From the NASA Web Site:
"OVATION: An empirical model of the intensity of the aurora. The model uses solar wind conditions and the IMF at the L1 point as inputs.
The Display: Shows the intensity and location of the aurora as expected for the time shown at the bottom of the map. This forecast is based on current solar wind conditions and the average time for the solar wind to propagate from the ACE satellite at L1 to Earth.
The model produces an estimate of the intensity of the aurora. In this product a linear relationship between intensity and viewing probability is assumed. This relationship was validated by comparison with data from the UVI instrument on the NASA POLAR Satellite .
The sunlit side of Earth is indicated by the lighter blue of the ocean. The sub-solar point is also shown as a yellow dot but only if the sub-solar point is in the view of the choosen image. The day-night line or terminator is shown as a yellow line. Note that the aurora will not be visible during daylight hours and it may be an hour or more before sunrise or after sunset that the aurora can be seen from the ground."