Numerous Bidirectionally Propagating Plasma Blobs near the Reconnection Site of a Solar Eruption

Magnetic reconnection is a fundamental process that is involved in various solar activities (e.g., Dere et al., 1991; Innes et al., 1997; Shibata et al., 2007; Tian et al., 2014a; Li et al., 2016a; Xue et al., 2016; Shen et al., 2017; Li et al., 2018a; Tian et al., 2018b, a; Huang et al., 2019; Srivastava et al., 2019; Hou et al., 2021b, a; Madjarska et al., 2022; Cheng et al., 2023; Wei et al., 2023). When a large-scale solar eruption occurs, the closed magnetic field is severely stretched out by the ejected structure. As a result, a current sheet generally forms, that is a narrow region across which the magnetic field changes rapidly (see Lin & Forbes, 2000; Priest & Forbes, 2002; Lin et al., 2003; Forbes et al., 2006). Magnetic reconnection can cause the magnetic field reconfiguration and the release of magnetic energy stored in the system beforehand. Many observational features related to the reconnection process have been observed (Lin et al., 2015), e.g., current sheets (e.g., Ciaravella et al., 2002; Reeves & Golub, 2011; Liu et al., 2013; Seaton et al., 2017; Warren et al., 2018; Chitta et al., 2021; Ding et al., 2024), inflows (e.g., Yokoyama et al., 2001; Lin et al., 2005; Narukage & Shibata, 2006; Li & Zhang, 2009; Savage et al., 2012), sunward outflows (e.g., Tian et al., 2014b; Sun et al., 2015), anti-sunward outflows (e.g., Wang et al., 2007; Chae et al., 2017), and bidirectional outflows (e.g., Savage et al., 2010; Su et al., 2013; Zhu et al., 2016; Yan et al., 2018). The plasma that surrounds the current sheets can be heated to temperatures of several or tens of millions of Kelvin (e.g., Su et al., 2013; Li et al., 2018b; Reva et al., 2023). The current sheets of solar eruptions may also experience transverse oscillations (e.g., Li et al., 2016b; Wang et al., 2022).

Theoretical studies and magnetohydrodynamic simulations have suggested that the onset of plasmoid instability plays a crucial role in fast energy releasing in current sheets (e.g., Bhattacharjee et al., 2009; Karpen et al., 2012; Ni et al., 2015a; Ye et al., 2019; Zhang et al., 2022). Plasma blobs (blob-like hot plasma) are believed to be an observable proxy of the occurrence of plasmoid instability in current sheets of solar eruptions. However, imaging fine structures within the current sheets of solar eruptions is very hard, especially using original intensity images. White-light observations have shown some anti-sunward propagating blobs appearing along current sheets (e.g., Ko et al., 2003; Lin et al., 2005; Liu et al., 2010; Song et al., 2012; Guo et al., 2013; Ling et al., 2014; Kwon et al., 2016; Schanche et al., 2016; Webb & Vourlidas, 2016; Lee et al., 2020). The white-light blobs usually are far away from the reconnection site and appear as diffuse structures with a width of $\geqslant$10 Mm in post coronal mass ejection (CME) current sheets. After the launch of the Solar Dynamics Observatory (SDO, Pesnell et al., 2012), the extreme ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly (AIA, Lemen et al., 2012) revealed sporadic small-scale plasma blobs propagating along current sheets of solar eruptions (Takasao et al., 2012; Kumar & Cho, 2013; Liu, 2013; Zhu et al., 2016; Gou et al., 2019; Dai et al., 2018; Lu et al., 2022; Kumar et al., 2023). Kumar & Cho (2013) reported the first simultaneous EUV and radio observations of several bidirectionally moving plasma blobs in a solar flare. Among recurring bidirectional outflows along the current sheet associated with a C2.0 flare, Zhu et al. (2016) found only two sunward propagating plasma blobs. Gou et al. (2019) found that anti-sunward moving plasma blobs merged into a larger blob that later evolved into a CME bubble. With the combination of EUV and white-light observations, Patel et al. (2020) carried out a statistical research on the bidirectionally propagating blobs during the post-impulsive phase of a long-duration solar eruption (Yu et al., 2020). They identified only 20 sunward plasma blobs and 16 anti-sunward ones in the enhanced EUV intensity images during a period of $\sim$2 hours. Recently, Kumar et al. (2023) reported a series of bidirectionally propagating plasmoids continuously released for about 30 minutes in a vertical flare current sheet behind an erupting flux rope. However, characterizing the role of plasmoid instability in the energy release of magnetic reconnection of solar eruptions is still a challenge.

In this study, we analyze multi-passband EUV images of a current sheet during an eruptive solar flare. Around the flare peak time, we identify more than one hundred compact, bidirectionally propagating plasma blobs along the current sheet, investigate their physical properties, and track their subsequent temporal evolution. We describe the observations in Section 2, present the analysis results in Section 3, and the discussion in Section 4. We summarize our findings in Section 5.

On November 19, 2022, a solar eruption was simultaneously detected by AIA on board SDO, the Solar Upper Transition Region Imager (SUTRI, Bai et al., 2023; Wang et al., 2023) on board the Space Advanced Technology demonstration satellite (SATech-01), the Hard X-ray Imager (HXI, Su et al., 2019; Zhang et al., 2019) on board the Advanced Space-Based Solar Observatory (ASO-S, Gan et al., 2019, 2023), and the Geostationary Operational Environmental Satellite (GOES). We mainly used the EUV images taken by AIA and SUTRI, to analyze the dynamics of the current sheet of the solar eruption. The AIA EUV images with different response temperatures have a pixel size of 0.6${}^{{}^{\prime\prime}}$ and a cadence of 12 s. The SUTRI 465 Å images capture the full-disk transition region at a temperature regime of $\sim$0.5 MK (Tian, 2017) and have a pixel size of 1.2${}^{{}^{\prime\prime}}$ and a cadence of 30 s. The SUTRI 465  Å and AIA 304  Å images were aligned using a linear Pearson correlation analysis. The AIA 1600 Å passband with a pixel size of 0.6${}^{{}^{\prime\prime}}$ and a cadence of 24 s, is also used to identify the flare ribbons of the solar eruption.

For this study, we reconstructed HXI image in the energy range of 20–30 keV with a pixel size of 2${}^{{}^{\prime\prime}}$ using a HXI Clean algorithm and the subcollimator groups G3 to G10. The HXI image provides a spatial resolution of $\sim$6.5${}^{{}^{\prime\prime}}$. The data observed 48 h after the imaging time is taken as background. Only the HXI 20–30 keV emisstion near 12:53 UT was reconstructed with a 20-second integration time, while the rest of the data did not have reliable quality. The images between HXI and AIA were aligned by using reference structures, such as flare ribbons and brightenings above flare loops.

For the identification of an associated CME of the solar eruption, we used the coronagraph images taken by the Large Angle Spectroscopic Coronagraph (Brueckner et al., 1995) on board the Solar and Heliospheric Observatory (SOHO, Thompson et al., 1998).

Previous studies of current sheets during solar eruptions mostly focus on limb observations in white-light and EUV passbands (e.g., Reeves & Golub, 2011; Savage et al., 2012; Liu, 2013; Yan et al., 2018; Cheng et al., 2018; Patel et al., 2020). Sporadic plasma blobs have been found in the post-CME current sheets (e.g., Ko et al., 2003; Lin et al., 2005; Liu et al., 2010; Song et al., 2012; Guo et al., 2013; Ling et al., 2014; Schanche et al., 2016; Chae et al., 2017; Lee et al., 2020). Here, we report a case study that reveals detailed evolution of many fine structures in the current sheet around the peak of an eruptive flare. In the EUV images, during about 24 minutes, at least 108 compact plasma blobs propagate bidirectionally from the reconnection site, which rises with a velocity of $\leqslant$69 km s^-1. In addition, we investigated the temporal variation of the blob number around the flare peak time, which has not been provided by the previous studies.

In general, direct observation of fine structures in a current sheet is very difficult due to the limited resolution of current instruments. In this study, we have identified more than one hundred compact plasma blobs with an average width of $\sim$1.7 Mm in a current sheet of a solar eruption from EUV observations. Previous AIA observations typically revealed only several plasmoids or blobs in current sheets of solar eruptions (Zhu et al., 2016; Dai et al., 2018; Gou et al., 2019). Patel et al. (2020) identified 20 sunward and 16 anti-sunward plasmoids using the enhanced AIA 131 Å images, but in about two hours during the post-impulsive phase of the solar eruption. French et al. (2019) also investigated the current sheet of this solar eruption using spectropolarimetric data. They found that small-scale magnetic field structures on scales of $\leqslant$6 Mm could be created by plasmoid formation during the reconnection process. These plasmoids are more diffuse and larger than the blobs identified in our study. With white-light observations, plasma blobs have also been identified in current sheets of solar eruptions (e.g., Guo et al., 2013; Lee et al., 2020; Patel et al., 2020), which are far away from the reconnection site as diffuse structures and two orders of magnitude larger than ours.

From the AIA 211 Å images, we have measured the physical parameters for the plasma blobs. The blob width is in the range of 0.8–3.0 Mm with an average value of 1.7 Mm, which is similar to those reported by Dai et al. (2018) and Gou et al. (2019), and smaller than those reported by Zhu et al. (2016) and Patel et al. (2020). In contrast, from off-limb observations, the widths of the current sheets of solar eruptions generally range from 10 Mm to about 100 Mm at heights of 0.27–1.16 solar radii from the solar surface (see Lin et al., 2015). The propagating velocity of the blobs is estimated to be 50–550 km s^-1 with an average value of 190 km s^-1, which is consistent with that from the EUV observations (Zhu et al., 2016; Patel et al., 2020). In addition, the width of the plasma blobs identified in a current sheet of solar eruption seems to follow a power law distribution (Guo et al., 2013; Patel et al., 2020) and has a correlation between the width and velocity (Patel et al., 2020). For the blob width in this study, however, we do not find a power law distribution or a correlation with other parameters, which might be due to that only small plasma blobs evolving for only several minutes in the low corona are identified.

What roles the fine structures within a current sheet play during the energy release process of solar eruptions is still an open question. Numerical simulations of plasmoid instability have attempted to investigate the fine structures inside current sheets and their temporal behavior (e.g., Ni et al., 2015a; Ye et al., 2019). The observational features in this study appear to be consistent with some results of the numerical models (Ye et al., 2019, 2020, 2021; Zhang et al., 2022; Ye et al., 2023). In the numerical studies conducted by Ye et al. (2019) and Ye et al. (2023), the sunward flows and the the anti-sunward ones only appear below and above the principal X-points, respectively. This agrees well with our observational feature in the time-distance diagram and the animation. In addition, we found that the temporal variation of the blob number is similar to that of the AIA 211 Å light curve obtained from the flare loop region during the solar eruption. The result from the power spectrum analysis in our study shows a similarity to that of the numerical studies by Ye et al. (2019, 2020). The blob size is also similar to that in the numerical model conducted by Zhang et al. (2022). Our observational results suggest that plasmoid instability may play a key role in the energy release processes of this solar eruption.

The bidirectional flows or plasmoids have also been identified at one side of the principle X-point in other numerical studies (e.g., Ni et al., 2015a, b). The discrepancy between our observational and the latter numerical results might be explained as the following. First, above the principal X-point, the rising current sheet may compensate for possible sunward flows/blobs. Second, the current sheet is a three-dimensional structure. The LOS superposition effect and the projection effect may impact the apparent motions of the outflows within the current sheet in the plane of sky.

In this study, we have reported an on-disk observation of numerous fine structures in a current sheet around the peak time of an eruptive flare. The current sheet appears in the low corona connecting the flare loops and the rising filament, and involves at least 108 compact plasma blobs propagating in both sunward and anti-sunward directions near the reconnection site. The width, duration, projected velocity, temperature, and density of these blobs are $\sim$1.7$\pm$0.5 Mm, $\sim$79$\pm$57 s, $\sim$191$\pm$81 km s^-1, $\sim$10^6.4±0.1 K, and $\sim$10^10.1±0.3 cm^-3, respectively. Using the velocity ratio of inflows and outflows associated with the reconnection process, the reconnection rate is estimated to be 0.27–0.33. In the meantime, the reconnection site rises with a velocity of $\leqslant$69 km s^-1. We have also analyzed the change of the blob number with time during the flare peak time, which varies in a way similar to that of the AIA 211 Å light curve obtained from the flare loop region. These observational results suggest that plasmoid instability plays an important role in the energy release process of solar eruptions.