The planetary geomagnetic indices Kp and ap are widely used in space geophysics. They provide an estimate of maximum magnetic perturbation within a 3-hour period. Many geophysical properties are clearly related to the indices, through energy transfer from a common disturbance source, but direct correlation is often lacking because of poor matching between the frequency of sampling and the physical response functions. The index ap(&tgr;) is a simple accumulation of the linear ap calculated with an attenuation factor &tgr; included to take account of natural temporal relaxation. The case for ap(&tgr;) and the related Kp(&tgr;) is made using applications to the variability of the plasma environment in the ionosphere and inner magnetosphere. These examples of improved correlation suggest that time-weighted integration might profitably be applied to other indices. ¿American Geophysical Union 1987