Three-phase Evolution of a Coronal Hole. II. The Magnetic Field

Abstract

Abstract We investigate the magnetic characteristics of a persistent coronal hole (CH) extracted from EUV imagery using Heliospheric and Magnetic Imager filtergrams over the period 2012 February–October. The magnetic field, its distribution, and the magnetic fine structure in the form of flux tubes (FTs) are analyzed in different evolutionary states of the CH. We find a strong linear correlation between the magnetic properties (e.g., signed/unsigned magnetic field strength) and the area of the CH. As such, the evolutionary pattern in the magnetic field clearly follows a three-phase evolution (growing, maximum, and decaying) as found from EUV data (Part I). This evolutionary process is most likely driven by strong FTs with a mean magnetic field strength exceeding 50 G. During the maximum phase they entail up to 72% of the total signed magnetic flux of the CH, but only cover up to 3.9% of the total CH area, whereas during the growing and decaying phases, strong FTs entail 54%–60% of the signed magnetic flux and cover around 1%–2% of the CH’s total area. We conclude that small-scale structures of strong unipolar magnetic field are the fundamental building blocks of a CH and govern its evolution.