Jeffrey J. Love
2011
<p><span>Analysis is made of the geomagnetic-activity </span><i>aa</i><span> index and its source </span><i>K</i><span>-index data from groups of ground-based observatories in Britain, and Australia, 1868.0–2009.0, solar cycles 11–23. The </span><i>K</i><span> data show persistent biases, especially for high (low) </span><i>K</i><span>-activity levels at British (Australian) observatories. From examination of multiple subsets of the </span><i>K</i><span> data we infer that the biases are not predominantly the result of changes in observatory location, localized induced magnetotelluric currents, changes in magnetometer technology, or the modernization of </span><i>K</i><span>-value estimation methods. Instead, the biases appear to be artifacts of the latitude-dependent scaling used to assign </span><i>K</i><span> values to particular local levels of geomagnetic activity. The biases are not effectively removed by weighting factors used to estimate </span><i>aa</i><span>. We show that long-term averages of the </span><i>aa</i><span> index, such as annual averages, are dominated by medium-level geomagnetic activity levels having </span><i>K</i><span> values of 3 and 4.</span></p>
application/pdf
10.5194/angeo-29-1365-2011
en
European Geosciences Union
Long-term biases in geomagnetic K and aa indices
article