Method to estimate geomagnetic total intensity variations due to volcanic activities by using the stochastic differential method
Ikuko Fujii
Abstract
Aiming to obtain the long-term variation of volcanic origin as a residual, we have developed a statistical method to remove large-scale variations of core and ionospheric/magnetospheric origins from the geomagnetic total intensity variation by using the stochastic differential method. The total intensity at Kouzu-shima and the three vector components of the geomagnetic field at Kakioka were referred to for analyzing the total intensities at Miyake-jima in order to estimate the large- and small-scale variations including crustal magnetization and induction. The filter length in the stochastic differential equation was optimized by minimizing Akaike Information Criteria so that the procedure was objective and automatic. In addition, ionospheric and oceanic tidal variations that remained in the residual were removed by fitting a set of tidal frequency sinusoidals in a least squares sense with a robust procedure. The non-volcanic large-scale variations were considerably reduced compared to a simple difference from the data at Kouzu-shima.
A set of numerical codes for the new method with necessary pre- and post-processing was developed and the whole procedure has been almost entirely automated. The method has been applied to 1-minute values of four total intensities at Miyake-jima since 2002. We have been attempting to apply the method to data at other volcanoes, for instance, the Aso volcano and Kusatsu-shirane volcano.
Received 6 February 2004; Received in revised form 25 February 2004; Accepted 26 February 2004
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