A second strong CME hit the earth today at about 17:10-17:12 UT. This occurred while
Texas was directly sub-solar (local noon). An interesting test for the hypothesis that
the seismograms are recording actual ground motion associated with the collision
of the earth system with a CME (Coronal Mass Emission) from the sun is to look
at the Hockley, Texas seismogram at the moment of collision since it was directly
sub-solar at the time and would show the maximum ground motion (if any occurs).
Theoretically, the compression of the ionosphere and troposphere on the side facing
the sun (the sub-solar point) will lead to a depression in the earth below and the
first motion observed on the seismogram should be down. Looking at yesterday's
seismograms it does appear that those located on the side of the earth facing the
sun at the time of the storm commencement (College, Alaska, Spitzbergen) do show
first motion which is down, while those facing away from the incoming CME show
first motion which is up (New Zealand, Siberia, Mongolia). So it should be possible
to answer Roger's question regarding ground motion vs induced electrical currents
in the same way we determine focal mechanisms of earthquakes - by examination
of first motion on the seismograms.
BW also asked a question regarding what free oscillations of the earth are. Think
of it this way, BW. When you hit a drumhead with a drumstick, you create vibrations
at the point of impact, these sounds spread across the drum membrane and
are equivalent to seismic waves emanating from a single source. But longer wavelength
waves are also produced because the drumhead is tied down at the ends, the
vibrations bounce off the edges and set up standing waves whose wavelengths
are related to the size and shape of the drumhead. These standing waves are more
like free-earth oscillations. In Free-earth oscillations, the earth vibrates as a whole.
The wavelengths are much longer than those of seismic waves which are generally
about the length of the fault that produced them (or shorter). Many people people
think of this as similar to the ringing of a bell, the larger the bell, the deeper the
tones. In this case, the earth is one huge bell with extremely deep (inaudible)
tones. But also consider that a bell can be caused to vibrate by a strong wind
passing by it or by striking it with a hammer - I hope you get the drift.

Back to the main topic.
The CME arrived today about 17:11 while Hockley Texas was sub-solar. To see
the effect on apparent ground motion look at the USGS on-line seismometer
for IU/HKT and come to your own conclusions.

Seismometer readings for the day at:
http://aslwww.cr.usgs.gov/Seismic_Data/heli2.htm

Minute-by-minute GOES geomagnetic field data at:
http://sec.noaa.gov/ftpdir/lists/geomag/20011022_G8mag_1m.txt

• USGS on-line seismograms