The Moon
Polar Cap Absorption (PCA)
PCA happens when solar protons from solar flares enter
the earth's upper atmosphere in high geomagnetic
latitudes, often causing radio blackouts and increased
auroral activity. These periods of severe ionospheric
disturbance are often marked by Forbush decreases;
Forbush decreases are when the counting rate of
background galactic cosmic radiation has sudden
anomalous decreases—decreases which may not
recover and attain normal levels for hours or days.
In effect, it is as if there was a magnetic screening of
galactic cosmic radiation by the enhanced solar plasma.
It has been noted, but unexplained, that PCA events
and Forbush decreases seem to be ordered with the
lunar synodic period (29.5 days). When this research
was begun, it was expected that a 27.3-day period
would be found, indicating a link with solar rotation. It
was a surprise to scientists when, instead, results