Aa Indices.

The aa index is a simple global geomagnetic activity index, with units of 1 nT (nanotesla), which is produced from the K indices from two approximately antipodal observatories. At present these are Hartland observatory in the UK and Canberra observatory in Australia. The main advantage in using aa indices for research purposes is that the time series spans further back (to 1868) than any of the other planetary indices time series.Also, up to date values are produced and made available weekly, giving nearer to real time availability than any other planetary index. In between the weekly updates, BGS calculates estimated aa indices, providing real time "nowcasts" which are updated on an hourly basis. These estimates are clearly marked with the letters "Est". Although calculated by the same method, the aa indices available on this service are not the definitive values. These are published by the International Service for Geomagnetic Indices, CRPE/CNET - CNRS, 4 Avenue de Neptune, F-94107 Saint Maur, Cedex, France.

aa指数是一种简易的全球地磁活动指数，其单位为1纳特（nT），该指数由两个约位于地球两极的观测站K指数计算得出。目前，这两个观测站分别是位于英国的哈特兰德观测站和位于澳大利亚的堪培拉观测站。采用aa指数进行研究的显著优势在于，其时间序列的跨度远超其他行星指数时间序列（可追溯至1868年）。此外，aa指数的最新值每周更新并对外发布，相较于其他行星指数，其提供的数据更接近实时。在每周更新之间，英国地质调查局（BGS）计算预估的aa指数，提供实时‘预测’服务，这些预测每小时更新一次。这些估计值明确标有“Est”字母。尽管计算方法相同，但本服务提供的aa指数并非最终数值。这些最终数值由国际地磁指数服务（International Service for Geomagnetic Indices, CRPE/CNET - CNRS，地址：法国圣莫尔塞德克斯区，海神大道4号，邮编F-94107）发布。