Influence of coronal hole morphology on the solar wind speed at Earth

Abstract

It has long been known that the high-speed stream (HSS) peak velocity at Earth directly depends on the area of the coronal hole (CH) on the Sun. Different degrees of association between the two parameters have been shown by many authors. In this study, we revisit this association in greater detail for a sample of 45 nonpolar CHs during the minimum phase of solar cycle 24. The aim is to understand how CHs of different properties influence the HSS peak speeds observed at Earth and draw from this to improve solar wind modeling. The characteristics of the CHs of our sample were extracted based on the Collection of Analysis Tools for Coronal Holes (CATCH) which employs an intensity threshold technique applied to extreme-ultraviolet (EUV) filtergrams. We first examined all the correlations between the geometric characteristics of the CHs and the HSS peak speed and duration at Earth, for the entire sample. The CHs were then categorized in different groups based on morphological criteria, such as the aspect ratio, the orientation angle and the geometric complexity, a parameter which is often neglected when the formation of the fast solar wind at Earth is studied. Our results, confirmed also by the bootstrapping technique, show that all three aforementioned morphological criteria play a major role in determining the HSS peak speed at 1 AU. Therefore, they need to be taken into consideration for empirical models that aim to forecast the fast solar wind at Earth based on the observed CH solar sources.