Why measure the magnetic field at the Earth's surface? Wouldn't satellites be better suited for space-weather studies?
Satellites and ground-based magnetometers are important for making measurements of the Earth’s magnetic field. They are not redundant but are instead complementary. After executing several orbits of the Earth, satellites can provide good geographical coverage for data collection. Ground-based magnetometers are much less expensive than satellites, they are much easier to install and control than satellites, and, with an array of magnetometers, they can provide coverage from numerous locations simultaneously.
Another consideration is that satellites orbit the Earth either inside or above the ionosphere, the electrically conducting part of the Earth’s atmosphere. Since currents in the ionosphere contribute to the magnetic field, this means that the field measured by a satellite is somewhat different than the field measured at the Earth's surface.
Finally, it is at the surface of the Earth, where we live, that many of the effects of space weather are most important, so measurements from ground-based observatories will always play a critical role in space-weather studies.