Plasmoid-mediated reconnection in solar UV bursts

Abstract

Context. Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection. Aims. To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere. Methods. We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma-β at the height of this X point. Results. The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes. Conclusions. Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-β, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.

Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO

“…There, following flux emergence the minority polarity cancels with the majority polarity of opposite sign (Chitta et al 2017a). Another recent reconnection modelling shows a similar situation as drawn in our Figure 15, which is found to lead to a bi-directional jet (Peter et al 2019b).…”

Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere

“…4.1 for discussion. Pontin et al 2013;Cheung et al 2015;Chitta et al 2017b;Peter et al 2019). This is similar to the case discussed in Fig.…”

ALMA observations of transient heating in a solar active region

“…Numerical simulations have shown that the magnetic topology that determines the height of reconnection along with the line of sight to the reconnection site play a role in the visibility of these small-scale heating events across the wavelength spectrum (Hansteen et al 2019;Peter et al 2019;Ortiz et al Movie associated to Fig. 5 is available at https:// www.aanda.org 2020; Syntelis & Priest 2020).…”

“…For example, strong emission in the wings of Hα would imply reconnection in the lower atmosphere, whereas bright Mg ii h and k, Si iv 1393.7, 1402.8 Å or even extreme-ultraviolet (EUV) emission would originate in higher layers. Other key signatures such as recurrence, intermittency, and large flow velocities may be caused by the release of fast-moving sequences of magnetic islands (or plasmoids) and jets (e.g., Karpen et al 1995;Shibata & Tanuma 2001;Bhattacharjee et al 2009;Innes et al 2015;Ni et al 2015;Rouppe van der Voort et al 2017;Peter et al 2019), while turbulence has also been shown to potentially play a role in triggering fast reconnection in MFs (e.g., Chitta & Lazarian 2020).…”