The distinction between geomagnetic excursions and reversals

Two recent studies of the geomagnetic field in the last 1 Myr have found 14 excursions, large changes in direction lasting 5-10 kyr each, six of which are established as global phenomena by correlation between different sites. The older picture of the geomagnetic field enjoying long periods of stable polarity may not therefore be correct; instead, the field appears to suffer many dramatic changes in direction and concomitant reduction in intensity for 10-20 per cent of the time. During excursions the field may reverse in the liquid outer core, which has timescales of 500 yr or less, but not in the solid inner core, where the field must change by diffusion with a timescale of 3 kyr. This timescale is consistent with the remarkably uniform duration of well-dated excursions. The disparity of dynamical timescales between the inner and outer cores, a factor of 10, is consistent with the 10 excursions between full reversals.