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

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, \(aa_N\) and \(aa_S\). However, other hemispheric asymmetries in the \(aa\) index remain: in particular, the distributions of 3-hourly \(aa_N\) and \(aa_S\) 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, \(aa_{HN}\) and \(aa_{HS}\), which also include the secular drift corrections detailed in Paper 1. These are combined into a new, 'homogeneous' \(aa\) index, \(aa_H\). It is shown that \(aa_H\), unlike \(aa\), reveals the 'equinoctial'-like time-of-day/time-of-year pattern that is found for the \(am\) index.