Physics of the Earth and Planetary Interiors 115Ž. 1999 173±179 www.elsevier.comrlocaterpepi A model of virtual geomagnetic pole motion during reversals V.V. Kuznetsov ) Institute of Geophysics, Siberian Branch of Russian Academy of Sciences, Geophysical ObserÕatory, Koptyug AÕenue 3, 630090 NoÕosibirsk, Russian Federation Received 23 January 1998; received in revised form 10 August 1998; accepted 14 March 1999 Abstract A new model of virtual geomagnetic pole motion during a reversal of the Earth's magnetic field is proposed. The model is based on the idea that the geomagnetic field is the summation of the dipole field and the field of global magnetic anomaliesŽ. GMAs . At the time of a reversal, the dipole field changes its sign and the field of the GMAs remains non-zero. The geographic location of the global magnetic anomaliesŽ. Canadian, Brazilian, Siberian and Antarctic are close to the 908W and 908E meridians; this defines the paths of the virtual pole drift at the time of a reversal. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Virtual geomagnetic pole; Motion; Reversals 1. Introduction ber of suggested models is increasing, which shows that this problem of interest to the scientific commu- During the last 10 years, magneticians have taken nity, and for the present has not been resolved. Let an interest in a phenomenon discovered in palaeo- us consider the problem more closely. magnetism. It has been found that during a reversal, ClementŽ. 1991 has shown that the distribution of the virtual geomagnetic polesŽ. VGP move through VGP paths on the Earth's surface possessed symme- the Central Asia and Australia or through North and try during the Matuyama±Brunhes geomagnetic re- South AmericaŽ. see Fig. 1 . Both paths are close to versal. He found that VGPs were spread over the the 908 meridian in the eastern and western hemi- entire Earth's surface, but that the greatest number of sphere. Some papers are devoted to studying the determinations during the Matuyama±Brunhes geo- VGP drift directionsŽ Koci, 1990; Clement, 1991; magnetic reversal lay within the region 608W"308. Tric et al., 1991; Constable, 1992, 1993; Jackson, At the same time, the antipodal part of VGP occu- 1992; McFadden, 1992; Gubbins, 1993; Gubbins and pied a corridor along 1108E"308. Tric et al.Ž. 1991 Coe, 1993; Hollerbach and Jones, 1993; McFadden have studied the distribution of VGP drift trajectories et al., 1993. ; other papers consider possible drift during the Upper Olduvai reversal and they have models based on the concept of the dynamo mecha- selected two cases: the change of geomagnetic field nism of the geomagnetic field generation. The num- polarity from reverse to normalŽ. R™N and the change from normal to reverseŽ. N™R . The VGPs ) E-mail: [email protected] moved only through the American continents. The 0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0031-9201Ž. 99 00075-8 174 V.V. KuznetsoÕrPhysics of the Earth and Planetary Interiors 115() 1999 173±179 authors believe that in this case VGPs ``prefer'' to Our computer model permits us to change the move through the Americas but not through Asia and field source intensitiesŽ within the magnitudes fixed Australia. by the magnetic observatories closest to the pole. KociŽ. 1990 has noted an interesting peculiarity of and to choose the velocity of the magnetic poleŽ. MP VGP drift during the reversal that occurred after the drift to correspond to the velocity determined by Jaramillo epoch. The VGP drifted from the southern Canadian magneticians based on observations in hemisphere northward from 908Sto208S, at which 1974, 1984 and 1994. I have used this model and the point, its path was interrupted; VGP occurred again data of the magnetic observatories closest to the pole only at latitude 408N, moving along the 908E merid- in 1994 to estimate the location of the NMP in 1994. ian. Some authorsŽ Clement, 1991; Gubbins and Subsequent direct observations, made in the region Coe, 1993.Ž. think that at the time of a N™R of NMP by Newitt and BartonŽ. 1996 in May 1994, reversal the VGPs move along the ``Asian path''; have confirmed the correctness of my prediction during aŽ. R™N reversal they move along the Ž.Kuznetsov, 1996 and, thus, the validity of my ``American path''. model. The measured velocity of the recent motion The problem of the VGP-drift during reversals of the poles is 10±15 kmryearŽ Peddie and Zunde, has recently been considered in papers by Gubbins 1987; Newitt and Barton, 1996. which is more than and SarsonŽ. 1994 , McFadden and Merrill Ž. 1995 the drift velocity of the palaeopoles by a factor of and Fuller et al.Ž. 1996 . The authors of these papers 100 000Ž. Courtillot and Besse, 1987 . Note the fact have analysed previous attempts to explain peculiari- that during a reversalŽ which lasts about thousand ties in the VGP drift, which were based on the years. the MP must drift with a velocity on the order magnetic dynamo model. They have concluded that of 10 kmryear. Can the conclusion be made that a at the present time this phenomenon is not well reversal of the Earth magnetic field is happening at understoodŽ. Fuller et al., 1996 . the present time? Can the model of recent pole motion be used to explain the drift trajectories during the time of a reversal? In this paper I will try to 2. Model answer the second question. HopeŽ. 1959 was probably the first to postulate Four global magnetic anomaliesŽ. GMA are dis- that global magnetic anomalies influence the drift of tinguishable on a total intensity map of the geomag- the North Magnetic PoleŽ. NMP . To develop this netic field, namely CanadianŽ. CMA , Brazilian idea further, I decided to simulate the magnetic field Ž.BMA , Siberian Ž. SMA and South Ž Antarctic . in Arctic. It turned out that three magnetic field Ž.ŽAMA Barraclough, 1987 . As one can see from sources were necessary in order for the calculated Fig. 1, these anomalies are placed on the path of pattern of the magnetic field lines to reproduce the VGP motion during the time of a geomagnetic field real pattern, not two sources as Hope thought. I reversal. Using the ideas put forward in the model of speculated that the two sources were the Canadian the recent motion of MP, we will try to explain this and Siberian anomalies, but where was the third phenomenon. In doing so, we assume that when the source located? It turned out that the third source dipole component changes its sign, and that the determined the dipole field position and was located GMA field exists for a time period longer than the at the pointŽ. 708N, 2708E . In a similar manner, the polarity change period. According to our model, the field in Antarctic was calculated. Here, only two generation of the dipole part of geomagnetic field sources were necessary: the dipole field source and occurs on the inner core boundaryŽ in Bullen's F- the field of the SouthŽ. Antarctic anomaly. Both layer.Ž Kuznetsov, 1990 . In this layer, a phase sources were on the 1408E meridian at latitudes of transition happens and electric processes arise to approximately 858 and 508S, respectively. Thus, the accompany to it. A radial hydrodynamic stream of longitudes of the sources of the dipoleŽ. main geo- matter can occur. Let us imagine such stream of magnetic fieldŽ. 1408E and 2708E did not coincide matter in the form of a Rossby vortex with good with each other or with the geomagnetic pole longi- electric conductivity. With availability of the dipole tudeŽ. Kuznetsov, 1990 . magnetic field frozen in the stream, the vortex is the V.V. KuznetsoÕrPhysics of the Earth and Planetary Interiors 115() 1999 173±179 175 Fig. 1. Tracks of the VGP drift during a reversal and the locations of the global magnetic anomalies:Ž. 1 Canadian, Ž. 2 Brazilian, Ž. 3 Siberian andŽ. 4 Antarctic. source of a magnetic field which increaseŽ or de- geomagnetic field is equal to zero. Let us use some crease. the vertical component of geomagnetic field. simple examples to illustrate this. Imagine that there On a magnetic field map the vortex shows itself as a are two magnetic field sources, A and B. As a first GMA. The dipole part of geomagnetic field during approximation, assume that both sources have the the reversal period turns out to be equal to zero, after same polarity and their intensities have a spatial which the magnetic field generated by the Rossby distribution. Let us symbolise the distance between vortexŽ. the GMA field will be non-zero for some themŽ. in geographic degrees as k. Our task is to time. The time during which the generation of the determine the segment y Ž.which is the part of k , GMA field will continue is influenced by the electric where the value Hs0. The necessary equation is: conductivity of the F-layer matter. A sin yyB sin kyy s0. 2 To estimate the distribution of the H component Ž. Ž. on the Earth's surface, it is necessary to know the The solution of this equation is: depth at which the generation of GMA fields takes s r q r place, which is the stream location. According to ctg y Ž.A B cos k sin k. different estimatesŽ Alldridge and Stearns, 1969; If AsB, then yskr2, if As0, then ysk,if s Alldridge, 1980; Kuznetsov, 1990. , the depth x Bs0, then ys0. In a similar manner it is possible 0.2 R, where R is the Earth's radius.