|Budget Amount *help
¥7,800,000 (Direct Cost : ¥7,800,000)
Fiscal Year 1997 : ¥1,900,000 (Direct Cost : ¥1,900,000)
Fiscal Year 1996 : ¥5,900,000 (Direct Cost : ¥5,900,000)
Precise measurements of the Earth's magnetic field has been carried out only for less than 200 years since the time of Gauss. Even if we count the sparse measurements of inclination and declination before that, the data covers only several hundred years. On the other hand, the typical time constant of the geomagnetic field is that of the dipole decay (15,000 years). The fact that we cannot hope for a complete description of the geomagnetic field without having observations for several times this time constant can be made clear by the discovery of the magnetic reversals from paleomagnetism. Paleomagnetic data have the characteristics of (1) large errors as well as bad distribution, (2) mostly only directional information, and (3) a few million years may be spanned in a global data set. The head investigator succeeded to formulate the "Taylor Expansion Method" which can overcome these shortcomings of paleomagnetic data by statistical treatment.
Within the period of this project, this method was successfully applied to (1) magnetic field intensity and virtual dipole moment (Kono and Hiroi, EPSL,1996), (2)distributions of inclination and declination (Kono, JGG,1997), (3) the shape of distributions of magnetic field directions and virtual geomagnetic poles (Kono, PEPI,1997). These studies indicate that, apart from the dominant axial dipole term, the (2, 1) harmonics are important in the paleomagnetic field. Further, in the case of nonlinear data such as paleomagnetic directions, it was shown that inversiion of the mean values of the data will not lead to the mean Gauss coefficients. In the case of paleointensity data, these problems are avoided and the mean field can be obtained (Kono, Tanaka, and Tsunakawa, submitted to JGR). Thus far, the forward approach to the paleosecular variation was completed and now is the time for formulating the inversion of paleomagnetic data.