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Ann. Geophys., 16, 1097-1104, 1998
www.ann-geophys.net/16/1097/1998/
© European Geosciences Union 1998
Observations of the upper frequency cutoffs of the auroral kilometric radiation
J. Hanasz1, R. Schreiber2, H. de Feraudy3, M. M. Mogilevsky4, and T. V. Romantsova4
1Space Research Centre, P.A.S., Toruń , Poland
2Nicolaus Copernicus Astronomical Centre, Laboratory for Astrophysics, Toruń , Poland
3Centre d'Etude des Environnements Terrestre et Planétaire CNRS, Velizy, France
4Space Research Institute, R.A.S., Moscow, Russia
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) ยท Radio science (magnetospheric physics)
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