Blowout Surge due to Interaction between a Solar Filament and Coronal Loops

Magnetic reconnection is generally accepted as a key mechanism for the rapid release of magnetic energy and the energy conversion (Priest & Forbes 2000). In 2D models, anti-parallel magnetic field lines approach each other in a current or at a magnetic null point, and reconnect to form new field lines (Hirayama 1974; Shibata 1999; Priest & Forbes 2000; Shen et al. 2012a). Therefore, magnetic energy can be converted into thermal and kinetic energy in the plasma, and the release of magnetic energy even results in large-scale events (Zhu et al. 2014; Reeves et al. 2015; Li et al. 2016; Xue et al. 2016; Yang et al. 2017; Zheng et al. 2017). An interaction between an erupting filament and a coronal field is a common reconnection phenomenon in the solar atmosphere, which is revealed by previous observations (Zhu et al. 2014; Li et al. 2015, 2016; Reeves et al. 2015). Zhu et al. (2014) studied the interaction between an erupting solar filament and a nearby coronal hole, where one leg of the filament opened because of the interchange reconnection. They also see enhanced brightening and bi-directional flows caused by magnetic reconnection, which is thought to facilitate filament eruption. Intermittent plasma outflows and blobs occurring between filament the magnetic field and the overlying field may contribute to the fast eruption of the filament (Reeves et al. 2015). In addition, high-resolution observational studies indicate that magnetic reconnection is also evidenced between the internal structures of solar prominence (Shen et al. 2015).

To our knowledge, magnetic reconnection is also responsible for the production of solar jet/surge. In the process of reconnection, transient, impulsive, collimated flows of plasma are observed as bright (dark) features at high (low) temperatures, which often originates from a compact quasi-circular base. Generally, the hot and cool ejective events are referred to as jet and surge, respectively (Canfield et al. 1996). As an extension of the model proposed by Moore et al. (2010, 2013) to explain the jets, some observations suggest that the jets are caused by magnetic reconnection between the erupting filament and the ambient field (Sterling et al. 2015, 2016). The presence of filaments in jet source regions and their eruption during jet formation have been observed in many blowout jet events (Shen et al. 2011, 2012b; Li et al. 2015, 2017; Sterling et al. 2015, 2016; Hong et al. 2017).

As in large-scale events, there is some observational evidence that intermittent and fragmented reconnection plays a role in the dynamics of jet/surge events. Zhang et al. (2016) have reported multiple blobs in homologous solar coronal jets in a closed-field corona. Many brightening blobs originating from the junction between the jet spire and the arch-base are observed in the evolution processes of the jet (Zhang & Zhang 2017). The bright blobs are possibly magnetic islands that occur in the process of magnetic reconnection due to the tearing-mode instability (Furth et al. 1963; Kliem et al. 2000; Asai et al. 2004; Lin et al. 2005; Ni et al. 2012; Kumar & Cho 2013; Wyper et al. 2016). Recently, tearing and the associated formation of magnetic islands have been performed in 3D simulations of jets in the closed-field corona (Wyper et al. 2016). Wyper & DeVore (2016) previously adapted the kink-instability model (Pariat et al. 2009) to simulate the jets occurring in closed coronal loops.

In total, many observations and theoretical models basically describe and explain the evolution of the jet. However, due to the complexity of the coronal field, there are few works that understand the role of reconnection in the process of the jet formation. In this paper, we present a confined eruption of a filament with a fan-spine configuration on 2015 February 6. The reconnection between the eruption filament and the outer spine-like loops gives rise to a blowout surge, in which a bi-directional flow is believed to be the reconnection evidence.

The filament eruption is well covered by observations from Solar Dynamics Observatory (SDO; Pesnell et al. 2012) and Big Bear Solar Observatory (BBSO). For the SDO, the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) multi-wavelength images and the Helioseismic and Magnetic Imager (HMI; Schou et al. 2012) data are chosen to study the event. UV-EUV images have cadences of 24–12 s and a pixel size of 06, while HMI magnetograms have a pixel size of 05 and a cadence of 45 s. Hα line-center data is also obtained by BBSO. Its images have cadences of 60 s and a pixel size of 1''. X-ray images are reconstructed with the CLEAN algorithm using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al. 2002).

We observe a filament and the associated surge-like eruption of the filament material under a fan-spine configuration (Figure 1). At the edge of active region (12277), a very small filament is located nearly along the polarity inversion line (PIL). The magnetic field under the filament shows the obvious feature of a small negative polarity field encircled by the surrounding positive field (Figure 1(b)). The development of the filament results in a surge-like eruption accompanied with a remote brightening, as shown in Hα image (Figure 1(c)). Figure 1(d) displays the active region in the 94 Å channel after the flare onset (at 23:47 UT). We can see the connectivity highlighted by coronal loops within the active region, known as fan-spine topology (Figure 1(d)).

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In Figure 2, we show a slow rising of the filament and the subsequent surge erupting along spine-like loops. By superimposing the HMI magnetic field on an Hα image, one can see that the filament is located above the PIL, between the positive polarity and the negative polarity (Figure 2(c1)). It has an apparent length of only 3.5 × 10⁴ km and a width of about 2 × 10³ km. In the AIA 211 Å image, the filament is surrounded by some overlying loops, which likely connect the remote region of the fan-spine configuration (Figure 2(b)). In Figure 2(c2), a precursor brightening (denoted by a red ellipse) appears near the center of the filament at 22:53 UT, implying that magnetic reconnection has taken place and the filament has become unstable. After the activation, the filament makes a rising movement toward the northwest, indicated by the green arrow (Figures 2(a2)–(a3)). Some parts of the filament plasma are heated and become bright, as can be seen in the 304 Å image (Figure 2(a3)). As the rising filament collides with the outer spine-like field, the filament begins to change its eruption direction, extending along the outer spine-like loops (nearly westward) indicated by the purple arrow (Figure 2(c4)). Meanwhile, a circular ribbon (Figure 2(c3)) develops along nearly the entire PIL (highlighted by a purple dotted line), which may be related to null-point reconnection in the fan-spine magnetic topology (Antiochos 1990; Lau & Finn 1990; Wang & Liu 2012; Zhang et al. 2015; Hong et al. 2017; Li et al. 2017). Finally, the filament eruption is followed by a cool surge, which displays a curtain-like structure and grows to reach a width of ∼3 × 10⁴ km and a length up to ∼12 × 10⁴ km at 23:11 UT (as shown in Figure 2(c4)). A two-ribbon flare also appears at the surge base. The flare appears underneath the erupting filament (as shown in Figure 2(c4)), in agreement with classical flare models (Hirayama 1974; Shibata 1999). In the figure, the hard X-ray source is essentially right on top of the flare ribbons. In order to understand the relationship between the flare and the eruption, we present the height-time measurements of the filament eruption and the subsequent surge plasma, overlying them with a GOES flux curve (in Figure 2(d)). The average speeds of the rising filament and the eruptive surge are 60 km s⁻¹ and 140 km s⁻¹, respectively. We note that the GOES flux enhancement is simultaneous with the filament eruption in Figure 2(d). The flux increases (becomes enhanced) at one rate during the slow filament rise, and then at a markedly higher rate during the eruption.

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During the growth of the surge, the rare observational evidence and features for magnetic interaction between the erupting filament and the outer spine-like field are observed clearly. As the filament approaches the outer spine-like loops, some new small hot loops are observed. Figures 3(a1)–(a3) present the formation of these loops, "L3," at 94 Å, which correspond to a high temperature of about 7 MK. The formation of hot loops implies that the enveloped field of the filament reconnects with the outer spine-like field. Then the filament magnetic field is cut in the middle, as shown in the 304 Å images, and the two halves find new connections (Figures 3(b2)–(b3)). The northern end of the filament is connected with positive polarity somewhere below the fan surface, while the other end of the filament is opened to the outer spine-like field. This evolution is more easily seen in the animated version of Figure 3, available online. The scenario of the reconnection is described as below: The filament and the loops involved in the reconnection are labeled "L1" and "L2," respectively, as shown in Figure 3(b1). When the reconnection between the filament, "L1," and the overlying field, "L2," occurs, some new closed loops, "L3," and open field lines, "L4," are formed in Figure 3((a2); denoted by red dotted lines). The connectivity of the loops, "L3," is from the negative polarity N1 to the positive P1, while "L4" is from the another positive P2 to the remote negative polarity.

As the filament collides with the outer spine-like loops, the junction between them is heated by reconnection and becomes brightened (Figure 3(b2)). Some material, in the form of small bright blobs, emanates from a common site flowing along two directions, streaming up along the outer spine-like field and down along the eastern fan-like field (Figures 3(b2) and (b3)). The directions of this motion are denoted by yellow arrows in Figure 3(b2). In 1600 Å especially, a bright blob near the surge cusp is observed clearly, moving from the reconnection site along the direction of the surge eruption denoted by a white arrow (Figures 3(c2)–(c3)). To further inspect the dynamics of the bi-directional flow, we make spacetime plots (a slice marked by the red dashed line and arrow in Figure 3(b2)) constructed from AIA images in 304, 171, and 335 Å and display them in Figures 4(a)–(c). The flows in both the inward and outward directions are indicated as some streaks, with velocities derived by linear fits in Figure 4. The projected velocities of the upward flows have an average value of ∼130 km s⁻¹, while the inward flows is about ∼92 km s⁻¹. The former is faster than the latter, which is in line with previous observations (Liu et al. 2013; Zhu et al. 2014). This difference may be caused by the higher density of the surge base, resulting in a decelerated motion of the inward material.

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We find that a small-scale filament eruption is one origin of a blowout surge. The AIA 94 Å observation suggests that the filament could be enveloped by a 3D field with a fan-spine topology. Under a fan dome, the unstable filament slowly rises and expands to the northwest to collide with the outer spine-like loops. When the filament reconnects with the outer spine-like field, the plasma moves in the general direction of the surrounding coronal magnetic field (to the west), forming a surge with a wider spire. This is similar to the other flux rope/filament eruptions (Yang et al. 2012; Li et al. 2017), in which the erupted mass is trapped in closed coronal loops. In addition, the eruption is accompanied by a two-ribbon flare at the surge base and a remote brightening around the far-reaching end of the large loops.

This event provides rare observational evidence for magnetic interaction between an erupting filament and the outer spine-like field. In particular, small bright blobs appear at the reconnected position and propagate up along the outer spine-like field and down along the eastern fan-like field. In 1600 Å, we also observe a brightening plasma blob near the cusp of the surge. Based on the VAL-C model (Vernazza et al. 1981; Fontenla et al. 1993), the magnetic reconnection process between the filament and the coronal field may easily lead to the appearance of magnetic islands in the upper chromosphere (Bhattacharjee et al. 2009). According to recent numerical simulations of the jets (Yang et al. 2013; Ni et al. 2015), the magnetic islands are considered to be bright features with a higher temperature and density. Therefore, the observed blobs are likely to be the magnetic islands that relate to the tearing-mode instability (Furth et al. 1963; Kliem et al. 2000; Asai et al. 2004; Lin et al. 2005; Ni et al. 2012; Kumar & Cho 2013; Wyper et al. 2016). Li et al. (2016) observed an X-type structure that formed when the erupting filament encountered the loops. As parts of the filament in the current sheet gradually dispersed and disappeared, the magnetic topological structure of the filament will be changed and the filament field reconnected to its nearby coronal loops. Zhang & Zhang (2017) suggest that magnetic reconnections inside a jet not only heat and transfer the material, but also alter the magnetic topological structure. a similar magnetic reconnection scenario of jets is proposed by Zeng et al. (2016), who reported that reconnection occurred between the emerged magnetic flux and the unipolar field in a small-scale chromospheric jet. Bi-directional flows were observed across the separatrix regions, suggesting that the jet was produced due to magnetic reconnection. Another recent observations of the trains of plasma blobs in a recurrent jet are reported by Zhang et al. (2016). In our case, near the cusp of the surge the intermittent flows and bright blobs move along the field lines, suggesting that the magnetic reconnections between the filament and coronal field lines are in process. After that, the filament structure may be changed and the filament field reconnect with the surrounding loops.

We find that after the magnetic reconnection, the material contained by the filament is ejected along the outer spine-like loops, forming a surge. One leg of the filament becomes radial and the material in it ejects, while another leg forms new closed loops. We suggest that the erupting filament maintains successive reconnection near the cusp of the surge. Due to the reconnection that occurs between closed, high-density loops carried by the filament and the outer spine-like field, and low-density flux tubes, some plasma flows are mainly driven by strong thermal/magnetic pressure imbalances (Del Zanna et al. 2011).

In a standard blowout jet model proposed by Moore et al. (2010, 2013), there exists a filament that is involved in the reconnection and eruption. A magnetic reconnection between the filament and the open field lines is an essential ingredient for jet occurrence. In most cases, however, the magnetic situation is too dynamic and chaotic to determine whether the interaction between the filament and nearby open field lines is the primary trigger of the jetting. It is possible that most jets develop fast movements, and the early motion of filament strands is partially obscured by foreground material. It needs more case studies to confirm this point.

We thank an anonymous referee for providing detailed suggestions and comments that significantly improved the presentation of this paper. We are grateful to the AIA, HMI, and BBSO teams for data support. This work is supported by the Natural Science Foundation of China (11633008, 11333007, 11403098, 11503081, 11573012, 11503082, and 11473065), the CAS "Light of West China" Program, Foundation of Sichuan University of Science and Engineering grant 2015RC43, and QYZDJ-SSW-SLH012.