It occurs to me that you might have better luck getting this data from a place where it's already collected, the WAAS system. This is a set of ground stations that record this ionospheric disturbance and send correction factors up to the GPS satellites for rebroadcast to GPS receivers, which use the data to improve their accuracy.
There's a lot of WAAS data available at http://www.nstb.tc.faa.gov/RT_VerticalP ... nLevel.htm
, and pages that are linked from there. Also, there's a really interesting paper I found with Google that you might find very interesting for your project: http://www.ima.umn.edu/~toews/research/waasfilter.pdf
In general, Google is your friend
If you really want to record this data yourself, most of the commercially available GPS receivers are excellent. Make sure you get one that supports NMEA output. This is a widely used serial port communication standard implemented by most GPS receivers that are intended for hiking or boating (as opposed to car trips... those tend to be pretty dumbed down)). If your PC doesn't have a serial port, you might also need a USB-Serial converter, but those are cheap.
There's a pretty good description of what you'd need to do to hook it up to a PC here: http://www.windmill.co.uk/gps.html
, but that's just an example... I'm sure you can find better info if you search for it, possibly even in the GPS's documentation.
Personally, I've done a lot of geocaching, which involves using a GPS in a sort of "treasure hunting" game (see http://www.geocaching.com0
, and I've found that Magellan receivers are somewhat more accurate and pull in a signal more strongly under trees than Garmins, but the differences are minor, and the Garmins tend to be cheaper.
Ironically, because it will be less accurate, you'll want to either buy a GPS that does *not* have WAAS capability, or make sure that the one you buy can *disable* WAAS, because it will throw off your measurements, making them almost useless. The whole point of WAAS is to correct for the exact ionospheric disturbances that you're trying to measure. I would suggest looking on the Garmin and Magellan websites for documentation of their various models.
My preference would be for one that has WAAS, but can disable it, because that way after you're done with your experiment you can turn it back on and have a much more accurate receiver.