In the early days of space research, geomagnetic activity levels were quantified by the 3-hr Kp index evaluated using data from an array of middle-latitude magnetic observatories. In more recent times, the hourly average auroral electrojet AE index has been commonly used as an indicator of the level of magnetospheric activity. In order to qunatitatively compare correlations of particle and field signatures of solar-terrestrial interaction made using these two indices, it would be useful to have a qunatitative relationship between them. In this report, we provide such a relationship together with an evaluation of the circumstances under which the relationship is suspect. ¿American Geophysical Union 1991