Abstract
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(τ) is a simple accumulation of the linear ap calculated with an attenuation factor τ included to take account of natural temporal relaxation. The case for ap(τ) and the related Kp(τ) 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.
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