A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation

Lockwood, M. ORCID: https://orcid.org/0000-0002-7397-2172, Finch, I. D., Chambodut, A., Barnard, L. A. ORCID: https://orcid.org/0000-0001-9876-4612, Owens, M. J. ORCID: https://orcid.org/0000-0003-2061-2453 and Clarke, E. (2018) A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation. Journal of Space Weather and Space Climate, 8. A58. ISSN 2115-7251 doi: 10.1051/swsc/2018044

Abstract/Summary

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.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/80194
Item Type	Article
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	EDP Sciences
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