(Courtesy: Center for Integrated Space Weather Modeling)
The Boyle Index,
[Boyle et. al., J. Geophys. Res., 102, 111, 1997]
where v is the solar wind velocity in km/sec and B is the magnitude of the IMF, is a good predictor of the polar cap potential drop for times when the solar wind is steady and the index is 160 kV or less. For events where the Boyle index exceeds 160 kV, the polar cap potential generally saturates, only exceeding 200 kV in rare occasions. A Boyle index of 200 averaged over three hours will often yield a Kp index of 6 or higher; for 250 or more, the Kp index is often 7 or higher.
These data are derived from measurements taken by the ACE spacecraft, a component of the International Solar Terrestrial Program (ISTP) and SEC/NOAA for the planetary K index.
Click on the link BIvsKp for the draft of the paper accepted for publication in AGU Space Weather.
Graphs of other solar wind parameters over the last 24 hours can be viewed at the NOAA site.
Other links of interest:
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Page last updated 05/04/2010.