Analysis of magnetic disturbance due to buildings

In the National Space Development Agency of Japan (NASDA), a nonmagnetic space is required in the Spacecraft Magnetic Test Site (SMTS) to measure mainly magnetic moments of spacecraft. This is realized by means of the main coil of a three-axis Braunbek system combined with a nonmagnetic noise source space (NMNSS) of which the radius from SMTS is more than 300 m. A magnetically shielded room was not installed. Direct-current (DC) magnetic disturbance due to buildings in the Earth´s magnetic field and alternating-current (AC) disturbances due to human activities can decay sufficiently in such a large space. This paper describes DC magnetic disturbance due to a tall building, which is planned inside NMNSS. First, a method of analysis for three-dimensional magnetic field computation was verified by investigating the distribution of magnetic disturbance due to buildings of steel and concrete structure, which exist in NASDA. Secondly, magnetic disturbance ΔB due to the planned tall building was discussed. It was found that magnetic field concentrates at the core part of the top floor as an inlet and the office part of lower floors as an outlet. Consequently, considerably large ΔB with positive value was observed around them and ΔB with negative value was widely spread. ΔB at SMTS was -60 nT. When the height of the tall building was shortened to half, ΔB was reduced by two-thirds. This change may correspond to the difference of total surface area between them.