Martini et al., JGR, 2011

Abstract

Motivated by recent attempts to derive geomagnetic activity from hourly mean data in long-term studies, we test the recursive Kalman filter method to obtain the regular solar variation curve of the geomagnetic field. Using a simple algorithm we are able to assign a quiet-day curve to every day separately, without the need for additional input parameter(s) to define the geomagnetically quiet days. We derive a digital counterpart A_hK of the analog range index Ak at the sub-auroral Sodankylä station, and compare it to the earlier digital estimate Ah and the local Ak index. We find that the new method outperforms the former estimate in every aspect studied, and provide a robust, straightforward manner to estimate and verify the manually scaled Ak index, based on readily available hourly values. The model is independent of sampling, thus for shorter-term studies where high-sampling data is available, more accurate estimates can also be obtained when needed. Therefore, in contrast to other recent approaches, we do not provide a method to quantify irregular activity directly, but derive the actual quiet-day curves in the traditional manner. In future applications the same algorithm may be used to define a wide variety of geomagnetic indices (such as Ak, Dst or AE).