Long-term biases in geomagnetic <i>K</i> and <i>aa</i> indices

• Main Author: J. J. Love
• Format: Article
• Language: English
• Published: Copernicus Publications 2011-08-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/29/1365/2011/angeo-29-1365-2011.pdf

Analysis is made of the geomagnetic-activity <i>aa</i> index and its source <i>K</i>-index data from groups of ground-based observatories in Britain, and Australia, 1868.0–2009.0, solar cycles 11–23. The <i>K</i> data show persistent biases, especially for high (low) <i>K</i>-activity levels at British (Australian) observatories. From examination of multiple subsets of the <i>K</i> 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 <i>K</i>-value estimation methods. Instead, the biases appear to be artifacts of the latitude-dependent scaling used to assign <i>K</i> values to particular local levels of geomagnetic activity. The biases are not effectively removed by weighting factors used to estimate <i>aa</i>. We show that long-term averages of the <i>aa</i> index, such as annual averages, are dominated by medium-level geomagnetic activity levels having <i>K</i> values of 3 and 4.