The initiation of a solar streamer blowout coronal mass ejection arising from the streamer flank

Coronal mass ejections (CMEs) appear as bright transient features propagating in white-light coronagraph observations, and are generally believed to be the most violent eruptions in the solar system. CMEs release large amounts of magnetized plasma and energy from closed magnetic fields in the inner corona, which can drive space weather near the Earth. Hence, the origin and the initiation of CMEs are key to predicting potential space weather catastrophes. Currently, popular CME-triggering mechanisms include the breakout model (Antiochos et al. 1999) in which the sheared core fields inflate and reconnect with the overlying arcades (see Chen et al. (2016) for latest observations on breakout reconnection), the tether-cutting model (Moore et al. 2001) in which the reconnections between sheared arcades build and release a flux rope, and MHD flux-rope instabilities such as the torus/kink instability models (Török et al. 2004; Kliem & Török, 2006; Chen et al. 2007; Kliem et al. 2010). The details of the initiation mechanisms for CMEs can be found in some comprehensive reviews (Forbes et al. 2006; Chen 2011; Shibata & Magara 2011; Aulanier 2014; Janvier et al. 2015).

During the propagation from the inner corona to interplanetary space, CMEs usually encounter and interact with streamers that are tracers of the global coronal magnetic field configuration and the interfaces between opposite-polarity open fluxes (Low 1994; Chen et al. 2010). As a particular class of CMEs, streamer blowout (SBO) CMEs were first named by Sheeley et al. (1982) and characterized by a gradual swelling of the overlying streamer and an embedding flux-rope structure. SBO CMEs have a slow average speed of $\sim$390 km s^-1, and are long-lived events with an average duration of 40.5 hr, according to the statistics results of 909 events in 1996–2015 (Vourlidas & Webb 2018). SBO CMEs do not correlate with sunspot number and dominate during solar minimum. The characteristics suggest that SBO CMEs arise from extended polarity inversion lines (e.g., quiet-Sun and polar crown filaments) away from active regions (ARs), and are triggered by flux-rope instabilities or magnetic reconnections that release magnetic energy accumulated by differential rotation (Vourlidas & Webb 2018).

Successful eruptions arising from the streamer flank compose a special subset of CMEs in which the erupting core field containing a flux-rope structure gives rise to an SBO CME. If the eruption from the streamer flank is on a small scale, the CME will appear as a narrow streamer puff that was named by Bemporad et al. (2005). It has been suggested that the SBO CME arising from the streamer flank requires a special magnetic configuration consisting of three independent lower arches and the overlying large streamer arcade, and its initiation always involves the external and internal magnetic reconnections of the erupting fields (Bemporad et al. 2005; Moore & Sterling 2007). On the other hand, due to the initial location under one streamer leg, the strong influence of the streamer open fields can make a successful eruption experience a nonradial movement before traveling out along the radial streamer (Jiang et al. 2009; Yang et al. 2012). This nonradial trajectory of the eruptions of high-latitude filaments naturally arises from the multipolar flux systems, and the deflection toward the streamer belt (heliospheric current sheet) has been confirmed by the numerical simulations (Török et al. 2011; Zuccarello et al. 2012; Lynch & Edmondson 2013.

However, the evolution of SBO CMEs in the lower corona is rarely reported, and the initiation mechanism of SBO CMEs still remains unresolved, which is likely due to observational limitations. Here we report a SBO CME on 2019 August 12-13 that arose from the streamer flank with clear observational signatures of magnetic reconnections, and sheds light on the initiation mechanism for SBO CMEs.

The SBO CME was well captured in the east limb by the Large Angle and Spectrometric Coronagraph (LASCO; Brueckner et al. 1995) C2 and C3. The filament eruption in the lower corona, as the precursor to the CME, was clearly recorded by observations from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012). We mainly employ the AIA passbands of 193 Å (Fe XII, $\sim$1.6 MK), 171 Å (Fe IX, $\sim$0.6 MK), and 304 Å (He II, $\sim$0.05 MK). Each AIA image covers the solar full disk up to 0.5 $R_{\odot}$ above the limb, with a pixel resolution of 0.$"$6 and a cadence of 12 s. In addition, we also check the filament eruption in the Extreme Ultraviolet Imager (EUVI; Howard et al. 2008) on board the spacecraft of Solar-Terrestrial Relations Observatory (STEREO: Kaiser et al. 2008) ahead (-A) of the Earth.

We also capture the faint trajectories of the rising filament and the enveloping coronal structures using the persistence mapping technique (Thompson & Young 2016) in which the value at each pixel is set as the maximum at this same pixel among a set of images. The erupting structures are also identified by the processed normalized-intensity images, in which the pixel intensity at the center time of every two hours is normalized by values at the same pixel in the 2 hr sequential images. The magnetic field lines are extrapolated with the potential field source surface (PFSS; Schrijver & De Rosa 2003) package SolarSoftWare. The kinematics of the filament and enveloping coronal structures are obtained by the time-slice approach.

The white-light observations from LASCO captured a bright CME (Figure 1), and the PFSS extrapolated field lines confirm the configuration of a streamer enwrapping three sets of arches beneath, and a polar crown filament lies under the north arches in the streamer flank (Figure 2). As the precursor to the CME, the filament eruption in the lower corona is well recorded in AIA images. The filament rose slowly toward the solar equator with a writhing motion, and the ascent speed of the enveloping fields accelerated from $\sim$3 to $\sim$8 km s^-1 (Figure 3). The obvious signatures, a gradual swelling of the overlying streamer for $\sim$19 hr and a well-structured flux rope embedding in the CME, are consistent with the characteristics of typical SBO CMEs (Vourlidas & Webb 2018).

Interestingly, the SBO CME originated from a polar crown filament that located in the streamer flank far from ARs. In the special configuration, the overlying streamer is strong enough to make the erupting core rise nonradially (Jiang et al. 2009; Yang et al. 2012), and the nonradial movement can lead the filament fields to encounter and interact with neighboring coronal fields. The initiation mechanism for an eruption arising from the streamer flank was suggested to involve external and internal reconnections 15 yr ago (Figure 3 in Bemporad et al. 2005), but the observational evidence of external reconnections is rarely reported, to our knowledge. In a few cases of the similar eruptions arising from the streamer flanks (Moore & Sterling 2007; Yang et al. 2012) only coronal dimmings were found as the indirect signature of external reconnections. In this Letter, the external reconnections between the rising filament fields and coronal loops beneath are supported by the deformation of the bottom shape of the enveloping flux rope from the smooth concave to the dip (the red arrow in Figure 4(a)-(b)), the very slow inflows ($\sim$1.8 km s^-1) that make the filament fields and coronal loops beneath get close to each other (Figure 4(c)), and the persistent brightenings around the interface between the filament fields and coronal loops beneath (red arrows in bottom panels of Figure 4). The signatures of associated coronal dimmings were too weak to identify with both SDO and STEREO-A’s perspectives, which is possibly because of the weakness of the eruption. Following external reconnections, the filament finally erupted, and there appeared a Y-shaped structure implying a detachment of the stretched erupting core and the post-eruption loops in the filament source (blue and green arrows in bottom panels of Figure 4) as a result of internal reconnections of the stretched filament fields.

Both external and internal reconnections occurred. What did the external reconnection contribute to the successful release of the SBO CME? Jiang et al. (2009) reported that the eruption of an AR filament from the streamer flank directly blew the large streamer arcades open, and formed a narrow CME without external reconnections. It likely indicates that enough energy from the erupting core fields is the key to break through the large closed arcade of the overlying streamer, and AR filaments always store enough magnetic energy. The event duration is one important parameter for SBO CMEs, and follows a trend toward long durations for less energetic events. For this case, the short duration of 19 hr indicates an energetic event. Moreover, the polar crown filament exhibited a writhed inverse-$\gamma$-shaped structure during the ascent, which implies an amount of accumulated free energy and a kink instability before the rising. Kink instability can be crucial for triggering an eruption (Török et al. 2004). However, the rising speed of filament fields remained slow ($\sim$3 km s^-1) during the writhing motion (panels (b) and (j) in Figure 3) and began to accelerate to a higher value ($\sim$8 km s^-1; dotted lines in Figure 3) nearly simultaneously with the beginning of the inflows (vertical dashed lines in Figures 3 and 4). This likely indicates the kink instability is not the key to trigger the eruption, and external reconnection plays an important role in the initiation of the SBO CME.

Therefore, based on extrapolated PFSS field lines (Figure 2) and AIA observations (Figure 3-4), we propose a possible scenario for the initiation of the SBO CME in the schematic (Figure 5). In the pre-eruption magnetic configuration (panel (a)), the streamer consists of open fields (green and red lines) and a large arcade (blue loops), and three lower arches (pink, black, and orange) underneath the large streamer arcade. The northernmost arches (pink) involve the rising filament fields rising from the steamer north flank. The open fields root in the north positive polar region and in the south negative polar region, respectively, checked by magnetograms from the SDO. From north to south, the polarities are positive, negative, positive, and negative (green pluses and red minuses). During the ascent (panel (b)), because of the guidance of the north open fields, the northernmost rising filament fields move southward and interact with the southernmost arches (orange). The interaction contributes to the external reconnections (the red cross) of the filament fields, and the external reconnections results in a newly formed central arch (the red loop) and a curved large arcade (the cyan loop). Due to the decrease of the confinement, the filament rises faster, the opposite field lines of the stretched filament tail quickly get closer, and internal reconnections (the green cross) occur. Consequently, the filament erupts, accompanying with the formation of a closed flux-rope structure (the pink circle) detached from the solar surface and a lower arch (the green loop) at the filament source (panel (c)). Moreover, after the complete detachment of the filament fields from the solar surface, the streamer arcades are stretched sequentially, and then another reconnection occurs between the approaching legs of streamer arcades, which transfer the flux into the CME from below. On the other hand, through the slow lift of the filament fields, the open fields are slowly blown out, and the outer layer of the large arcades becomes open into the solar wind, serving as an indicator of the flux loss from above (panels (b)-(c)).

The scenario of the configuration and evolution of the slow SBO CME are similar to that in Figure 3 of Bemporad et al. (2005). However, the pre-eruption magnetic topology (Figure 5(a)) of three polarity inversion lines (yellow dotted lines) and four closed flux systems (blue, pink, black, orange) is exactly the multipolar flux distribution of the magnetic breakout CME initiation scenario (Antiochos et al. 1999) that demands at least one coronal null point at the intersection of these distinct flux systems. The associated separatrix boundaries and the overlying coronal null point are indicated by the black dashed lines and the red circle, respectively, in Figure 5(a). This overlying null point will be distorted and stretched out into a current sheet when the pink arcade is energized with the presence of sheared or twisted filament channel fields. That current sheet will facilitate reconnection between the various flux systems. In fact, the external reconnection proposed by Bemporad et al. (2005) is essentially the breakout reconnection noted by Antiochos et al. (1999), and the internal or tether-cutting reconnection in Moore et al. (2001) has a similar topology to the flare reconnection underneath the erupting core in the standard CSHKP model (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976).

All the results show that the initiation for the SBO CME rising from the streamer flank involves both external and internal reconnections. Are the external reconnections necessary for this SBO CME? There is an open question about how long this internal reconnection proceeds slowly (as a result of flux cancellation or converging flows) and builds up twist and energy in the flux-rope prior to the transition to the fast reconnection that causes the main acceleration and eruption. If there is no external reconnection, the polar crown filament will keep rising slowly, and accumulate much more magnetic energy via the differential rotation at a longer duration, and lead to internal reconnection at a higher altitude. As a result, the following CME likely leave weak or no low coronal signatures, making it a stealth CME (Robbrecht et al. 2009; Lynch et al. 2016) that is likely a subset of SBO CMEs. Hence, we suggest that external reconnections are important rather than necessary to induce the eruption onset in advance. Better observations of SBO CMEs are necessary to understand their initiation mechanisms.