3-Phase Evolution of a Coronal Hole, Part I: 360{\deg} remote sensing and in-situ observations

Abstract

We investigate the evolution of a well-observed, long-lived, low-latitude coronal hole (CH) over 10 solar rotations in the year 2012. By combining EUV imagery from STEREO-A/B and SDO we are able to track and study the entire evolution of the CH having a continuous 360$\deg$ coverage of the Sun. The remote sensing data are investigated together with in-situ solar wind plasma and magnetic field measurements from STEREO-A/B, ACE and WIND. From this we obtain how different evolutionary states of the CH as observed in the solar atmosphere (changes in EUV intensity and area) affect the properties of the associated high-speed stream measured at $1$AU. Most distinctly pronounced for the CH area, three development phases are derived: a) growing, b) maximum, and c) decaying phase. During these phases the CH area a) increases over a duration of around three months from about $1 \cdot 10^{10} \mathrm{km}^{2}$ to $6 \cdot 10^{10} \mathrm{km}^{2}$, b) keeps a rather constant area for about one month of $> 9 \cdot 10^{10} \mathrm{km}^{2}$, and c) finally decreases in the following three months below $1 \cdot 10^{10} \mathrm{km}^{2}$ until the CH cannot be identified anymore. The three phases manifest themselves also in the EUV intensity and in in-situ measured solar wind proton bulk velocity. Interestingly, the three phases are related to a different range in solar wind speed variations and we find for the growing phase a range of $460-600$~km~s$^{-1}$, for the maximum phase $600-720$~km~s$^{-1}$, and for the decaying phase a more irregular behavior connected to slow and fast solar wind speed of $350-550$~km~s$^{-1}$.