OBSERVATIONS OF THE UPPER FREQUENCY CUTOFFS OF THE AURORAL KILOMETRICRADIATION

Intense auroral kilometric radiation (AKR) is being frequently observe d with POLRAD from the Auroral Probe (Interball-2). Observations of th e abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are repo rted. The UFCs can be observed at a frequency range from 300 to 700 kH z, corresponding to AKR generation altitudes from approximately 4800 t o 2100 km, and are distributed in magnetic local time (MLT) hours simi larly to the AKR events, with a maximum at 1 h MLT. The observed frequ ency extent of the UFCs is less than or equal to 12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested tha t the UFC may be associated with an abrupt switching on of the generat ion mechanism, when the electron density becomes sufficiently low insi de a plasma depletion at an altitude where the ratio off,,lf,, crosses some threshold value. The steepness of the UFCs can imply a non-linea r process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are someti mes, though very seldom (<10%), accompanied by narrow band (less than 4 kHz) ''ridges'' of radiation observed at the cutoff frequency. They are smoothly drifting in frequncy for several minutes. The power densi ty of radiation in the ''ridge'' can be up to 2 orders of magnitude st ronger than in the accompanying wide band emission of AKR. The ''ridge '' at UFC can imply either energy concentration at the source bottom, or focusing, if specific conditions for the escape of the radiation ar e assumed.