Geomagnetic polarity dating
The Earth's Main Field dominates over the interplanetary magnetic field in the area called the magnetosphere.
The magnetosphere is shaped somewhat like a comet in response to the dynamic pressure of the solar wind.
This portion of the geomagnetic field is often referred to as the Main Field.
The Main Field varies slowly in time and can be described by mathematical models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM).
D is considered positive when the angle measured is east of true north and negative when west.
D and I are measured in units of degrees, positive east for D and positive down for I.