The background image for these pages is created
from satellite-derived free air gravity anomalies (gray-scale) and color contours of the
Earth's geomagnetic inclination, declination and field strength.
Satellite-derived free air gravity anomalies
Ocean surfaces represent a gravitational
equipotential surface. Free air gravity anomalies are then derived by calculating the rate
of change in elevation of this surface. The area of the image is the central Atlantic
Ocean with the Lesser Antilles Islands and Venezuela located to the west, and Africa to
the East. The Mid-Atlantic Ridge (short north-south segments) and related transform faults
and fracture zones (generally east-west) are well defined through the center of the area.
The Earth's gravity field is monopolar so it
always points down towards the Earth's center. But the magnetic field is dipolar, which
means that the direction of the field changes with geographic position. At the geomagnetic
poles the field is vertical or perpendicular to the Earth's surface (see diagram below).
At the geomagnetic equator it is horizontal or parallel to the Earth's surface, and
oriented north-south. Geomagnetic inclination and declination describe the magnetic field
vector with respect to geographic location. Geomagnetic field strength generally is
strongest near its poles and weakest over its equator.
Tasa Graphic Arts has a lot of cool
Below is an "unwashed" version
of the background image.
- Geomagnetic inclination (10º red contours)
increases from -20º in the south to 50º in the north.
- Geomagnetic declination (5º blue contours)
decreases to -15º through the center of the area and increases westward and eastward to
-10º and -5º respectively.
- Geomagnetic field strength (5,000 nT magenta
contours) increases from 30,000 nT in the south to 45,000 nT in the north.