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 both important for making measurements of the Earth’s magnetic field. They are not redundant but are instead complementary:

• Satellites provide good geographical coverage for data collection.
• Ground-based magnetometers are much less expensive and much easier to install than satellites. An array of magnetometers provides coverage from numerous locations simultaneously. 

Another consideration is that satellites orbit the Earth either inside or above the ionosphere, which is the part of the Earth’s atmosphere that's electrically conductive. Since currents in the ionosphere contribute to the magnetic field, this means that the field measured by a satellite is somewhat different from the field measured at the Earth's surface. 

Many of the effects of space weather are most important at the surface of the Earth--where we live--so measurements from ground-based observatories will always play a critical role in space-weather studies.

Learn more: USGS Geomagnetism Program