地球非偶极磁场对虚地磁极计算的影响

古地磁学使用的虚地磁极（VGP）是在地心偶极磁场假设下计算的,由于地球非偶极磁场的存在，VGP一般不同于真地磁极（RGP）.为了定量检验非偶极磁场对VGP的影响，本文利用国际参考地磁场模型IGRF 1900～2000，在全球5°×5°的“虚拟测点”网格上计算了VGP和RGP的位置，并求出两种磁极的经纬度偏差和二者的角距离.结果表明，南极地区VGP与GP的角距离最大，可达26°，南大西洋和欧亚大陆北部最大达到24°和18°，其余地区一般小于15°.VGP对RGP的偏差与地磁场分布有关：在非偶极磁场较弱的地区（如太平洋半球），纬度偏差一般不大（≤10°），但是在主要地磁异常区（如南大西洋和南极地区），VGP对RGP的纬度偏差可达25°.VGP对RGP的经度偏差要比纬度偏差大得多，例如在欧亚大陆北部地区，经度偏差分布在-180°到180°的大范围内.

Effect of non-dipole field on VGP estimation

The virtual geomagnetic pole (VGP) calculated under the assumption of a central dipole field is generally different from the real geomagnetic pole (RGP) since effect of the non dipole field. In order to quantitatively examine effects of the non dipole field on VGP determination, the VGP and RGP are calculated at 5°×5° virtual sites by using the IGRF 1900～2000 models. The results show significant departures of VGP from RGP in some major magnetic anomaly regions. The latitude departures of VGP from the RGP are as large as 25° in these anomaly regions, although they are acceptable in most sites on the earth's surface (less than 10°), mainly confined in the regions with a weak intensity of the non dipole field, such as Pacific Ocean. The longitude departures of VGP from RGP are much more remarkable, scattering in a wide range from -180° to 180° in Eurasian continent. The angle distance etween VGP and the RGP are generally within a range of 15°, except the Antarctica, South Atlantic Ocean and Eurasia regions.