Identification of broad-band waves above the auroral acceleration region: Cluster observations
- 1Swedish Institute of Space Physics, Box 537, SE-751 21 Uppsala, Sweden
- 2Dept. of Astronomy and Space Physics, Uppsala University, Box 515, SE-751 20 Uppsala, Sweden
- 3The Blacket Laboratory, Imperial College, Prince Consort Road, London SW7 2BW, London, UK
- 4CETP/CNRS, University of Versailles Saint Quentin, 10–12 Av. De l’Europe, 78140 Vélizy, France
Abstract. We investigate broad-band emissions at frequencies above the ion gyrofrequency on auroral field lines at geocentric distances of about 4.5 Earth radii. Observations by the Cluster satellites are used to study the wave characteristics and to determine the wave modes involved. All events include some bursts of broad-band emissions with a substantial component of the electric field parallel to the geomagnetic field. Studying the polarization of the emissions we find that linear waves in a homogeneous plasma can be used to theoretically describe the observations.
The broad-band emissions include short bursts of ion acoustic waves, and longer periods of ion Bernstein and Electrostatic Ion Cyclotron (EIC) waves. All waves occur during the same event within a few seconds, with EIC waves as the most common. Theoretically, there is no sharp limit between these wave modes and they can be described by the same dispersion surface. These emissions are closely associated with low-frequency Alfvén waves, indicating a possible generation mechanism.
Key words. Magnetospheric physics (auroral phenomena; electric fields; plasma waves and instabilities)