HR: 1340h
AN: GP33A-0925    [Abstracts]
TI: Constraint on the heterogeneity of mantle conductivity structure obtained from the geomagnetic jerks
AU: * Nagao, H
EM: nagao@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Shimizu, H
EM: shimizu@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Utada, H
EM: utada@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AB: It is well-known that the occurrence time of each geomagnetic jerk in the southern hemisphere delays a few years comparing with the northern hemisphere (e.g., Alexandrescu et al [1996], Nagao et al. [2002, 2003]). Two physical mechanisms are possible for this interesting phenomenon; the distribution of the magnetic field at the core-mantle boundary (CMB) and/or the effect of the mantle conductivity on the magnetic field diffusion. Although which mechanism is significant is still controversial, it is valuable to obtain a constraint on the mantle conductivity that can explain this occurrence time lag. Backus [1983] derived theoretically, in the case of 1-D mantle, several characteristic time constants of a jerk related to the mantle conductivity, in which significant ones are delay time and smoothing time. We have been developing a tool using the Kalman filter, which enables us to determine automatically the amplitude, occurrence time, smoothing time of a jerk, and estimation error in each determined parameter. We apply this tool to the eastward component of geomagnetic monthly means obtained at worldwide geomagnetic observatories, and find that the distribution of amplitude has a spherical harmonic component of S₂² and that the occurrence time in the African region delays a few years comparing with the other regions. When a jerk is assumed to occur simultaneously at the whole CMB, the mantle conductance is required to be an order of 10⁹S in order to explain this time lag. This conductance is much larger than the upper constraint estimated from several decades variation in the length-of-day (LOD) on the basis of the electromagnetic core-mantle coupling theory. Therefore, a lateral heterogeneity is to be taken into account in order to satisfy both the jerk occurrence time lag and the LOD variation. We carry out a numerical experiment in the wave number and frequency domains using the MIDM (Koyama et al. [2002]) of how a magnetic field (especially in the case of S₂²) input from the CMB diffuses in the 3-D mantle, and obtain a transfer function between an input at the CMB and the corresponding output at the Earthfs surface. We can construct a synthetic but realistic time-variant magnetic field model utilizing this transfer function, and obtain the time constants of a jerk. We will discuss in the presentation a constraint on the heterogeneity of the mantle conductivity structure by comparing the time constants of the jerks estimated from the data analysis with those obtained from the numerical experiment.
DE: 1515 Geomagnetic induction
DE: 1555 Time variations: diurnal to decadal
DE: 1560 Time variations: secular and longer
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting