In order to directly compare the solar wind measurements from ACE with the corresponding ionospheric radar measurements, and because the statistical model pattern used in APL FIT is keyed to the IMF, we must determine the amount of time to delay the ACE measurements to allow for propagation to the ionosphere. This time delay, or lag time, between ACE and the ionosphere depends on the solar wind speed and density and can range from ~30 min to longer than 90 min.
The sensitivity to errors in the determination of the lag time is greatly reduced by selecting time periods with quasi-stable solar wind and IMF conditions. A nominal value for the lag time is found by applying a relatively standard technique, whereby the lag time is comprised of three parts: the solar wind advection time , the magnetosheath transit time , and the Alfvén transit time along magnetic field lines from the subsolar magnetopause to the ionosphere .
The three components are