Spherical Cap Harmonic Analysis of the Geomagnetic Field in the Beijing‐Tianjin‐Hebei Region

Abstract

Abstract Three‐component absolute surveys of the geomagnetic field were carried out at 45 field sites in the Beijing‐Tianjin‐Hebei region in 2002. The survey data were reduced. The reduce time was 16:00‐18:00 (UT) on May 5, 2002. The RMS values after reducing are respectively better than 1.5nT (for total intensity F) and 0.5' (for declination D and inclination I ). Taking the International Geomagnetic Reference Field (IGRF2000) as the geomagnetic normal field in this region, the spherical cap harmonic model (BTHASCH) of geomagnetic anomalous field in the Beijing‐Tianjin‐Hebei region is calculated. The pole of the spherical cap is at 39.5°N and 117.0°E and its half‐angle of the spherical cap is 4°. During calculating the model, the truncation order of the spherical cap harmonic function is 1‐10. According to comprehensive comparison of the results, the final truncation order is 5 and the spherical cap harmonic model (BTHGRF) in the Beijing‐Tianjin‐Hebei region is established. According to this model, geomagnetic anomalous field charts (Δ X , Δ Y , Δ Z , Δ F , Δ D , Δ I ) and geomagnetic charts ( X , Y , Z , F , D , I ) in the Beijing‐Tianjin‐Hebei region are drawn.