The following is a short list of observatories that measure the local magnetic field where they are located on a daily or even minute-by-minute basis. By plotting their data you can watch for geomagnetic disturbances related to auroral/geomagnetic storms produced by solar storms. Also, the IGRF model is a mathematical representation of the Earth's magnetic field on average and may be explored to see how it changes with altitude and geographic location near the Earth's surface.