Wideband bursts of auroral kilometric radiation and their association with UV auroral bulges

Impulsive wideband bursts of auroral kilometric radiation (AKR) observed from the Interball 2 spacecraft are characterized by a rapid rise of intensity (< l min) over a wide frequency range from 30 to 900 kHz, followed by a decay over a few minutes. Their source regions expand rapidly both upward and downward along the auroral field lines. The expansion velocity is determined 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 small as 2 km s−1 (at altitudes below 3000 km), much smaller than the velocity 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 Alfvén waves. In the lower frequency range, below 200 kHz (> 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.