Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko
Abstract. Ion acoustic waves were observed between 15 and 30 km from the centre of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft during its close flyby on 28 March 2015. There are two electron populations: one cold at approximately 0.2 eV and one warm at approximately 4 eV. The ions are dominated by a cold (a few hundredths of eV) distribution with a bulk speed of (3–3.7) km/s. Near closest approach the propagation direction was within 50 degrees from the direction of the bulk velocity, leading to a Doppler shift of the waves that in the spacecraft frame cover a frequency range up to approximately 4 kHz. The wave power decreased over cometocentric distances from 24 to 30 km. The main difference between the plasma at closest approach and in the region where the waves are decaying is the absence of a significant current in the latter.