Abstract
Abstract. Intense auroral kilometric radiation (AKR) is being frequently observed with POLRAD from the Auroral Probe (Interball-2). Observations of the abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are reported. The UFCs can be observed at a frequency range from 300 to 700 kHz, corresponding to AKR generation altitudes from approximately 4800 to 2100 km, and are distributed in magnetic local time (MLT) hours similarly to the AKR events, with a maximum at 1 h MLT. The observed frequency extent of the UFCs is ≤12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested that the UFC may be associated with an abrupt switching on of the generation mechanism, when the electron density becomes sufficiently low inside a plasma depletion at an altitude where the ratio of fpe/fce crosses some threshold value. The steepness of the UFCs can imply a non-linear process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are sometimes, 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 density of radiation in the "ridge" can be up to 2 orders of magnitude stronger 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 are assumed.Key words. Magnetospheric physics (auroral phenomena; plasma waves and instabilities) · Radio science (magnetospheric physics)
Highlights
Interball-2, the second spacecraft of the Interball project, called the Auroral Probe, is aimed at studying dissipation processes of the energy transferred to the auroral region along the open magnetic ®eld lines from the magnetospheric tail
We have found 515 cases of upper frequency cutos (UFCs) in spectrograms recorded by POLRAD in the ®rst year of Interball-2 operation, from September 4, 1996 to October 8, 1997
Steepness of the UFC suggests that the growth rate of emission increases rapidly when the ratio of fpeafe inside the plasma depletion decreases below some threshold value
Summary
Interball-2, the second spacecraft of the Interball project, called the Auroral Probe, is aimed at studying dissipation processes of the energy transferred to the auroral region along the open magnetic ®eld lines from the magnetospheric tail. We present some spectral structures observed with POLRAD, characteristic by their steep cutos at the upper frequency limit of the AKR spectra, that, to our knowledge, have not been previously reported. Like those shown, indicated by horizontal bars above the spectrogram, are dierent They are single narrow drifting bands located at the upper cuto frequency of the AKR, accompanied by a weaker wide emission at lower frequencies. It can be noticed by comparison with the instrumental frequency pro®le, that its observed bandwidth is about 4 kHz (1 step of SFA).
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