The new digital Ah index of geomagnetic activity at Alibag and other stations

Abstract

The geomagnetic activity carries important information about various parameters of the near-earth space and the solar magnetic activity on short as well as on long-term scales. In short or moderately long-term studies, the Kp/Ap index is a widely used reliable measure of geomagnetic activity. On the long-term scale, the aa index has, till recently, been the only index offering a sufficiently long registration for centennial studies. However, the long-term robustness of aa index was seriously questioned, and the early registrations until the first decades of the last century are not available in digital form in sufficiently high resolution. Therefore, the aa index cannot be verified, nor its probable error be reassuringly quantified. Here we further verify the incorrectness of aa, using a recently introduced digital measure of the geomagnetic activity, the Ah index calculated at the Indian Alibag station. Global Ah from a number of long-running stations can be used to reliably extend the Ap series by roughly 30 years, allowing the study of geomagnetic activity for more than a century at the three-hourly resolution. Local Ah indices can also be calculated at any latitudinal region.