A Simple Mean-Field Diagnostic from Stokes V Spectra

It is shown that the diagnostics from an observed circularly polarized line in a rapidly rotating star are directly interpretable, not in terms of the observed Stokes V profiles, but in terms of its antiderivative with respect to wavelength (in velocity units if preferred). This also leads to a new mean-field diagnostic that is just as easily obtained as the standard ``center of gravity'' approach, and is less susceptible to cancellation if the line-of-sight field changes sign over the face of the star.