IRISSi iv LINE PROFILES: AN INDICATION FOR THE PLASMOID INSTABILITY DURING SMALL-SCALE MAGNETIC RECONNECTION ON THE SUN

Abstract

Our understanding of the process of fast reconnection has undergone a dramatic change in the last 10 years driven, in part, by the availability of high-resolution numerical simulations that have consistently demonstrated the break-up of current sheets into magnetic islands, with reconnection rates that become independent of Lundquist number, challenging the belief that fast magnetic reconnection in flares proceeds via the Petschek mechanism that invokes pairs of slow-mode shocks connected to a compact diffusion region. The reconnection sites are too small to be resolved with images but these reconnection mechanisms, Petschek and the plasmoid instability, have reconnection sites with very different density and velocity structures and so can be distinguished by high-resolution lineprofiles observations. Using IRIS spectroscopic observations we obtain a survey of typical line profiles produced by small-scale events thought to be reconnection sites on the Sun. Slit-jaw images are used to investigate the plasma heating and re-configuration at the sites. A sample of 15 events from two active regions is presented. The line profiles are complex with bright cores and broad wings extending to over 300 km s −1 . The profiles can be reproduced with the multiple magnetic islands and acceleration sites that characterise the plasmoid instability but not by bi-directional jets that characterise the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.

Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere

“…The transition region spectra associated with these events show profiles that are common in classical explosive events (Innes et al 2015). We see profiles that are strongly Doppler shifted to the blue or red (Figure 3), as well as profiles that are more centered around 0kms −1 with very broad wings up to 100 kms −1…”

“…We see evidence for substantial heating to transition region temperatures leading to highly broadened, non-Gaussian Si IV profiles observed with IRIS. Such profiles had previously been associated with reconnection mediated by the plasmoid instability based on idealized numerical simulations (Innes et al 2015). This has now been placed on a solid footing by the direct observational evidence of tiny blob-like features, and by advanced numerical simulations of UV bursts that show remarkable similarities to the observations.…”

“…The synthetic Si IV profiles are narrower than the observed ones, likely because the numerical resolution of the current simulation is not high enough to capture the multitude of plasmoids that in more idealized simulations (Innes et al 2015), with much higher resolution leading to bright and broader spectral profiles. This is supported by numerical experiments in which the resolution of the current simulation is increased by a factor of two.…”

“…These profiles are non-Gaussian and often show triangular-shaped profiles, which Innes et al (2015) and Guo et al (2017) have suggested are caused by reconnection driven by the plasmoid instability. The spatial pattern of these strongly broadened explosive event type profiles in Si IV is illustrated in Figure 3, which shows a multitude of non-Gaussian, flat-topped or triangular-shaped profiles, sometimes centered around 0kms −1 (yellow), as well as strongly blueshifted and redshifted profiles in the vicinity.…”

Observations of Multiple Blobs in Homologous Solar Coronal Jets in Closed Loop

“…Such intermittent magnetic plasmoids or blobs created by magnetic reconnection have also been observed in the large-scale current sheet (CS) driven by CMEs and the smallscale CS associated with chromospheric jets (Asai et al, 2004;Lin et al, 2005;Takasao et al, 2012;Singh et al, 2012;Kumar & Cho, 2013;Lin et al, 2015). They are generally explained by the tearing-mode instability (TMI) of the thin CS where a series of magnetic islands are recurrently created during magnetic reconnection (Furth et al, 1963;Drake et al, 2006;Bárta et al, 2008;Innes et al, 2015), which makes electron acceleration more efficient and dynamic (Kliem et al, 2000). Recently, multidimensional, MHD numerical simulations of TMI have been significantly improved, thanks to the rapid advancement of powerful supercomputers.…”