Light Bridge in a Developing Active Region. I. Observation of Light Bridge and its Dynamic Activity Phenomena

TL;DR

This paper investigates the structure and dynamic activity phenomena of a light bridge in a developing active region using multi-instrument solar observations, revealing magnetic reconnection and magnetoconvective processes as key drivers.

Contribution

It provides detailed observational insights into the magnetic and dynamic properties of a light bridge in a developing sunspot, linking activity phenomena to magnetic reconnection and convection.

Findings

Light bridge has a weak, horizontal magnetic field transported by convective upflow.

Recurrent chromospheric brightenings are caused by magnetic reconnection.

Dark surges are cool plasma ejections from reconnection regions.

Abstract

Light bridges, the bright structures that divide the umbra of sunspots and pores into smaller pieces, are known to produce wide variety of activity events in solar active regions (ARs). It is also known that the light bridges appear in the assembling process of nascent sunspots. The ultimate goal of this series of papers is to reveal the nature of light bridges in developing ARs and the occurrence of activity events associated with the light bridge structures from both observational and numerical approaches. In this first paper, exploiting the observational data obtained by Hinode, IRIS, and Solar Dynamics Observatory (SDO), we investigate the detailed structure of the light bridge in NOAA AR 11974 and its dynamic activity phenomena. As a result, we find that the light bridge has a weak, horizontal magnetic field, which is transported from the interior by large-scale convective upflow and is surrounded by strong, vertical fields of adjacent pores. In the chromosphere above the bridge, a transient brightening occurs repeatedly and intermittently, followed by a recurrent dark surge ejection into higher altitudes. Our analysis indicates that the brightening is the plasma heating due to magnetic reconnection at lower altitudes, while the dark surge is the cool, dense plasma ejected from the reconnection region. From the observational results, we conclude that the dynamic activity observed in a light bridge structure such as chromospheric brightenings and dark surge ejections are driven by magnetoconvective evolution within the light bridge and its interaction with surrounding magnetic fields.