A new forecasting index Pch, which is a function of the intensity in the central area ([10°E,10°W], [30°N, 30°S], Figure 1) in SOHO EIT 284 Å images of the solar corona (Pch=Σ1/b, in which b is the brightness of the pixels inside the area), has been developed to predict the daily solar wind speed 3 days in advance.
The Pch factor reflects both the area and brightness effects of CHs. It shows that in periods of low CME activity the Pch factor can improve forecasting accuracy. The Pch index can eliminate an important bias in the forecasting process, which is present in the methods using CH area as input parameter due to the fact that the CH boundary cannot be unambiguously determined from observations, since Pch does not depend on the CH boundary determination (Luo et al., 2008).
Now the Pch index is given based on SDO/AIA images once a day, and the area of each coronal hole is given in percentage of the visible disk.