At a low activity comet the plasma is distributed in an asymmetric way. The hybrid simulation code Amitis is used to look at the spatial evolution of ion velocity distribution functions (VDFs), from the upstream solar wind to within the comet magnetosphere where the solar wind is heavily mass-loaded by the cometary plasma. We find that the spatial structures of the ions and fields form a highly asymmetric, half-open induced magnetosphere. The VDFs of solar wind and cometary ions vary drastically for different locations in the comet magnetosphere. The shape of the VDFs differ for different species. The solar wind protons show high anisotropies that occasionally resemble partial rings, in particular at small cometocentric distances. A second, decoupled, proton population is also found. Solar wind alpha particles show similar anisotropies, although less pronounced and at different spatial scales. The VDFs of cometary ions are mostly determined by the structure of the electric field. We perform supplementary dynamic particle backtracing to understand the flow patterns of solar wind ions that lead to these anisotropic distributions. This tracing is needed to understand the origin of cometary ions in a given part of the comet magnetosphere. The particle tracing also aids in interpreting observed VDFs and relating them to spatial features in the electric and magnetic fields of the comet environment.

No spacecraft visiting a comet has been equipped with instruments to directly measure the static electric field. However, the electric field can occasionally be estimated indirectly by observing its effects on the ion velocity distribution. We present such observations made by the Rosetta spacecraft on 19th of April 2016 when comet 67P was at a low outgassing rate and the plasma environment was relatively homogeneous. The ion velocity distributions show the cometary ions on the first half of their gyration. We estimate the bulk drift velocity and the gyration speed from the distributions. By using the local measured magnetic field and assuming an E x B drift of the gyrocentre, we get an estimate for the average electric field driving this ion motion. We analyse a period of 13h, during which the plasma environment does not change drastically. We find that the average strength of the electric field is 0.21mV/m. The direction of the electric field is mostly anti-sunward. This is in agreement with previous results based on different methods

Proton plasma asymmetries between the hemispheres of Venus’ dayside magnetosheath lying downstream of the quasi-perpendicular ($q_\perp$) and quasi-parallel ($q_\parallel$) sides of the bow shock are characterized using measurements taken by a mass-energy spectrometer. This characterization enables comparison to analogous Earth studies, thereby providing insight as to which plasma phenomena, such as turbulent particle heating, contribute in creating the observed plasma asymmetries in planetary magnetosheaths. A database of dayside bow-shock crossings along with magnetosheath proton densities, bulk speeds, temperatures, and magnetic-field strengths is manually constructed by selecting measurements taken during stable solar-wind conditions. Ratios of these magnetosheath proton parameters are calculated as functions of distance from the central meridian and the upstream Alfvén Mach number to quantify the $q_{\perp/\parallel}$ asymmetries. The density and bulk-speed exhibit $q_\parallel$-favored asymmetries, mirroring those observed at Earth, whereas the magnetic-field strength reveals no significant asymmetry despite expectations based on simulations. The temperatures perpendicular ($T_\perp$) and parallel ($T_\parallel$) to the background magnetic field have $q_\perp$-favored asymmetries while the temperature anisotropy $T_\perp / T_\parallel$ exhibits a $q_\parallel$-favored asymmetry. This trend is opposite to that seen at Earth, suggesting that the different spatial scales of the two planets’ magnetosheaths may affect the impact of turbulent processes on global plasma properties.

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.

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.

We investigate sunward planetary ions in the Martian magnetotail that potentially reduce the amount of escaping ions. The global properties of sunward flows in the Martian magnetotail are characterized, based on over 13-years of ion data (May 2007–December 2020) collected by the ASPERA-3 instrument on Mars Express. We find that sunward flows mainly occur in the vicinity of the crustal fields, implying that crustal fields may play a key role in producing such flows. The occurrence rate and sunward flux are higher during solar maximum rather than solar minimum. However, we identify a relatively low occurrence rate of sunward flows and low sunward flux, suggesting that sunward flows have negligible influence on total ion escape at Mars. This is different from those at Venus, where sunward flows can significantly decrease the total escape rates of ions.

We present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low-activity comet at a heliocentric distance of 2.8 AU on April 19th, 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualisation method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well-known partial rings of cometary pickup ions at high-activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy-angle dispersion and manifest as partial rings in velocity space.