Low-Latitude Coronal Holes at the Minimum of the 23rd Solar Cycle

TL;DR

This study analyzes low-latitude coronal holes during the 2007 solar minimum, revealing larger areas, persistent features, and a complex magnetic field structure with dominant multi-pole components compared to the 1996 minimum.

Contribution

It provides a detailed comparison of coronal hole characteristics and magnetic field structures between two solar minima, highlighting the increased multi-pole dominance in 2007.

Findings

Larger coronal hole areas in 2007 compared to 1996.

Presence of five persistent coronal holes over multiple rotations.

The 2007 minimum had a significantly weaker dipole component.

Abstract

Low and mid-latitude coronal holes (CHs) observed on the Sun during the current solar activity minimum (from September 21, 2006, Carrington rotation (CR) 2048, until June 26, 2009 (CR 2084)) were analyzed using {\it SOHO}/EIT and STEREO-A SECCHI EUVI data. From both the observations and Potential Field Source Surface (PFSS) modeling, we find that the area occupied by CHs inside a belt of $\pm 40^\circ$ around the solar equator is larger in the current 2007 solar minimum relative to the similar phase of the previous 1996 solar minimum. The enhanced CH area is related to a recurrent appearance of five persistent CHs, which survived during 7-27 solar rotations. Three of the CHs are of positive magnetic polarity and two are negative. The most long-lived CH was being formed during 2 days and existed for 27 rotations. This CH was associated with fast solar wind at 1 AU of approximately 620$\pm 40$ km s$^{-1}$. The 3D MHD modeling for this time period shows an open field structure above this CH. We conclude that the global magnetic field of the Sun possessed a multi-pole structure during this time period. Calculation of the harmonic power spectrum of the solar magnetic field demonstrates a greater prevalence of multi-pole components over the dipole component in the 2007 solar minimum compared to the 1996 solar minimum. The unusual large separation between the dipole and multi-pole components is due to the very low magnitude of the dipole component, which is three times lower than that in the previous 1996 solar minimum.