Magnetic reconnection is a key process in astrophysical and space plasmas that converts magnetic energy into other forms of energy. It drives gamma-ray bursts, solar flares, and accelerates particles. A fast reconnection is necessary to explain the explosive nature of flares and bursts. However, conditions for the onset of fast reconnection are a topic of intense debate. Simulations of magnetic reconnection in 2D have shown that the current sheet is unstable to the formation of magnetic islands or plasmoids that accelerate reconnection (Bhattacharjee et al. 2009). At the same time, high-resolution 3D simulations suggest the development of turbulence in the reconnection layer (Kowal et al. 2019), which drives fast reconnection (Lazarian & Vishniac 1999). To shed light on the trigger of fast reconnection, we exploited high-cadence spectroscopic observations of reconnection-driven microflares recorded by the Interface Region Imaging Spectrograph (IRIS) (De Pontieu et al. 2014).

Turbulent reconnection in a microflare

IRIS observed a microflare in the core of active region (AR) 12567 on 2018 July 18, at a high-cadence of 1.4 s, covering the whole event that lasted for about 4 minutes. This allowed us to investigate the plasma properties of the microflare at its onset. In particular, we used the Si IV emission line at 1403 Å to study the plasma properties. To this end, we computed the integrated intensity of the Si IV line and its excess line width (beyond its nominal thermal line width of 6.6 km s-1 at equilibrium formation temperature of 0.08 MK), as a function of time at each spatial pixel along the slit that crossed the center of the microflare.

Figure 1 – Microflare observed in the core of AR 12567. Top panels are IRIS SJI 2796 Å snapshots showing the AR core before and during a microflare event. The cyan coloured contour outlines the microflare loop system. The slanted red line marks a segment across the microflare used to determining the loop width. The dark vertical line is the location of the IRIS slit. Bottom panel shows the time series of Si IV 1403 Å integrated intensity (black) and the excess line width (determined by a single Gaussian fit to the Si IV spectral profiles; red) at the center of the loop system.

The microflare is seen as a 14 Mm long loop system in the AR core (top panels in Figure 1). The integrated Si IV line intensity shows more than two orders of magnitude fluctuation over the course of the microflare (Figure 1c). During the initial stages of the microflare when the intensity is still close to the background level, we observed broad Si IV emission lines, that exhibited persistent excess line widths in the range of 40 to 60 km s-1 (around 01:54 UT). These excess widths are comparable to local sound speed of about 50 km s-1 at 0.1 MK. Furthermore, these widths are a factor of 2 to 3 larger than those observed in active regions under quiescent conditions (De Pontieu et al. 2015). These excess widths should arise from either unresolved non-thermal motions or superimposed bulk plasma flows, with a broad distribution of line-of-sight velocities, confined to the microflaring loop. Thus, the observed broad emission lines characterize the turbulent flows in the loop system. The line widths further increase to more than 80 km s-1 followed by a rapid increase in intensity resulting in the observed microflare (after 01:56 UT).

Concurrent with the enhanced line widths, we observed that the spatial width of the microflaring loop also increases (Chitta & Lazarian 2020). This correlation suggests that the turbulent motions play a role in broadening of the loop. Furthermore, we found a close quantitative agreement between the observed spatial width of the loop and the observationally constrained width of outflow region as predicted by the theory of turbulent reconnection (Lazarian & Vishniac 1999).

Conclusions

We propose that the injection of turbulence into the reconnection layer increases the width of the outflow region. The latter increases the level of turbulence, establishing a positive feedback. Our observations point to a scenario of fast reconnection onset in a current sheet broadened by turbulence (also see Priest & Heyvaerts 1974 and Heyvaerts et al. 1977 for a discussion on the onset of turbulence in a current sheet). Overall, our observations are consistent with the theory of turbulent reconnection (Lazarian & Vishniac 1999). Future higher-resolution observations would be useful to understand the role of turbulent versus plasmoid reconnection at different stages of the development of various flares on the Sun.

Based on the recently published article by Chitta, L. P., & Lazarian, A. The Astrophysical Journal Letters (2020), DOI: https://doi.org/10.3847/2041-8213/ab6f0a

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