Development of the autocorrelation function of the magnetic field in evolution of a solar active region

ABSTRACT Time variations of the autocorrelation function (ACF) of the magnetic field for an active region (AR), NOAA 13354, were investigated. A 5-d set of magnetograms was acquired with the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. No sudden response in ACF to an individual flare onset in the corona was revealed. We found that on scales below approximately 30–40 Mm, all ACFs during the time period from 2023 June 28 to 2023 July 02 follow the same exponential law with the slope of about −0.02. On scales larger than 40 Mm, the ACFs vary with time, showing a clear tendency of progressive enhancement inside an interval of about 40–80 Mm. The appearance and reinforcement of large-scale interactions are obvious. These changes in the ACFs are contemporary with the enhancement of complexity of the AR (towards a multipolar structure) and with the increase in the 3-d-average flare index. Such changes in the photosphere accompanying an increase in flare activity can be seen as evidence of the significant role of large-scale magneto-morphological reconstructions and connections in creating flare conditions in the corona.