Proton temperature anisotropies in the plasma environment of Venus

Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T ⊥) and parallel (T ‖) to the background magnetic field. We calculate average values of T ⊥ and T ‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T ⊥/T ‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near-subsolar magnetosheath (T ⊥/T ‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere

Cometary ion drift energy and temperature at comet 67P-Churyumov/Gerasimeko

The Ion Composition Analyzer (ICA) on the Rosetta spacecraft observed both the solar wind and the cometary ionosphere around comet 67P/Churyumov-Gerasimenko for nearly two years. However, observations of low energy cometary ions were affected by a highly negative spacecraft potential, and the ICA ion density estimates were often much lower than plasma densities found by other instruments. Since the low energy cometary ions are often the highest density population in the plasma environment, it is nonetheless desirable to understand their properties. To do so, we select ICA data with densities comparable to those of Rosetta's Langmuir Probe (LAP)/Mutual Impedance Probe throughout the mission. We then correct the cometary ion energy distribution of each energy-angle scan for spacecraft potential and fit a drifting Maxwell-Boltzmann distribution, which gives an estimate of the drift energy and temperature for 3521 scans. The resulting drift energy is generally between 11--18 eV and the temperature between 0.5--1 eV. The drift energy shows good agreement with published ion flow speeds from LAP during the same time period and is much higher than the cometary neutral speed. We see additional higher energy cometary ions in the spectra closest to perihelion, which can either be a second Maxwellian or a kappa distribution. The energy and temperature are negatively correlated with heliocentric distance, but the slope of the change is small. It cannot be quantitatively determined whether this trend is primarily due to heliocentric distance or spacecraft distance to the comet, which increased with decreasing heliocentric distance.Comment: 9 pages, 10 figure

Upper limit of the solar wind protons backscattering efficiency from Comet 67P/Churyumov-Gerasimenko

Context. Solar wind ions backscattering is a fundamental plasma-surface interaction process that may occur on all celestial bodies exposed to the solar wind and lacking a significant atmosphere or magnetosphere. Yet, observations have been limited to the regolith-covered Moon and Phobos, one of the Martian moons. Aims. We aim to expand our knowledge of the process to include comets by investigating the backscattering of solar wind protons from the surface of comet 67P/Churyumov-Gerasimenko. Methods. We used one of the ion spectrometers on board ESA s Rosetta spacecraft to search for evidence of backscattered solar wind protons from the cometary surface. The signal of interest was expected to be very weak and several statistical treatments of the data were essential to eliminate any influence from background noise and instrumental effects. Due to limited knowledge of the signal location within the observed parameter space, we conducted a statistical analysis to identify the most probable conditions for detecting the signal. Results. No significant solar wind backscattered protons were ever observed by the instrument. The statement applies to the large spectrum of observation conditions. An upper limit of the backscattered proton flux is given, as well as an upper limit of the backscattering efficiency of 9 A 104. Conclusions. The surface of comet 67P/Churyumov-Gerasimenko distinguishes itself as a notably weak reflector of solar wind protons, with its backscattering efficiency, at most, as large as the lowest observed backscattering efficiency from the lunar regolith

The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta

Spacecraft charging is problematic for low‐energy plasma measurements. The charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. The Ion Composition Analyzer (RPC‐ICA) on board the Rosetta spacecraft is suffering from this effect. RPC‐ICA was measuring positive ions in the vicinity of comet 67P/Churyumov‐Gerasimenko, covering an energy range of a few eV/q to 40 keV/q. The low‐energy part of the data is, however, heavily distorted by the negatively charged spacecraft. In this study we use the Spacecraft Plasma Interaction Software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (FOV) of the instrument. The results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. Below this energy the FOV is often heavily distorted, but the distortion differs between different viewing directions. Generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles

The Influence of Varying Spacecraft Potentials and Debye Lengths on In Situ Low-Energy Ion Measurements

Low‐energy ions are difficult to measure, mainly due to spacecraft charging. The ions areattracted to or repelled from the charged surface prior to detection, which changes both the energy andtravel direction of the ions. This results in distortions of the data, and the changed travel directions distort the effective field of view (FOV) of the instrument performing the measurements. The ion composition analyzer (RPC‐ICA) was measuring positive ions down to an energy of a few eV around comet67P/Churyumov‐Gerasimenko. Low‐energy ions play important parts in processes in the cometary environment, but the FOV of RPC‐ICA has been shown to get severely distorted at low ion energies. Several factors are believed to affect the distortion level. In this study we use the Spacecraft Plasma Interaction Software (SPIS) to investigate the influence of varying spacecraft potentials and Debye lengths on the FOV distortion of RPC‐ICA. We show that the distortion level is dependent on the Debye length of the surrounding plasma, but the sensitivity varies substantially between different viewing directions of the instrument. We also show that a small nonlinearity exists in the relation between FOV distortion, ion energy, and spacecraft potential, mainly caused by the photoemission and bulk flow of the cometary plasma

Rosetta spacecraft potential and electron temperature in the coma of comet 67P

International audienceWe present and compare measurements of the spacecraft potential (VS/C) of ESA's Rosetta spacecraft during its ∼2 year stay in the inner coma of comet 67P/Churyumov-Gerasimenko, by the Rosetta Langmuir probe (RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. VS/C was mostly negative, driven so by the high (∼5-10 eV) electron temperature of the coma photoelectrons. LAP only picks up a portion of the full VS/C since the two probes, mounted on booms of length 2.2 and 1.6 m, respectively, are generally inside the potential field of the negatively charged spacecraft. Comparison of the minimum energy of positive ions collected by ICA shows that this portion varies between 0.7 and 1 VS/C, with generally good correspondence between the two instruments except when local ion production is weak and accelerated ions dominate the flux. We map VS/C and its evolution during the mission and use it to constrain the electron density ne and temperature Te in the inner coma. Comparison of VS/C measurements to electron density measurements by the Mutual Impedance Probe (RPC-MIP) allows more accurate estimates of the electron temperature and a cross-calibration of the neand Te measurements by the different instruments

Rosetta spacecraft potential and electron temperature in the coma of comet 67P

International audienceWe present and compare measurements of the spacecraft potential (VS/C) of ESA's Rosetta spacecraft during its ∼2 year stay in the inner coma of comet 67P/Churyumov-Gerasimenko, by the Rosetta Langmuir probe (RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. VS/C was mostly negative, driven so by the high (∼5-10 eV) electron temperature of the coma photoelectrons. LAP only picks up a portion of the full VS/C since the two probes, mounted on booms of length 2.2 and 1.6 m, respectively, are generally inside the potential field of the negatively charged spacecraft. Comparison of the minimum energy of positive ions collected by ICA shows that this portion varies between 0.7 and 1 VS/C, with generally good correspondence between the two instruments except when local ion production is weak and accelerated ions dominate the flux. We map VS/C and its evolution during the mission and use it to constrain the electron density ne and temperature Te in the inner coma. Comparison of VS/C measurements to electron density measurements by the Mutual Impedance Probe (RPC-MIP) allows more accurate estimates of the electron temperature and a cross-calibration of the neand Te measurements by the different instruments

Flow directions of low-energy ions in and around the diamagnetic cavity of comet 67P

The flow direction of low-energy ions around comet 67P/Churyumov–Gerasimenko has previously been difficult to constrain due to the influence of the spacecraft potential. The Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA) on Rosetta measured the distribution function of positive ions with energies down to just a few eV/q throughout the escort phase ofthe mission. Unfortunately, the substantial negative spacecraft potential distorted the directional information of the low-energy data. In this work, we present the flow directions of low-energy ions around comet 67P, corrected for the spacecraft potential using Particle-In-Cell simulation results. We focus on the region in and around the diamagnetic cavity, where low-energy ions are especially important for the dynamics. We separate between slightly accelerated ‘burst’ features and a more constant ‘band’ of low-energy ions visible in the data. The ‘bursts’ are flowing radially outwards from the nucleus with an antisunward component while the ‘band’ is predominantly streaming back towards the comet. This provides evidence of counter-streaming ions, which has implications for the overall expansion velocity of the ions. The backstreaming ions are present also at times when the diamagnetic cavity was not detected, indicating that the process accelerating the ions back towards the comet is not connected to the cavity boundary