Time-Domain Simulation of Electromagnetic Induction: Transient Disturbances due to Mantle Plume

Mantle electrical conductivity can be measured by studying the frequency-dependent electromagnetic (EM) responses to long-period external EM fields. If an upwelling hot plume exists, the lateral temperature contrast across a hot plume is estimated to be in the range of 10² to 10³ K. This temperature contrast is translated into electrical conductivity contrast of a factor of several or greater in the upper mantle, and much larger than that observable in seismic properties (within a few percent). We carried out computer simulations using a time-domain 3-D finite difference code of EM induction to test the sensitivity associated with a plume like conductivity anomaly in the upper mantle. We imposed external plane field that oscillates with a period of 50,000 to 130,000 sec assuming its very long wave length as compared with the dimension of conductivity anomaly. The prominent periods of several hours to one day or two are characteristic for magnetic substorms that can sample the upper mantle depths well. Results show that the time-domain simulations have advantages to capture transient disturbances in induction associated with plume like anomalies. We interprete that the most important effect with the presence of a plume with much higher conductivity than the surrounding mantle is the expulsion and thus reflection of the external EM field. The second important effect of a plume surrounded by less conductive matters is their ability to allow the external EM fields to slowly penetrate into mantle with disturbances. The sensitivity of induction is controled by the combination of the frequency band of the external fields and the conductivity structure. We also examine geomagnetic data observed at seperate stations with a spatial interval of ~100 km or so to check if the data have quality to distinguish different EM responses predicted by the computer simulations.