Impulsive wideband bursts of auroral kilometric radiation (AKR) observed from the Interball 2 spacecraft are characterized by a rapid rise of intensity (6000 km), the bursts show an upward source expansion, often simultaneous with the downward expansion of the same source below 3000 km. We observe a close association of the AKR bursts with the fast expansion of the auroral bulges recorded with the UV imager on Polar. It is concluded that the bursts are triggered during the rapid development of auroral substorms. The bursts show a microstructure, which consists of multiple microbursts lasting sometimes less than 6 s. They indicate that the source regions are filamented. This filamentation may be the counterpart of a filamentary nature of the currents closing the current wedge, flowing parallel to the geomagnetic field. ¿ 2001 American Geophysical Union