  The solar wind influence on geospace can be described as the sum of a
directly driven component, or dayside reconnection, and an unloading component,
associated with the release of magnetic energy via nightside reconnection. The
two processes are poorly correlated on short time scales, but exactly equal
when averaged over long time windows. Because of this peculiar property,
regression models of ionospheric electrodynamics that are based on solar wind
data are time scale specific: Models derived from 1 min resolution data will be
different from models derived from hourly, daily, or monthly data. We explain
and quantify this effect on simple linear regression models of various
geomagnetic indices. We also derive a time scale-dependent correction factor
that can be used with the Average Magnetic field and Polar current System
model. Finally, we show how absolute estimates of the nightside reconnection
rate can be calculated from solar wind measurements and geomagnetic indices.
