The Formation of an Atypical Sunspot Light Bridge as a Result of Large-scale Flux Emergence

Abstract

We use a combination of full-disk data from the Solar Dynamics Observatory and high-resolution data from the Dunn Solar Telescope (DST) to study the formation, structure, and evolution of an atypical light bridge (LB) in a regular sunspot. The LB results from the emergence of magnetic flux with one footpoint rooted in a pore outside the parent sunspot that appears about 17 hr before the LB. The pore has a polarity opposite to that of the sunspot and recedes from it at a speed of about 0.4 km s−1. This is accompanied by the development of an elongated magnetic channel in the outer penumbra that triggers the formation of the LB when it reaches the inner penumbral boundary. The LB is a nearly horizontal structure with a field strength of about 1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85 km s−1 along its entire length. The emergence of the LB leads to dynamic surges in the chromosphere and transition region about 13 minutes later. We derive the photospheric and chromospheric structure of the LB in the DST data from spectral line parameters and inversions of He i at 1083 nm, Si i at 1082.7 nm, Ca ii IR at 854 nm, and H α at 656 nm and speckle-reconstructed imaging at 700 and 430 nm. The LB shows an elongated filamentary shape in the photosphere without lateral extrusions. The thermal inversion of Ca ii IR reveals the LB to be about 600–800 K hotter than the umbra. Different sections of the LB are elevated to heights between 400 and 700 km. Our results indicate that LB formation is part of a flux emergence event with the LB envelope reaching a height of about 29 Mm before dissolving after about 13 hr. We conclude that the existence of persistent, large-scale photospheric blueshifts in LBs is the most likely criterion for distinguishing between flux emergence events and overturning convection in field-free umbral intrusions.

How rare are counter Evershed flows?

“…The lifetimes of the CEFs found in the literature range from 1 hr to approximately 12 hr (e.g., Louis et al 2014;Siu-Tapia et al 2018;Louis et al 2020). Figure 5 presents the histogram of the lifetimes of the CEFs from our study.…”

“…This Evershed flow (see Solanki 2003 for a review) was detected more than a century ago (Evershed 1909). In the past decade, there have been a few reports of peculiar flows directed toward the umbra (Kleint & Sainz Dalda 2013;Louis et al 2014;Siu-Tapia et al 2017;Guglielmino et al 2017Guglielmino et al , 2019Louis et al 2020). These socalled counter Evershed flows (CEFs) have also been observed in one magnetohydrodynamic (MHD) simulation (Siu-Tapia et al 2018).…”

“…Counter Evershed flows studied in the literature have been found in complex active regions (ARs). Recently, Louis et al (2020) reported the appearance of a light bridge associated with a CEF. Common to these works, each of which analyzed only one sunspot, is that CEFs were described as "anomalous," "unusual," or "atypical" flows.…”

Heating of the solar chromosphere in a sunspot light bridge by electric currents

“…Only the two scans at 20:44 UT and 20:55 UT were used here. The DST setup and data are described in detail in Louis et al (2020), hereafter LBC20.…”

Homologous flaring activity over a sunspot light bridge in an emerging active region

“…3g). If the homologous nature of the flares were the result of small-scale loops emerging in the LB and reconnecting with the overlying umbral field (Tian et al 2018), one would expect blue-shifts either in localized patches or along the body of the LB (Louis et al 2020). With HMI's native spatial resolution of ∼320 km, we do not find evidence for opposite-polarity fields above uncertainties within the LB, though we do not exclude the possibility of smallscale velocity and magnetic inhomogeneities having been present (Louis et al 2009(Louis et al , 2015aLouis 2015).…”