The list here contains the times and the potential drops observed by F13 and F15 
along their polar crossing track.  Since F13 is in the roughly dawn-dusk 
orientation, we generally take those observations as a proxy for the true cross 
polar cap potential drop.  The F15 data are included for completeness and to show 
that the observed potentials along that track are lower than the potentials 
observed along the F13 track.  Note also that because of the tilt of the magnetic 
dipole relative to the orbital plane, there is a period from roughly 0200 to 0800 
UT every day where the satellites do not go above 75 degrees magnetic latitude. 
Thus the F13 passes during that time observe potential that are much lower than 
the true cross polar cap potential.  Under nominal conditions both F13 and F15 
only skim the edge of the potential pattern during this time and observe potential 
drops close to zero.  During this storm, the potential pattern has expanded 
greatly so both satellites observe significant potential drops.  However, how 
close the F13 observations during this time are to the true cross polar cap 
potential drop is uncertain.   See Hairston et al., 2003 for more information 
on the observations during this storm. Last, the times given here are the midpoint 
times of the satellite's crossing of the potential pattern and are within a minute 
or two of when the satellite reached the highest magnetic latitude.  A data 
dropout during the northern pass of F13 at the start of 1 April resulted in no 
potential drop measurement being available for that pass. 

Hairston, M. R., T. W. Hill, and R. A. Heelis, Observed saturation of the 
ionospheric polar cap potential during the 31 March 2001 Storm, Geo. Res. 
Letts., 30, doi:10.1029/2002GL015894, 2003