Abstract
Using in situ measurements from different instruments on board the Rosetta spacecraft, we investigate the properties of the newly discovered low-frequency oscillations, known as singing comet waves, that sometimes dominate the close plasma environment of comet 67P/Churyumov-Gerasimenko. These waves are thought to be generated by a modified ion-Weibel instability that grows due to a beam of water ions created by water molecules that outgass from the comet. We take advantage of a cometary outburst event that occurred on 2016 February 19 to probe this generation mechanism. We analyze the 3D magnetic field waveforms to infer the properties of the magnetic oscillations of the cometary ion waves. They are observed in the typical frequency range (~50 mHz) before the cometary outburst, but at ~20 mHz during the outburst. They are also observed to be elliptically right-hand polarized and to propagate rather closely (~0−50°) to the background magnetic field. We also construct a density dataset with a high enough time resolution that allows us to study the plasma contribution to the ion cometary waves. The correlation between plasma and magnetic field variations associated with the waves indicates that they are mostly in phase before and during the outburst, which means that they are compressional waves. We therefore show that the measurements from multiple instruments are consistent with the modified ion-Weibel instability as the source of the singing comet wave activity. We also argue that the observed frequency of the singing comet waves could be a way to indirectly probe the strength of neutral plasma coupling in the 67P environment.
Highlights
For about two years, the groundbreaking ESA/Rosetta mission (Glassmeier et al 2007a; Taylor et al 2017) escorted comet 67P/Churyumov-Gerasimenko, while previous cometary missions were limited to flybys
We took advantage of high time-resolution magnetic field measurements and a newly created high time-resolution density dataset obtained from the cross-calibration of RPC-MIP and RPC-LAP instruments, which allowed us to cover the frequency range of the cometary ion mode
– they propagate mostly parallel to oblique (∼0−50◦) to the background magnetic field;
Summary
The groundbreaking ESA/Rosetta mission (Glassmeier et al 2007a; Taylor et al 2017) escorted comet 67P/Churyumov-Gerasimenko (hereafter 67P), while previous cometary missions were limited to flybys. To benefit from the high time resolution of the density time series obtained from RPC-LAP voltage or current and the trustworthiness of RPC-MIP densities, we cross-calibrated the 17 ms (i.e., 60 Hz) time-resolution RPCLAP floating potential measurements with RPC-MIP absolute density estimates and obtained a plasma density time series (hereafter called RPC-MIP/LAP density) with a time resolution sufficient for the analysis of plasma oscillations conducted in this paper For this purpose, we considered a simple charging model of the Rosetta orbiter where the incoming cometary electron current collected by the spacecraft body is balanced by the outgoing photoelectron current associated to the UV ionizing flux on the spacecraft, while the ion and secondary current contribution is considered negligible (Odelstad et al 2015). After the analysis of magnetic field oscillations during the outburst event of 2016 February 19 at comet 67P, the plasma pressure variations are investigated using these cross-calibrated densities
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