RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3003, doi:10.2205/2005ES000175, 2005

Magnetic Moment Decline and Restoration. The Duration of Inversions

[8]  Using their own paleomagnetic data, different authors estimate a decline in the intensity of the old magnetic field Hold (or of the magnetic moment M ) during the reversal to be 3 to 10 (and more) times. This difference of their estimates is associated mainly with the absence of any distinct level from which this decline can be estimated. Moreover, it should be taken into consideration that these investigators derive their data of the absolute decline of the magnetic field intensity as a result of studying igneous rocks, because in the case of using sedimentary rocks one can deal only with the variations of the parameters associated with paleointensity. Nevertheless, it should be recognized that the data obtained from the studies of different objects usually show some general agreement.

2005ES000175-fig01
Figure 1
[9]  The paleomagnetic records available show that some Hold variations took place both before and after the reversal, and that one can judge about the average M level with a stationary field only after the averaging of the recorded variations. Moreover, as stated by some authors [Meynadier et al., 1994, 1998; Thibal et al., 1995; Valet and Meynadier, 1993], to name but a few, the Hold variations in the periods between the inversions show "an asymmetric saw-tooth pattern'': the Hold intensity grows abruptly after the end of the inversion, and then, following high-magnitude variations, declines slowly toward the beginning of the next reversal. The general decline in this case amounts to 1.5-2 times, the variation magnitude being as high as ~0.5 H of the average value and the typical variation times ranging from a few hundred to a few thousand years (Figure 1). At the same time, some authors believe that this variation of the parameters characterizing the behavior of the stationary (one-polarity) field might have been associated with the stable viscous magnetization of the rocks [Hartl and Tauxe, 1996; Laj et al., 1996], to name but a few.

[10]  The analysis of the paleointensity data variation during and in the vicinity of the reversals showed that the average decline of the magnetic field during its reversals had been as high as seven times [Gurarii, 1988]. This estimate was obtained for the reversals of the last 15 million years, using the data published prior to 1986. As new data were published, including the detailed descriptions of the near-reversal variations of the geomagnetic field, this estimate did not experience any substantial changes [Gurarii et al., 2000a; Hartl and Tauxe, 1996], to name but a few.

[11]  At the same time, an interesting result was obtained, which calls for its verification at the present-day level of data accumulation. It appears that the coefficient of the magnetic moment decline during reversals is controlled by the M value of the stationary field before its reversal [Petrova and Sperantova, 1986]. S. I. Braginskii advanced the suggestion (during some oral discussion) that various parts of the magnetic (dipole and nondipole) field show their different reactions to the reversal. The pre-reversal dipole field of a variable intensity declines slowly almost to zero, and a new field arises in an opposite direction, the nondipole field varying to a significantly lower degree. In other words, no dipole field could exist during the fairly long time of the pole switching. This view offered by S. I. Braginskii agrees with the conclusions advanced by Gurarii [1988], Clement [2004], and other authors, and is of great interest in terms of the physical nature of the dipole and nondipole field.

[12]  As follows from the estimates of most of the authors, the time interval of some low M existence was notably longer than the time of the magnetic field reversal. The modal values of the Late Cenozoic reversals, estimated from the data available prior to 1986, suggested the average duration of the reversals to be 7-8 thousand years (the individual estimates varying from 4 to 25 thousand years), the time interval of the declining magnetic moment being 1.5 to 2.0 time longer, that is, embraced a time interval of up to 16 thousand years [Gurarii, 1988]. Merrill and McFadden [1999] estimated the time necessary for a complete reversal to be 1 to 8 thousand years. Clement [2004] concluded that the period of the four latest reversals had been 7 thousand years. This author noted that the duration of the field sign change had varied as a function of the latitude of the study area, which agrees with the simple model assuming that the dipole field declined to zero during its reversal and recovered again in the continuous presence of some nondipole field. This allows one to assume that the average time of a dipole field absence during inversions (if this is correct) was at least shorter than 7-8 thousand years.


RJES

Citation: Gurarii, G. Z. (2005), Geomagnetic field reversals: Main results and basic problems, Russ. J. Earth Sci., 7, ES3003, doi:10.2205/2005ES000175.

Copyright 2005 by the Russian Journal of Earth Sciences

Powered by TeXWeb (Win32, v.2.0).