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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).

Coronal holes; Solar wind

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