Introduction to Geomag
Geomagnetism is the study of the Earth's magnetic field. This includes the fields produced by the Earth as well as those interacting with the Earth. Internal dynamo processes within the Earth create slowly changing magnetic fields. The continuous flow of particles and fields from the Sun (called the solar wind) interacts with the Earth's magnetic field. Strong, transient impulses due to solar events produce ground-level magnetic disturbance, by affecting the complex current systems in and near the Earth's atmosphere. This means that the magnetic field on the Earth's surface is different from location to location, and that it changes over time. Some of these changes are slow, such as the centuries-long drift in the direction of the magnetic field; some of these changes are quick, such as the sudden enhancement during a geomagnetic storm that can occur in minutes.
The use of geomagnetism for navigation began with the use of compasses for orientation. A compass is a simple magnetic instrument that points to magnetic north. Magnetic north is skewed from geographic north by an angle called declination. To navigate accurately, it is necessary to understand how declination varies over distance and time. This is why there is a long history of compiling magnetic information and maps. This information is still important today because magnetic field variations, like those seen during a geomagnetic storm, can affect GPS, directional drilling, electric power grids and other systems.
Magnetic observatories were first established in the 19th century through the influence of Alexander von Humboldt and Carl Friedrich Gauss. Since then, magnetic measurement has advanced significantly, progressing from simple visual readings using magnetic survey instruments to automatic electronic recording. Modern observatories provide very accurate long-term measurements of the ground-level magnetic field every second. There are two ways to look at this data. Both of them measure F as the total magnitude of the magnetic field.
The first is with geographic coordinates:
- X - North component. The magnitude of the magnetic field in the northern direction.
- Y - East component. The magnitude of the magnetic field in the east direction.
- Z - Downward component. The magnitude of the magnetic field toward the ground.
The second is with relative coordinates:
- H - Horizontal intensity. The magnitude of the magnetic field parallel to the ground.
- D - Declination angle. The angle between geographic north and magnetic north.
- I - Inclination angle. The angle between the vertical direction and F.
- Love, J. J., 2011. Magnetic observatory, McGraw-Hill Encyclopedia of Science & Technology, 10, 307-310.
- Love, J. J. & Finn, C. A., 2011. The USGS Geomagnetism Program and its role in space weather monitoring, Space Weather, 9, S07001, doi:10.1029/2011SW000684.
- Love, J. J., Feb. 2008. Magnetic monitoring of Earth and space, Physics Today, 61, 31-37.
- Love, J. J., Applegate, D. & Townshend, J .B., 2008. Monitoring the Earth's Dynamic Magnetic Field, USGS Fact Sheet, 2007-3092.