AKR bursts and substorm field line excitation

Abstract

A new manifestation of the Auroral Kilometric Radiation is put in evidence through the observations of the POLRAD wave experiment of the INTERBALL mission, the Wide Band Bursts of the AKR, or WBB AKR. It consists in bursts of radiation with a very broad bandwidth, typically 100 – 800 kHz. The whole frequency range is excited at once or in less than few minutes. This corresponds to the excitation of long segments of auroral field lines in a time often less that one minute. The sources of the emission are stretched along the field lines between altitudes ranging from 2000 to 20000 km. The relationship of these bursts to the development of auroral bulges, in the UV spectral range, is shown by the comparison of the POLRAD wave observations and those of the UV imager of the POLAR mission. It is shown that these bursts are generated during fast expansion of the auroral bulge. An accurate timing of the burst events is made with the time evolution of the frequency integrated wave energy flux. It shows that the bursts have a rise time of few minutes which is followed by an exponential relaxation with a characteristic time of few tens of minutes. The source of the bursts first expands along the field lines then it shrinks during the relaxation phase. The bursts are triggered few minutes before the maximum intensity of the UV auroral bulge. It is also shown that the bursts actually consist of a large number of individual broad bandwidth elements, each, lasting less than few seconds.