DISCRETE WAVE-PACKETS AT THE PROTON CYCLOTRON FREQUENCY AT COMET P/HALLEY

We present in this letter the first experimental evidence for wave packets near the local proton cyclotron frequency in the plasma environment of a comet. The observations have been made by the GIOTTO magnetometer experiment (MAG) around comet P/Halley on both sides of closest approach. The waves are always left-handed in the spacecraft frame, elliptically or nearly circularly polarized and they propagate at small singles from the ambient magnetic field. Their period in the spacecraft frame always closely fits the local proton cyclotron period. The possible generation mechanisms of these waves are discussed. They can be consistently interpreted as waves generated by a resonant helical beam instability fed by the cometary pick-up protons. These waves are intrinsically right- handed waves the plasma rest frame and are anomalously Doppler-shifted from the plasma frame to the spacecraft frame because their phase velocity is small compared to the local solar wind speed. A generation by a resonance with heavy (water group) ions introducing a temperature effect is also discussed but is less satisfactory.