Magnetic Reconnection at the Earliest Stage of Solar Flux Emergence

Abstract

On 2016 September 20, the Interface Region Imaging Spectrograph observed an active region during its earliest emerging phase for almost 7 hours. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory observed continuous emergence of small-scale magnetic bipoles with a rate of ∼10 16 Mx s −1 . The emergence of magnetic fluxes and interactions between different polarities lead to frequent occurrence of ultraviolet (UV) bursts, which exhibit as intense transient brightenings in the 1400Å images. In the meantime, discrete small patches with the same magnetic polarity tend to move together and merge, leading to enhancement of the magnetic fields and thus formation of pores (small sunspots) at some locations. The spectra of these UV bursts are characterized by the superposition of several chromospheric absorption lines on the greatly broadened profiles of some emission lines formed at typical transition region temperatures, suggesting heating of the local materials to a few tens of thousands of kelvin in the lower atmosphere by magnetic reconnection. Some bursts reveal blue and red shifts of ∼100 km s −1 at neighboring pixels, indicating the spatially resolved bidirectional reconnection outflows. Many such bursts appear to be associated with the cancellation of magnetic fluxes with a rate of the order of ∼10 15 Mx s −1 . We also investigate the three-dimensional magnetic field topology through a magneto-hydrostatic model and find that a small fraction of the bursts are associated with bald patches (magnetic dips). Finally, we find that almost all bursts are located in regions of large squashing factor at the height of ∼1 Mm, reinforcing our conclusion that these bursts are produced through reconnection in the lower atmosphere.

A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating. III. 3D Simulations and Atmospheric Response

“…So, our results demonstrate that, in cancelling regions, a wide spectrum of ejections can be produced, with a wide range of different temperatures and sizes associated with Ellerman bombs, UV bursts and IRIS bombs and other hot and cool ejections (Yang et al 2013;Vissers et al 2015;Reid et al 2016;Rutten 2016;Nelson et al 2017;Rouppe van der Voort et al 2017;Young et al 2018;Tian et al 2018;Chen et al 2019;Guglielmino et al 2019;Ortiz et al 2020;Tiwari et al 2019;Huang et al 2019;Madjarska 2019). The predicted spatial offsets and time delays for hot and cool ejections are in accordance with observational findings.…”

A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating

“…Three key roles are played by the interaction distance, which may be written as 19 Mx for B 0 = 100 G. These computed maximum reconnection heights are consistent with those from magnetic field extrapolations for chromospheric bursts (e.g., Chitta et al 2017a;Tian et al 2018).…”

Solar Ultraviolet Bursts

“…The null point of the fan-spine system was found to be at 500 km, and the reconnection driving the UV burst was suggested to be due to shearing at the null point, driven by the motion of the MMF. Zhao et al (2017) and Tian et al (2018b) combined vector magnetogram data from HMI with a MHD relaxation technique (Zhu et al 2013(Zhu et al , 2016 to investigate the magnetic environment of UV bursts. Zhao et al (2017) distinguished UV bursts associated with bald patches and those with flux cancellation.…”