The Earth's Magnetic Field

    The dominate effect is the primary magnetic field of the earth. It is generally accepted that the primary field is generated by electrical currents flowing in the molten metallic inner layer of the earth. Another factor influencing the observed magnetic field is the magnetism of natural features in the earth's crust, rocks, metallic ores, etc. The residual magnetic fields from cultural objects, railroads, metal buildings, fences, etc. can be included in this group.

    The third factor is the magnetic field generated by currents flowing in the ionized layers of the earth's atmosphere induced by streams of particles (protons) originating from the sun. The influence of the sun on the earth's magnetic field is well proven. The diurnal declination variations of typically many minutes of arc are caused as the earth faces the particle stream from the sun and then, as night approaches, is shielded by the mass of the earth itself. This effect makes early morning or late afternoon the" best" time for compass readings. The declination plot attached to this page shows a minute by minute declination recording for a 55+ hour period in early 1991. The recording was made at the observatory at Fredericksburg VA, one of many observatories in operation around the world. The large swings are 12 hours apart showing the effect of the earth shielding the station from the sun. The plot also shows the annual average declination for the area from the math model used in the MAGDEC program.

    Magnetic ”storms” are well correlated with the appearance of a large number of sun spots on the solar disk. Because of the lower speed of the particles from the sun spots or flares associated with the sun spots, the onset of magnetic storms, both timing and intensity, can be predicted with a high degree of certainty. NOAA's Solar Environment Laboratory provides this service free of charge. The forecast is available in a recorded message at 303-497-3235.