Lower-Hybrid waves observed by Rosetta at comet 67P

Electric field measurements from cometary environments are very rare, but can provide important information on how plasma waves help fashion the plasma environment. We investigate the plasma wave activity observed in the electric field measurements obtained by the Langmuir probe instrument (RPC-LAP) onboard ESA's Rosetta spacecraft, which followed the comet 67P/Churyumov-Gerasimenko in its orbit around the sun for over two years in 2014-2016. We focus on waves in the range 1-30 Hz, roughly corresponding to the lower-hybrid frequency range. Here, electric field oscillations close to the local H₂O⁺ lower hybrid frequency are common. Especially large wave amplitudes are often observed at or near pronounced plasma density gradients, and a linear instability analysis shows that conditions are often favourable for wave growth by the lower hybrid drift instability. However, the association to density gradients is not ubiquitous and other instabilities are likely needed as well to explain the observed wave activity, e.g. the two-stream instability between solar wind protons and cometary pick-up ions. Close to peak activity of the comet however, the solar wind flow was entirely diverted and excluded from the inner parts of the coma, where the spacecraft was. Here, we instead propose that an ion/ion streaming instability between cold newborn cometary ions and heated heavy ions that were picked up earlier, plays an important role for generating the waves observed in the lower hybrid frequency range. We compare theoretical conditions for growth of these instabilities to observed conditions in the plasma at 67P. This investigation helps to clarify the role and importance of these plasma waves in the cometary plasma environment. They can, for example, heat or cool plasma populations, produce supra-thermal electrons, reduce plasma anisotropies and gradients, couple different plasma species, and provide anomalous resistivity.