Summary of internal field variations secular variation due to core fluctuations with time scales of years to millions of years, including reversals. Physics of the dynamo is clear, details uncertain but numerical modeling and new observations make progress likely. A complete observatory has both types to get the complete vector field, since Backus showed that a total field instrument was not sufficient. Early observatories often recorded only field declination and dip.
Field strength is more difficult to recover from the distant past., except by archaeo magnetism.
Proton precession measures field magnitude but not direction. A 3 axis flux gate plus a total field instrument is standard on satellites and can be used on aircraft with orientation scheme, though total field is more common. Dip and declination change over time, allowing magnetic stratigraphy as an application of paleomag but the external field variations show a big variety of time scales in contrast to the secular variation and reversal features of the main field. Magnetic observatories on the surface record the vector field variations. Rapid variations, daily variations, magnetic storms.