Geomagnetic secular variation
![Estimated declination contours by year, 1590 to 1990 (click to see variation).](/Images/godic/202501/15/Earth_Magnetic_Field_Declination_from_1590_to_19902555.gif")
Geomagnetic secular variation refers to changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere.