Anomaly of the geomagnetic Sq variation in Japan: effect from 3-D subterranean structure or the ocean effect?

Many years ago Rikitake et al. described the anomalous behaviour of the vertical component Z of the geomagnetic solar quiet (Sq) daily variation field at observatories in central and northern Japan - namely about 2 hr shift of the local noontime peak towards morning hours. They suggested that this anomaly is associated with the anomalous distribution of electrical conductivity in the mantle beneath central Japan. Although a few works have been done to confirm or argue this explanation, no clear answer has been obtained so far. The goal of this work is to understand the nature of this anomaly using our 3-D forward solution. The conductivity model of the Earth includes oceans of laterally variable conductance and conducting mantle either spherically symmetric or 3-D underneath. Data from six Japanese observatories at four seasons for two different years of the solar cycle are analysed. As an inducing ionospheric (Sq) current system, we use those provided by the Comprehensive Model (CM4) of Sabaka et al. Our analysis clearly demonstrates that 3-D induction in the ocean is responsible for the anomalous behaviour of Z daily variations in this region. We also show that the effects from a suite of 3-D mantle models that include mantle wedge and subducting slab are minor compared with the ocean effec

Evolution of the current system during solar wind pressure pulses based on aurora and magnetometer observations

Additional file 2: Figure S2. Same as Additional file 1 but for the July 14, 2012, event

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 11 (2010): Q09003, doi:10.1029/2010GC003101.This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.Japanese participation in the Marianas experiment was supported by Japan Society for the Promotion of Science for Grant-In-Aid for Scientific Research (15340149 and 12440116), Japan-U.S. Integrated Action Program and the 21st Century COE Program of Origin and Evolution of Planetary Systems, and by the Ministry of Education, Culture, Sports, Science, and Technology for the Stagnant Slab Project, Grant-in Aid for Scientific Research on Priority Areas (17037003 and 16075204). U.S. participation was supported by NSF grant OCE0405641. Australian support came from Flinders University. T. M. is supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Deep Ocean Exploration Institute

doi:10.1016/S0377-0273(03)00119-7

Abstract Time changes in electrical resistivity were observed before the 1986 eruption of Izu^Oshima volcano, Japan. The aim of this paper is to try to interpret these observed changes in terms of the evolution of a conducting body that simulates the ascending magma by finite difference numerical modeling. Before the time changes were examined, it had been shown that the present numerical model well reproduces the spatial characteristics of the observed apparent resistivities. After some trials involving forward calculation, the time changes, observed during a few months before the eruption, were found to correspond to the formation of a small magma reservoir several hundred meters below the summit. The volume of this reservoir was estimated to be 5U10 6 m 3 , which is in good agreement with the volume of magma drained back from the conduit after the eruption, as estimated from repeated gravity surveys. By comparing the modeling results and observations, the mean ascending velocity of the magma head was estimated to be about 100 m per month during the ten months before the eruption.

A Direct Inversion Method for Two-dimensional Modeling in the Geomagnetic Induction Problem

報告番号: 乙08558 ; 学位授与年月日: 1987-09-28 ; 学位の種別: 論文博士 ; 学位の種類: 理学博士 ; 学位記番号: 第8558号 ; 研究科・専攻: 理学系研究

自然電磁場変動を用いた電磁誘導法における平面波近似と平板地球近似について

When applying electromagnetic sounding methods, such as the magnetotelluric method, studies are usually carried out by: (1) treating the inducing field as spatially uniform and (2) treating the Earth as a semi-infinite conductor with a plane surface. These assumptions are both approximations of electro-magnetic induction caused by the incidence of a laterally non-uniform inducing field into the conducting spherical Earth. Although the basic theoretical concept was established many decades ago, the physical conditions for these two approximations are not fully and systematically understood, and some confusion appears in the literature. Therefore, the basic formulation of electromagnetic induction in both spherical and Cartesian coordinate systems is re-examined and the conditions for systematically deriving the two approximations are clarified. The results reveal that the solutions for the two coordinate systems are consistent with each other at an appropriate limit and that the two approximations result in neither indefinite nor non-unique problems, as suggested by some previous studies, if appropriate approximation conditions are applied.通常マグネトテルリク法をはじめとする電磁誘導法では，（1）地球を半無限導体とみなし，平坦な地表より上方から与えられる（2）空間的に一様な電磁場変動が起こす電磁誘導を扱うことが多い．（1）と（2）はそれぞれ，空間的に非一様な電磁場変動が，球体の地球内部に生じる電磁誘導の近似的取り扱いである．電磁誘導法の理論的枠組みが確立してから長い年月が経過しているが，この 2つの近似の成り立つ物理的条件は系統的に理解されているとは言いがたく，文献には混乱も見られる．小論では，球座標系および直交座標系における電磁誘導問題の基本式を再吟味することにより，2つの近似の条件がどのように導かれているのかを明らかにすることを目指す．得られた結果は，それぞれの座標系での解には適切な極限において必ず整合性があることを示した．すなわち，近似の条件を適切に与えさえすれば，過去の文献で指摘されたような「不定性」や「非一意性」などの問題は生じないことがわかった