Centennial geomagnetic activity studied by a new, reliable long-term index

We reanalyse geomagnetic activity during the last century using a recently proposed AhAh index which modifies the K   index method appropriate for using hourly data in long-term (centennial) studies. We calculate the local AhAh index for six stations from different latitudes whose observations cover most of the previous century. We take into account and correct for the fact that the data sampling was changed from hourly spot values to hourly means in the early part of the last century. Since variability of spot values is larger, the early AhAh indices, without due correction, would remain artificially large. Using recent high-sampling data, we estimate the required correction to be about 20%, i.e., large enough to make a significant effect for long-term estimates. The AhAh index verifies that geomagnetic activity has increased during the last century at all stations. Also, the AhAh indices prove our earlier finding that the amount of centennial increase varies greatly with latitude, being largest at high latitudes, smaller at low latitudes and, quite unexpectedly, smallest at mid-latitudes. The centennial increase depicted by the aa index is roughly twice larger than that depicted by the AhAh index at mid-latitudes, and even larger than depicted by global AhAh indices. Moreover, both the Ap index and the AhAh indices verify that the scaling of the aa index was erroneously modified by a few nT in late 1950s, implying that the aa index must be revised. We also show that the AhAh index correlates extremely well with the Ap index, better than the aa index and much better than the recently proposed, not-K   based Inter-Hour Variability (IHV) index. Accordingly, the global AhAh index offers the most reliable extension of the Ap index by roughly 30 years, and is recommended to be used in centennial studies of geomagnetic activity instead of the aa or IHV indices. Also, the local AhAh indices can be used to extend the local K/ak indices to the centennial time scales.