Good point Roger. No I cannot be sure that at least a portion of the activity
on the seismometers is not due to induced electrical currents. This type of
induction, however, does not generally occur at the moment of commencement
of the storm, but after the electrical currents get organized in the ionosphere,
then there certainly are induced currents to worry about.
I am more inclined to believe we are seeing actual ground motion, however.
When a strong CME arrives from the sun, it compresses the upper atmosphere
on the sub-solar side of the earth bringing the troposphere limit up to 10
km closer to the earth in some storms while on the opposite side, the
upper atmosphere is torn away and a relative vacuum exists. This leads
to an atmospheric pressure wave forming which can easily induce ground
motion. The long-period form that is displayed on the seismograms suggests
something more akin to free-earth oscillation after great earthquakes. Induced
currents, it seems to me would be more short period than what is seen.
I think this is a worthy topic for debate, at any rate. These oscillations
are seen during every major sudden commencement geomagnetic storm
that I know of. It seems to me that the physics of the system are ideal
to set up free-earth oscillations.