Abstract
The IESP experiment implemented onboard the Interball auroral probe measures the six components (3B, 3E) of the waves in the ULF range: 0.1â10 Hz and from time to time 0â30 Hz. Two different kinds of waves have been observed in the auroral region at altitudes between 10 000 and 20 000 km: (1) electrostatic emissions which consist of quasi-monochromatic structures with frequencies above the oxygen gyrofrequency, superimposed on a wide band signal interpreted as a Doppler broadening, (2) electromagnetic wide band spectrum fluctuations. These emissions are interpreted as current-driven electromagnetic or electrostatic ion cyclotron waves. The electromagnetic/electrostatic character is controlled by the plasma parameter βi and by the O+ concentration.Key words. Magnetospheric physics · Auroral phenomena · Plasma waves and instabilities · Interball Auroral probe
Highlights
The auroral region above the ionosphere at an altitude of 32 Earth radii has already been investigated by the American satellites S3-3 and DE-1 and by the Swedish satellite Viking
Thanks to the magnetic and electric sensor implemented onboard the Interball Auroral probe, both electrostatic or electromagnetic waves have been detected in the auroral region above 2 RE
From an observational point of view one distinguishes two kinds ofuctuations: (1) electrostatic emissions which consist of quasi-monochromatic structures above the oxygen gyrofrequency, superimposed on a wide band signal interpreted as a Doppler broadening, (2) electromagneticuctuations with a wide band spectrum and a very bursty character
Summary
The auroral region above the ionosphere at an altitude of 32 Earth radii has already been investigated by the American satellites S3-3 and DE-1 and by the Swedish satellite Viking. While most of the waves and structures observed were interpreted as being electrostatic, in some cases the authors have concluded the existence of electromagnetic waves, generated by wave-particle interactions (Temerin and Lysak, 1984; Gustafsson et al, 1990). The IESP experiment which carries a complete set of instruments for measuring the components of the electromagnetic waves in the ULF range up to 30 Hz is well suited to disentangle the electromagnetic and electrostatic waves and to assess the potential role of KAWs at accelerating particles. The trajectory of the Interball Auroral probe is well adapted to exploring the region along the auroral ®eld lines where the dierent mechanisms of particle acceleration occur since it covers altitude ranges between 10 000 and 20 000 km. A tentative interpretation of the phenomena observed in the regions probed by Interball will be given in the third part
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