Wideband bursts of auroral kilometric radiation and their association withUV auroral bulges

Impulsive wideband bursts of auroral kilometric radiation (AKR) observed fr om the Interball 2 spacecraft are characterized by a rapid rise of intensit y (< 1 min) over a wide frequency range from 30 to 900 kHz, followed by a d ecay over a few minutes. Their source regions expand rapidly both upward an d downward along the auroral field lines. The expansion velocity is determi ned from the rapid broadening of the frequency range of their leading edge (sometimes more than 100 kHz s(-1)). The drifts can be negative or positive . Above 500 kHz the inferred downward source expansion is sometimes as smal l as 2 km s(-1) (at altitudes below 3000 km), much smaller than the velocit y of the ion sound waves. In the medium frequency range 200 - 500 kHz (3000 - 6000 km) the expansion velocities are usually large, sometimes more than 3000 km s(-1), comparable to the velocity of the Alfven waves. In the lowe r frequency range, below 200 kHz (<greater than> 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 r apid development of auroral substorms. The bursts show a microstructure, wh ich 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.