Relation Between Coronal Hole Areas on the Sun and the Solar Wind Parameters at 1 AU

We analyze the relationship between the coronal hole (CH) characteristics on the Sun (area, position, and intensity levels) and the corresponding solar wind parameters (solar wind speed v, proton temperature T, proton density n, and magnetic field strength B) measured in situ at 1 AU with a 6-h time resolution. We developed a histogram-based intensity thresholding method to obtain fractional CH areas from SOHO/EIT 195 Å images. The algorithm was applied to 6-h cadence EIT 195 Å images for the year 2005, which were characterized by a low solar activity. In calculating well-defined peaks of the solar wind parameters corresponding to the peaks in CH area, we found that the solar wind speed v shows a high correlation with correlation coefficient cc=0.78, medium correlation for T and B with cc=0.41 and cc=0.41. No significant correlation was found with the proton density n. Applying an intensity-weighted CH area did not improve the relations, since the size and the mean intensity of the CH areas are not independent parameters but strongly correlated (cc=− 0.72). Comparison of the fractional CH areas derived from GOES/SXI and SOHO/EIT and the related solar wind predictions shows no systematic differences (cc=0.79).