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 kBTe≈0.2 eV and one
warm at kBTe≈2 eV. The ions are
dominated by a cold (a few hundredths of electronvolt) distribution of
water group ions with a bulk speed of (3–3.7) km s−1. A
warm kBTe≈6 eV ion population,
which also is present, has no influence on the ion acoustic waves due
to its low density of only 0.25 % of the plasma density.
Near closest approach the propagation direction was within
50∘ from the direction of the bulk velocity. The waves, which
in the plasma frame appear below the ion plasma frequency
fpi≈2 kHz, are Doppler-shifted to the
spacecraft frame where they cover a frequency range up to
approximately 4 kHz.
The waves are detected in a region of space where the
magnetic field is piled up and draped around the inner part of the
ionised coma. Estimates of the current associated with the magnetic
field gradient as observed by Rosetta are used as input to
calculations of dispersion relations for current-driven ion acoustic
waves, using kinetic theory. Agreement between theory and observations
is obtained for electron and ion distributions with the properties
described above. The wave power decreases 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. Wave
observations and theory combined supplement the particle measurements
that are difficult at low energies and complicated by spacecraft
charging.
Kinetic Theory of Ion Acoustic Waves Observed at Comet 67P/Churyumov–Gerasimenko
