We report the detection of large-amplitude, quasi-harmonic density fluctuations with associated magnetic field oscillations in the region surrounding the diamagnetic cavity of comet 67P. Typical frequencies are ~0.1 Hz, corresponding to ~10 times the water and ≲0.5 times the proton gyro-frequencies, respectively. Magnetic field oscillations are not always clearly observed in association with these density fluctuations, but when they are, they consistently have wave vectors perpendicular to the background magnetic field, with the principal axis of polarization close to field-aligned and with a ~90° phase shift w.r.t. the density fluctuations. The fluctuations are observed in association with asymmetric plasma density and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. This is a new type of waves not previously observed at comets. They are likely Ion Bernstein waves, and we propose that they are excited by unstable ring, ring-beam or spherical shell distributions of cometary ions just outside the cavity boundary. These waves may play an important role in redistributing energy between different particle populations and reshape the plasma environment of the comet.

In early June 2015 the Ion and Electron Sensor (IES) on board the Rosetta spacecraft (SC) observed troughs in the ion measurements at about 200 km from the comet. The troughs coincided with measurement results of two other instruments on board Rosetta: the peaks of the neutral gas density measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the peaks of the electron density measured by the Langmuir and Mutual Impedence Probe Instruments, (LAP and MIP) and the most negative levels of the Spacecraft potential also measured by LAP. We propose that the dips in the ion measurements are the result of charge exchange reactions between the ions and the neutral population emitted by the comet nucleus. Measurements from the Ion Composition Analyzer (ICA) on board Rosetta show that these ions are mostly water ions.

We show that an ion-ion cross-field streaming instability between cold newborn cometary ions and heated heavy ions that were picked up upstream is likely a contributing source of observed lower hybrid (LH) waves in the inner coma of comet 67P/Churyumov-Gerasimenko. Electric field oscillations in the LH frequency range are common here, and have previously been attributed mainly to the lower-hybrid drift instability (LHDI), driven by gradients associated with observed local density fluctuations. However, the observed wave activity is not confined to such gradients, nor is it always strongest there. Thus, other instabilities are likely needed as well to explain the observed wave activity. Several previous works have shown the existence of multiple populations of cometary ions in the inner coma of 67P, distinguished by differences in mass, energy and/or flow direction. We here examine two selected time intervals in October and November 2015, with substantial wave activity in the lower hybrid frequency range, where we identify two distinct cometary ion populations: a bulk population of locally produced, predominantly radially outflowing ions, and a more tenuous population picked up further upstream and accelerated back towards the comet by the solar wind electric field. These two populations exhibit strong relative drifts ($\sim$20 km/s, or about 5 times the pickup ion thermal velocity), and we perform an electrostatic dispersion analysis showing that conditions should be favorable for lower hybrid wave generation through the ion-ion cross-field instability.

Wakes behind spacecraft caused by supersonic drifting positive ions are common in plasmas and disturb in situ measurements. We concentrate on observations of the electric field with double-probe instruments. When the equivalent spacecraft charging is small compared to the ion drift energy the wake effects are caused by the spacecraft body and can be compensated for. We discuss examples from the Cluster spacecraft in the solar wind, including statistics of the direction, width and electrostatic potential of wakes, and compare with an analytical model. When the equivalent positive spacecraft charging is large compared to the ion drift energy, an enhanced wake forms. In this case observations of the geophysical electric field with the double-probe technique becomes extremely challenging. Rather, the wake can be used to estimate the flux of cold (eV) positive ions. We discuss such examples from the Cluster spacecraft in the low-density magnetospheric lobes. For an intermediate range of parameters, when the equivalent charging of the spacecraft is similar to the drift energy of the ions, also the charged wire booms of a double-probe instrument must be taken into account. We discuss an example of these effects from the MMS spacecraft near the magnetopause. We find that the observed wake characteristics provide information which can be used for scientific studies. An important example is the enhanced wakes used to estimate the outflow of ionospheric origin in the magnetospheric lobes to about 10^26 cold (eV) ions/s, constituting a large fraction of the mass outflow from planet Earth.