A curve fitting procedure for mathematically representing the auroral oval is described. It is shown that the Feldstein ovals and quiet auroral ovals of the night side in DMSP photographs can be approximated by an offset circle, plus a small (≲ 1° magnitude) Fourier component in corrected geomagnetic coordinates. Parameters are introduced, Θ and (θo, ϕo), that indicate the dynamic motion of the auroral oval size and center location, respectively. Using the parameter Θ, an analysis of several DMSP photographs shows a strong correlation between the southward interplanetary magnetic field (−Bz) and the size of the auroral oval.