J. Space Weather Space Clim.
Volume 8, 2018
|Number of page(s)||17|
|Published online||07 December 2018|
A homogeneous aa index: 2. Hemispheric asymmetries and the equinoctial variation
Department of Meteorology, University of Reading, Whiteknights Campus, Earley Gate, PO Box 243, Reading
RG6 6BB, UK
2 Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, Oxfordshire, UK
3 Institut de Physique du Globe de Strasbourg, UMR7516, Université de Strasbourg/EOST, CNRS, 5 rue René Descartes, 67084 Strasbourg Cedex, France
4 British Geological Survey, Edinburgh EH14 4AP, UK
* Corresponding author: firstname.lastname@example.org
Accepted: 19 October 2018
Paper 1 (Lockwood et al., 2018) generated annual means of a new version of the aa geomagnetic activity index which includes corrections for secular drift in the geographic coordinates of the auroral oval, thereby resolving the difference between the centennial-scale change in the northern and southern hemisphere indices, aaN and aaS. However, other hemispheric asymmetries in the aa index remain: in particular, the distributions of 3-hourly aaN and aaS values are different and the correlation between them is not high on this timescale (r = 0.66). In the present paper, a location-dependant station sensitivity model is developed using the am index (derived from a much more extensive network of stations in both hemispheres) and used to reduce the difference between the hemispheric aa indices and improve their correlation (to r = 0.79) by generating corrected 3-hourly hemispheric indices, aaHN and aaHS, which also include the secular drift corrections detailed in Paper 1. These are combined into a new, “homogeneous” aa index, aaH. It is shown that aaH, unlike aa, reveals the “equinoctial”-like time-of-day/time-of-year pattern that is found for the am index.
Key words: Space climate / Space weather / Geomagnetism / Space environment / Historical records
© M. Lockwood et al., Published by EDP Sciences 2018
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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