The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ¿1 (¿2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm-associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ~1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ~300 kHz, corresponding to an upper limit of the AKR source altitude of ~4700 km. The entire AKR frequency band expands to ~64--650 kHz within a few minutes after onset, corresponding to a wider source altitude of ~2100--12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase. ¿ 2000 American Geophysical Union