Electrostatic turbulence in the high latitude ionosphere

Two components of the Extremely Low Frequency (ELF) electric field are measured on board the low altitude (400 to 2000 km) polar orbiting satellite Aureol-3. At high latitude, the main wave-field component observed in the frequency range up to a few hundred Hertz is the electrostatic turbulence. The frequency spectrum measured in the satellite reference frame usually follows a power lay (E squared alpha E(sub 0) f sup -alpha), thus allowing to describe the turbulence with two parameters (amplitude and spectral index). These characteristic parameters will be discussed in relation with the possible free energy sources: field-aligned current, particle precipitation, electron density gradient. The relation between the frequency spectrum and the wave number spectrum is not unambiguous. However, under the hypothesis of time-independent turbulence, it is possible to deduce, from the measurement of two different components of the electric field, the anisotropy of the wave number spectrum of the turbulence. The method used for this determination will be described and applied to physical situations observed in the polar cap, in the auroral zone, and in sub-auroral regions.