Long-term biases in geomagnetic <i>K</i> and <i>aa</i> indices
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...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2011-08-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/29/1365/2011/angeo-29-1365-2011.pdf |
| Summary: | 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. |
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| ISSN: | 0992-7689 1432-0576 |