<P> In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation . This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 9--12 years . However, with the Sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas reversals on Earth seem to occur during periods of low field strength . </P> <P> Some scientists, such as Richard A. Muller, think that geomagnetic reversals are not spontaneous processes but rather are triggered by external events that directly disrupt the flow in the Earth's core . Proposals include impact events or internal events such as the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones or the initiation of new mantle plumes from the core - mantle boundary . Supporters of this hypothesis hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field . Because the magnetic field is stable in either the present North - South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that half the recoveries become reversals . However, the proposed mechanism does not appear to work in a quantitative model, and the evidence from stratigraphy for a correlation between reversals and impact events is weak . There is no evidence for a reversal connected with the impact event that caused the Cretaceous--Paleogene extinction event . </P> <P> Shortly after the first geomagnetic polarity time scales were produced, scientists began exploring the possibility that reversals could be linked to extinctions . Most such proposals rest on the assumption that the Earth's magnetic field would be much weaker during reversals . Possibly the first such hypothesis was that high energy particles trapped in the Van Allen radiation belt could be liberated and bombard the Earth . Detailed calculations confirm that if the Earth's dipole field disappeared entirely (leaving the quadrupole and higher components), most of the atmosphere would become accessible to high energy particles, but would act as a barrier to them, and cosmic ray collisions would produce secondary radiation of beryllium - 10 or chlorine - 36 . An increase of beryllium - 10 was noted in a 2012 German study showing a peak of beryllium - 10 in Greenland ice cores during a brief complete reversal 41,000 years ago which led to the magnetic field strength dropping to an estimated 5% of normal during the reversal . There is evidence that this occurs both during secular variation and during reversals . </P> <P> Another hypothesis by McCormac and Evans assumes that the Earth's field disappears entirely during reversals . They argue that the atmosphere of Mars may have been eroded away by the solar wind because it had no magnetic field to protect it . They predict that ions would be stripped away from Earth's atmosphere above 100 km . However, paleointensity measurements show that the magnetic field has not disappeared during reversals . Based on paleointensity data for the last 800,000 years, the magnetopause is still estimated to have been at about 3 Earth radii during the Brunhes - Matuyama reversal . Even if the internal magnetic field did disappear, the solar wind can induce a magnetic field in the Earth's ionosphere sufficient to shield the surface from energetic particles . </P>

During the last 4 million years how many times has earth reversed polarity