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.