Northern and Southern Hemisphere Polar Cap Indices: to what extent do
they agree and to what extent should they agree?
Abstract
The IAGA-endorsed Polar Cap indices for the northern and southern
hemispheres, PCN and PCS, are compared for 1998-2021. Potential effects
of the slightly-different, and changing, magnetic coordinates of the two
magnetic stations employed, Thule (Qaanaaq) in Greenland and Vostok in
Antarctica, are investigated. It is shown that the agreement in overall
behaviour of the two indices is very close indeed but that PCS
consistently correlates slightly better with solar wind parameters than
PCN. Optimum lags for these correlations are 19 min. for 1-min. data and
37 min. for hourly averages. The correlations are significantly higher
with the predicted magnetopause reconnection voltage, which is a linear
predictor of PCN and PCS for all 1-hour data and for all but the largest
0.1% of 1-min. values. The indices show lower correlation and marked
non-linearity (tending to saturation) at all levels with the estimated
magnetopause reconnection electric field or the estimated power input
into the magnetosphere. The PCN index is shown to correlate closely with
the transpolar voltage measured by the northern-hemisphere SuperDARN
radar network and both PCN and PCS clearly show the Russell-McPherron
effect of dipole tilt and the Y-component of the interplanetary magnetic
field. However the patterns in time-of-year and Universal Time (UT) are
complicated by lobe reconnection during northward-IMF, the effect of
which on the indices is shown to be predominantly a summer hemisphere
phenomenon and gives UT dependence on the IMF Y-component that is
predicted theoretically.