Secular variation and excursions of the Earth's magnetic field

Abstract

Several different models for the secular variation of the earth's magnetic field have been proposed. None of the proposed models fits well the observational evidence about the secular variation, which calls for an increase in the nondipole field contribution toward virtual geomagnetic pole (VPG) scatter of a factor of 2 on going from equator to pole, a symmetric distribution of VGP's at all latitudes (and hence a nonsymmetric distribution of directions), and a drifting portion of the non‐dipole field significantly greater than the standing portion. A new model for secular variation is introduced which fits the observed data better than the others. The secular variation data from over 1000 lava flows from Iceland are discussed, and it is shown that the low‐latitude VGP's do not fit in with a Fisher distribution. Instead, the whole distribution of VGP's is well fit by a combination of about 0.9 of a Fisher distribution and about 0.1 of a random distribution. The low‐latitude poles also come from lavas which have a lower than normal intensity of magnetization. These low‐latitude poles are caused either by excursions of the earth's magnetic field or during times when the earth's magnetic field is undergoing a reversal. By looking at the statistics of when these low latitude poles occur in the section, it is possible to infer that either there are many undetected reversals or there is a high chance (about 40%) that when the dipole field decreases in intensity (before a possible reversal), it can increase in intensity with the same polarity that it originally had.