I was confused by the discussion of GPS in relation to Total Electron Content, unaware that GPS signals can and are used to measure this. The following website explains how it's done. Pretty slick. One of those "why didn't I think of that" moments.

http://www.ips.gov.au/Satellite/6/2

Here's some websites that give real time TEC maps.

http://www.ips.gov.au/Satellite/2/1/5
http://www.swpc.noaa.gov/ustec/
http://www.swpc.noaa.gov/ustec/images/index.html

And here's a page with an animation of the TEC disturbance caused by the Tohoku Earthquake. Keep in mind these are ionospheric disturbances CAUSED by the quake, not preceding it.

http://www.seg.nict.go.jp/2011TohokuEarthquake/index.html

Brian