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.