A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation. (arXiv:1811.09815v1 [physics.space-ph])

A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation. (arXiv:1811.09815v1 [physics.space-ph])
<a href="http://arxiv.org/find/physics/1/au:+Lockwood_M/0/1/0/all/0/1">Mike Lockwood</a>, <a href="http://arxiv.org/find/physics/1/au:+Finch_I/0/1/0/all/0/1">Ivan D. Finch</a>, <a href="http://arxiv.org/find/physics/1/au:+Chambodut_A/0/1/0/all/0/1">Aude Chambodut</a>, <a href="http://arxiv.org/find/physics/1/au:+Barnard_L/0/1/0/all/0/1">Luke A. Barnard</a>, <a href="http://arxiv.org/find/physics/1/au:+Owens_M/0/1/0/all/0/1">Mathew J. Owens</a>, <a href="http://arxiv.org/find/physics/1/au:+Clarke_E/0/1/0/all/0/1">Ellen Clarke</a>

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.

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.

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