without extended polarity inversion lines, they previous work that has suggested stealth CMEs might were both formed in a magnetic field configuration that in some way be associated with open magnetic field re- was extended in altitude, as found by Robbrecht et al. gions, the quiet sun, or empty filament channels. Our (2009); D’Huys et al.(2014); Alzate and Morgan(2017). findings support previous work by Alzate and Morgan For example, STEREO data for the 27 October 2009 (2017) who showed imaging processing techniques are event showed that the underside of the erupting struc- essential in searching for the origins of CMEs that leave ture (as determined from the concave-up feature) was no obvious signatures on disk. clearly visible at 0.5R above the photosphere during The identification of low coronal signatures associated its slow rise phase, with the transition to the fast rise with the CSHKP model enables us to not only identify phase occurring at ∼1R . These values are similar to the source region of stealth CMEs, but also to analyse those found in Robbrecht et al.(2009). Such a high al- the evolution of each region in the time leading up to titude means that the erupting structure originated in the eruption for comparison with CME theories. Models a region with lower plasma density and weaker mag- that have specifically been suggested as being relevant netic field than is usually found for active region CMEs. to stealth CMEs are those of streamer blowout CMEs The eruptions did not originate in the core active region (Howard and Harrison 2013) that invoke differential ro- magnetic field of NOAA active regions 11029 and 11165 tation as the mechanism that energises the magnetic that is responsible for the dominant active region EUV field system to bring it to an eruptive state (Vourlidas or soft X-ray emission. This high altitude location in and Webb 2018). In this scenario, the pre-eruptive mag- turn leads to CMEs that have low accelerations due to netic field configuration could be that of an arcade or the low magnetic field strength. a flux rope. In the streamer blowout numerical model For both events COR1-A stack plots show the prop- of Lynch et al.(2016), the pre-eruptive magnetic field agation of the underside of the stealth CMEs, not the configuration is that of a sheared arcade energised via leading edge of the erupting structure. However, it was photospheric shearing motions within a multipolar field not possible to identify the underside of each erupting configuration. The shearing motions lead to breakout structure in the EUVI-A stack plots, presumably due to reconnection above the central arcade followed by flare weak plasma emission in the 195A˚ waveband associated reconnection within the sheared arcade, which forms a with a low plasma density due to their high-altitude. It flux rope and accelerates the CME. The key role of pho- is notable that when the stealth CME of 27 October 2009 tospheric flows in the above mentioned model implies is in its slow-rise phase (that is, before the reconnection 16 O’Kane et al.

Figure 10. Top : Annotated GOES curve for the time leading up to the stealth CME of 27th October 2009. Active region 11029 was the only active region on disk at this time. The dashed lines represent C-class flares from AR 11029. The light blue shaded region represents the time of the slow rise phase of the stealth CME and the light red shaded region represents the CME fast rise phase. Bottom : Annotated GOES curve for the time leading up to the stealth CME of 3rd March 2011. There were multiple active regions in the northern hemisphere during this time, which contribute to the GOES light curve. The light red shaded region represents the stealth CME fast rise phase, the slow rise phase was not captured. associated with the fast-rise phase sets in) a flux rope is with a concave-up structure, indicating the presence of already present. The flux rope is identified through the a flux rope, during the fast rise phase. This is expected concave-up structure seen in STEREO coronagraph im- since regardless of the pre-eruptive configuration, flare ages and this observation is supportive of a pre-eruptive reconnection within a sheared arcade will always build flux rope having formed. Both events show a clear cavity a flux rope. Stealth CMEs from active regions 17

The challenge now is to try and discern whether any come from active regions with a short polarity inversion aspects of the evolution of NOAA active region 11029 line oriented in the north-south direction, and therefore that produced the 27 October 2009 event can be linked not significantly acted on by differential rotation. How- to the formation of the flux rope prior to its slow-rise ever, the active regions in which both events originated phase. There is increasing observational support for the exhibited flux emergence, which could have been the importance of the role of magnetic reconnection in the process by which the energy was injected. formation of eruptive structures. Observationally this The question raised by this study is then whether is manifested by (confined) flaring or flux cancellation stealth events represent the high-altitude part of a spec- that is able to transform a sheared arcade into a flux trum of CMEs, related to flux rope formation by high- rope. The height of the reconnection then determines altitude magnetic reconnection. Structural changes oc- the height at which the underside of the flux rope is cur above the core field of both active regions, and located from photosphere/chromosphere (Chintzoglou we suggest that the stealth CMEs originate from mag- et al. 2015) into the corona (e.g. (James et al. 2017; netic field whose footpoints are embedded either side of James et al. 2018)). SDO/HMI and SOHO/MDI images each active region. The vertical extent of each eruptive of both stealth CME source regions in this study showed structure presents additional challenges in terms of re- no major or sustained flux cancellation in the time lead- constructing the magnetic field configuration from pho- ing up to eruption, nor were any S-shaped plasma emis- tospheric magnetic field extrapolations and numerical sion structures observed in the the AIA, EIT or EUVI modelling to capture the formation of the structure. passbands that may have indicated the formation of a The availability of 195 A˚ data from both SOHO/EIT flux rope that could then have risen above the active re- and STEREO/EUVI for the first stealth CME event gion core. However, NOAA active region 11029 showed means that it presents an interesting case study for in- weak flaring in the hours leading up to the stealth CME vestigating the possible role instrument capabilities play, (table 2, top panel figure 10). The flaring will have been as highlighted by Howard and Harrison(2013). De- due to reconnection in the corona and this reconnection spite applying the MGN image processing techniques to could have produced the flux rope in the corona. Even SOHO EUV data, little could be observed in comparison though there is no observational support for the presence to the STEREO EUV data. Observational limitations of a pre-eruptive flux rope for the stealth CME event of are clearly shown in this case, as more features could be 3rd March 2011, it is notable that flaring and flare rib- distinguished in the STEREO/EUVI data. Considera- bons are observed in NOAA active region 11165 in the tion should therefore be given to temperature response, hours leading up to the eruption. Both stealth CME dynamic range and image cadence during operation in source regions show asimilar evolution in this regard. the development of future EUV imagers. The key as- Previous studies of stealth CMEs have suggested that pect is to design telescopes that are able to detect CMEs they may be sympathetic eruptions triggered by a re- with weak signatures in EUV. This study supports the configuration of overlying field, and therefore removal of growing focus on the so-called middle corona and the stabilising flux, due to a nearby CME. Indeed, the 27 need for instrumentation that can capture the evolu- October 2009 event had an eruption from a nearby re- tion of structures with faint EUV emission. In the gion 11.5 hours before its slow-rise phase was observed, future, with more appropriate instrumentation, what and the event of 3 March 2011 had an adjacent eruption would have been classed as a stealth CME in the SOHO 31 hours before its CME-related dimming was observed or SDO era may no longer be the case. In addition, we (table 2, bottom panel figure 10). In the absence of ex- find that the side-on view provided by STEREO was tended polarity inversion lines and sustained shearing crucial in identifying, and studying the evolution of, the motions, a sympathetic eruption suggests the presence stealth CME source regions that were challenging to ob- of a pre-existing flux rope in quasi-equilibrium (Török serve from above due to the dominance of emission from et al. 2011), which is what we find for the event of 27 the active region core. Nonetheless, investigating what October 2009. causes these events to have such weak signatures, but A fundamental aspect of CMEs is that they are known still produce magnetic structures that escape the Sun to be the result of an energy storage and release process remains to be investigated further, and will aid over- (see Green et al. 2018, for an overview). As discussed all understanding of the physical processes involved in above, theoretical and modelling work on stealth CMEs CME initiation. has proposed that the energy injection is provided by slow shearing motions created by photospheric differential rotation. However, in this study the stealth CMEs 5. CONCLUSIONS 18 O’Kane et al.

This study used advanced image processing techniques JRO thanks the STFC for support via funding given to identify and study the source regions of two stealth in her PHD studentship. JRO also thanks Mathew West CMEs that were observed in multi-spacecraft corona- and Marilena Mierla for useful discussions at the Royal graph data on 27 October 2009 and 3 March 2011. We Observatory Belgium. LMG acknowledges support find that both stealth CMEs originated in active region through a Royal Society University Research Fellowship areas as opposed to the quiet sun or in filament channels, and through the Leverhulme Trust Research P‘roject contrary to the previous studies on these stealth events Grant 2014-051. DML acknowledges support from the (Kilpua et al. 2014; Pevtsov et al. 2011). However, the European Commission’s H2020 Programme under the erupting structures were not formed in the core active following Grant Agreements: GREST (no. 653982) and region field, but likely at altitudes of ∼0.5R above the Pre-EST (no. 739500) as well as support from the Lever- photosphere. The energy injection appears not to be hulme Trust for an Early-Career Fellowship (ECF-2014- the result of differential rotation but instead related to 792) and is grateful to the Science Technology and Facil- the emergence of new flux into the active region. In the ities Council for the award of an Ernest Rutherford Fel- event of 27 October 2009 we find observational support lowship (ST/R003246/1). HASR acknowledges support for the presence of a flux rope formed by reconnection from the STFC consolidated grant (ST/P000533/1). in the corona during or before the slow rise phase of the The authors thank the anonymous referee whose com- CME. The flux rope may have been destabilised as a ments helped to improve the paper. SDO is a mission sympathetic eruption following a nearby CME. We find of NASAs Living With a Star Program. STEREO is that the stealth CMEs of this study are no different to the third mission in NASAs Solar Terrestrial Probes other CMEs in that they show features of the standard program. SOHO is a mission of international coopera- model but at the lower energy end of the spectrum with tion between ESA and NASA. The authors thank the weaker signatures that current instrumentation can only SDO, STEREO, and SOHO teams for making their data just resolve. publicly accessible.

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