Here is a low-cost nanoTesla resolution magnetic field sensor. It monitors minute fluctuations in the earth's magnetic field caused by variations in the solar wind. A stable system this has been left running for months at a time - uploading data every few minutes to this site. The code is well tested and robust with a modular design permitting easy adaptation and development by students of both the software and hardware. The system has been described in a paper in Physics Education Solar wind monitor—a school geophysics project. The code, construction details and an updated paper can be downloaded here.
As the solar wind interacts with the upper atmosphere it distorts the earth weak magnetic field. Thus monitoring variations in the earth's field via a buried magnetometer perversely gives information on the strength of the solar wind and coronal mass ejections from the sun. During periods of high activity aurora may be visible.