Average solar active region: I. Intensities, velocities, and the photospheric magnetic field

TL;DR

This study constructs an average bipolar solar active region by ensemble averaging observational data, revealing typical evolution patterns of magnetic flux, velocities, and emissions, thus providing insights into AR development mechanisms.

Contribution

It introduces a novel ensemble averaging method to analyze the typical evolution of bipolar ARs, normalizing data to identify general trends beyond individual variability.

Findings

Average AR shows flux emergence, peak activity, and decay phases.

Leading polarity remains coherent longer than trailing polarity.

Enhanced atmospheric heating occurs in later AR phases.

Abstract

Context: Solar active regions (ARs) are key manifestations of the Sun's magnetic activity, displaying diverse spatial and temporal characteristics. Their formation and evolution play a crucial role in understanding the solar dynamo and space weather. While individual ARs exhibit significant variability, ensemble averaging offers a method to extract their typical properties and evolution. Aims: This study aims to construct an average bipolar AR using ensemble averaging of observational data. By normalizing ARs in space and time, we seek to identify general trends in the evolution of magnetic flux, velocity fields, and atmospheric emissions, providing insights into the underlying physical mechanisms governing AR development. Methods: We analysed a sample of bipolar ARs observed by the Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly aboard the Solar Dynamics Observatory. The ARs were selected based on strict criteria, ensuring clear polarity separation and emergence within 60 degrees of the solar central meridian. Normalisation procedures were applied to align ARs spatially and temporally before computing an ensemble average of various observables, including line-of-sight magnetograms, Dopplergrams, and multi-wavelength intensity maps. Results: The average AR exhibits a well-defined evolutionary pattern, with flux emergence followed by peak activity and subsequent decay. The leading polarity retains coherence longer than the trailing one, consistent with previous studies. Surface flow maps revealed a diverging outflow near the emergence site before the emerged AR is clearly visible in magnetograms. Atmospheric emission variations indicate enhanced heating above the AR in later phases, possibly due to persistent reconnection events. The ensemble averaging approach highlights systematic features of AR evolution that are often obscured by individual-case variability.