A homogeneous aa index, 2. Hemispheric asymmetries and the equinoctial variation

Lockwood, Mike; Finch, Ivan D.; Chambodut, Aude; Barnard, Luke A.; Owens, Mathew J.; Clarke, Ellen. 2018 A homogeneous aa index, 2. Hemispheric asymmetries and the equinoctial variation. Journal of Space Weather and Space Climate, 8, A58.

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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: Publication - Article
Digital Object Identifier (DOI):
ISSN: 2115-7251
Date made live: 09 Jan 2019 16:46 +0 (UTC)

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