The cross-polar cap potential (PCP) is a parameter that characterizes the level at which solar wind energy is coupled into the earth’s upper atmosphere. It is measured in the ionosphere by integrating the electric field along a line connecting maxima of the high-latitude convection pattern. A simple explanation for the source of the PCP is that, for southward interplanetary magnetic field (IMF) conditions, the polar cap field lines connect directly to the solar wind magnetic field and the solar wind electric field maps directly along them [e.g. Toffoletto and Hill, 1989]. If one knew the amount of open flux threading the polar cap and the solar wind magnetic field strength and velocity, it would be a simple matter to calculate the PCP. This is attractive since the upstream IMF and solar wind plasma are monitored continuously, and it is possible to observe the location of the open-closed field line boundary, which provides an estimate of the flux threading the polar cap. Unfortunately, the simple explanation is not necessarily right. Even more damning, the level of variability of the potential when plotted against solar wind parameters is quite large.
In this Journal Club, a recent study of the observed variability of the CCP will be presented along with discussion of the source of the CCP and the source of the variability..