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

Mike Lockwood,Luke A Barnard,Aude Chambodut,Mathew J Owens,Ellen Clarke,Ivan D Finch

doi:10.1051/swsc/2018044

Abstract

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.

Highlights

As discussed in Paper 1 (Lockwood et al, 2018), the aa index was devised by Mayaud (1971, 1972, 1980) to give a continuous, well-calibrated and homogeneous record of geomagnetic activity that extends back to 1868
*Corresponding author: m.lockwood@reading.ac.uk 1940; Cliver et al, 2002; Le Mouël et al, 2004), with equinoctial peaks in average values: this can be seen in Figure 1a for the northern hemisphere index, aaN, for the southern hemisphere index, aaS and for the average of the two, aa
There is a second annual geometric effect associated with the variable difference between the GSE and heliocentric RTN reference frames: this effect is in antiphase with the heliographic latitude effect, favouring solstices over the equinoxes in terms of giving southward IMF and geomagnetic activity

Summary

The aa and am indices

As discussed in Paper 1 (Lockwood et al, 2018), the aa index was devised by Mayaud (1971, 1972, 1980) to give a continuous, well-calibrated and homogeneous record of geomagnetic activity that extends back to 1868. There is a second annual geometric effect associated with the variable difference between the GSE and heliocentric RTN reference frames: this effect is in antiphase with the heliographic latitude effect, favouring solstices over the equinoxes in terms of giving southward IMF and geomagnetic activity It has no UT variation but is relatively small. Note that indices influenced by the substorm current wedge depend on the solar wind dynamic pressure PSW (= mSWNSWVSW2, where mSW is the mean ion mass, NSW the number density and VSW the speed of the solar wind), because it compresses the near-Earth geomagnetic tail and so modulates the nearEarth cross-tail current there for a given open magnetic flux content in the tail (Lockwood, 2013): Finch et al (2008) showed that a VSW2 dependence was present in the equinoctial pattern response but not in the directly-driven dayside response. As discussed in Paper 1, mid-latitude range indices respond primarily to the substorm current wedge, and so the results of Finch et al (2008) explain why it is the am index that displays the equinoctial pattern most clearly

The aims of the present paper

Methodology

Conclusions