Spencer: solar magnetic modulation of Earth’s cloud cover 10 times larger than TSI variation

“As I understand it, the [geomagnetic] Ap index is believed to be related to the level of cosmic ray activity reaching the Earth [which is modulated by the solar magnetic field]. …

If we do a scatterplot of the data (below), we get an average linear relationship of about 0.05 W per sq. meter increase in reflected sunlight per 1 unit decrease in Ap index. This is at least qualitatively consistent with a decrease in solar activity corresponding to an increase in cloud cover. …

But just how big is this linear relationship seen in the above scatterplot? From looking at a 70-year plot of Ap data (originally from David Archibald), we see that the 11-year sunspot cycle modulates the Ap index by at least 10 units. …

When the 10 Ap unit variations are multiplied by the 0.05 scale factor, it suggests about a 0.5 W per sq. meter modulation of global reflected sunlight during the 11 year solar cycle (as well as in monthly and yearly variations of geomagnetic activity). I calculate that this is a factor of 10 greater than the change in reflected sunlight that results from the 0.1% modulation of the total solar irradiance [TSI] during the solar cycle.

At face value, that would mean the geomagnetic [solar] modulation of cloudiness has about 10 times the effect on the amount of sunlight absorbed by the Earth as does the solar cycle’s direct modulation of the sun’s output.” “Geomagnetic Forcing of Earth’s Cloud Cover During 2000-2008?


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