ABSTRACT Spacecraft charging causes notorious issues for low-energy plasma measurements. The charged particles are accelerated towards or repelled from the spacecraft surface, affecting both their ...energy and travel direction. The latter results in a distortion of the effective field of view (FOV) of the instrument making the measurements. The Comet Interceptor mission, planned to be launched in 2029, will make a flyby of a long-period or interstellar comet that ideally is dynamically new. The mission comprises one main spacecraft A, developed by the European Space Agency (ESA), and two sub-probes B1 and B2, developed by the Japan Aerospace Exploration Agency and ESA, respectively. The low-energy plasma measurements made by Comet Interceptor will likely be affected by the spacecraft potential in the case of low relative flyby velocities. On probe B1, the Cometary Ion Mass Spectrometer (CIMS) of the Plasma Suite is an ion mass spectrometer, capable of measuring ions with energies down to 10 eV/q. In this work, we use the Spacecraft Plasma Interaction Software to study the influence of the spacecraft potential on the low-energy ion measurements to be made by CIMS in the inner cometary magnetosphere. The results show that the effective FOV of CIMS is distorted at low energies when the flyby velocity is low. The distortion level is highly geometry dependent, and the largest distortions are caused by the magnetometer boom. Furthermore, the results show that cold ions with bulk velocities in the range 1–10 km s−1, flowing both radially away from and inward towards the nucleus, are detectable by the instrument considering the nominal observation geometry.
Context. On 20 January 2015 the Rosetta spacecraft was at a heliocentric distance of 2.5 AU, accompanying comet 67P/Churyumov-Gerasimenko on its journey toward the Sun. The Ion Composition Analyser ...(RPC-ICA), other instruments of the Rosetta Plasma Consortium, and the ROSINA instrument made observations relevant to the generation of plasma waves in the cometary environment. Aims. Observations of plasma waves by the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) can be explained by dispersion relations calculated based on measurements of ions by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA), and this gives insight into the relationship between plasma phenomena and the neutral coma, which is observed by the Comet Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA-COPS). Methods. We use the simple pole expansion technique to compute dispersion relations for waves on ion timescales based on the observed ion distribution functions. These dispersion relations are then compared to the waves that are observed. Data from the instruments RPC-LAP, RPC-ICA and the mutual impedance probe (RPC-MIP) are compared to find the best estimate of the plasma density. Results. We find that ion acoustic waves are present in the plasma at comet 67P/Churyumov-Gerasimenko, where the major ion species is H2O+. The bulk of the ion distribution is cold, kBTi = 0.01 eV when the ion acoustic waves are observed. At times when the neutral density is high, ions are heated through acceleration by the solar wind electric field and scattered in collisions with the neutrals. This process heats the ions to about 1 eV, which leads to significant damping of the ion acoustic waves. Conclusions. In conclusion, we show that ion acoustic waves appear in the H2O+ plasmas at comet 67P/Churyumov-Gerasimenko and how the interaction between the neutral and ion populations affects the wave properties.
Context.
During its two-year mission at comet 67P, Rosetta nearly continuously monitored the inner coma plasma environment for gas production rates varying over three orders of magnitude, at ...distances to the nucleus ranging from a few to a few hundred kilometres. To achieve the best possible measurements, cross-calibration of the plasma instruments is needed.
Aims.
Our goal is to provide a consistent plasma density dataset for the full mission, while in the process providing a statistical characterisation of the plasma in the inner coma and its evolution.
Methods.
We constructed physical models for two different methods to cross-calibrate the spacecraft potential and the ion current as measured by the Rosetta Langmuir probes (LAP) to the electron density as measured by the Mutual Impedance Probe (MIP). We also described the methods used to estimate spacecraft potential, and validated the results with the Ion Composition Analyser (ICA).
Results.
We retrieve a continuous plasma density dataset for the entire cometary mission with a much improved dynamical range compared to any plasma instrument alone and, at times, improve the temporal resolution from 0.24−0.74 Hz to 57.8 Hz. The physical model also yields, at a three-hour time resolution, ion flow speeds and a proxy for the solar EUV flux from the photoemission from the Langmuir probes.
Conclusions.
We report on two independent mission-wide estimates of the ion flow speed that are consistent with the bulk H
2
O
+
ion velocities as measured by the ICA. We find the ion flow to consistently be much faster than the neutral gas over the entire mission, lending further evidence that the ions are collisionally decoupled from the neutrals in the coma. Measurements of ion speeds from Rosetta are therefore not consistent with the assumptions made in previously published plasma density models of the comet 67P’s ionosphere at the start and end of the mission. Also, the measured EUV flux is perfectly consistent with independently derived values previously published from LAP and lends support for the conclusions drawn regarding an attenuation of solar EUV from a distant nanograin dust population, when the comet activity was high. The new density dataset is consistent with the existing MIP density dataset, but it facilitates plasma analysis on much shorter timescales, and it also covers long time periods where densities were too low to be measured by MIP.
In this series of papers, we present statistical maps of mirror-mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. ...We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on extreme ultraviolet (EUV) solar flux levels (high and low) and, specific to Mars, on Mars Year (MY) as well as atmospheric seasons (four solar longitudes Ls).
We first use magnetic-field-only criteria to detect these structures and present ways to mitigate ambiguities in their nature. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft from November 2014 to February 2021 (MY32–MY35), we detect events closely resembling MMs lasting in total more than 170 000 s, corresponding to about 0.1 % of MAVEN's total time spent in the Martian plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 1 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. Detection probabilities are higher on average in low-solar-EUV conditions, whereas high-solar-EUV conditions see an increase in detections within the magnetospheric tail. We tentatively link the former tendency to two combining effects: the favouring of ion cyclotron waves the closer to perihelion due to plasma beta effects and, possibly, the non-gyrotropy of pickup ion distributions. This study is the first of two on the magnetosheaths of Mars and Venus.
In this series of papers, we present statistical maps of mirror-mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. ...We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on extreme ultraviolet (EUV) solar flux levels (high and low). The detection of these structures is done through magnetic-field-only criteria, and ambiguous determinations are checked further. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Venus Express (VEX) spacecraft from May 2006 to November 2014, we detect structures closely resembling MMs lasting in total more than 93 000 s, corresponding to about 0.6 % of VEX's total time spent in Venus's plasma environment. We calculate MM-like occurrences normalized to the spacecraft's residence time during the course of the mission. Detection probabilities are about 10 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. For solar maximum, the active region behind the bow shock is further inside the magnetosheath, near the solar minimum bow shock location. The ratios of the observations during solar minimum and maximum are slightly dependent on the depth ΔB/B of the structures; deeper structures are more prevalent at solar maximum. A dependence on solar EUV (F10.7) flux is also present, where at higher F10.7 flux the events occur at higher values than the daily-average value of the flux. The main dependence of the MM-like structures is on the condition of the bow shock: for quasi-perpendicular conditions, the MM occurrence rate is higher than for quasi-parallel conditions. However, when the shock becomes “too perpendicular” the chance of observing MM-like structures reduces again. Combining the plasma data from the Ion Mass Analyser (IMA on board Venus Express) with the magnetometer data shows that the instability criterion for MMs is reduced in the two main regions where the structures are measured, whereas it is still enhanced in the region between these two regions, implying that the generation of MMs is transferring energy from the particles to the field. With the addition of the Electron Spectrometer (ELS on board Venus Express) data, it is possible to show that there is an anti-phase between the magnetic field strength and the density for the MM-like structures. This study is Part 2 of a series of papers on the magnetosheaths of Mars and Venus.
Ion acoustic waves were observed between 15 and 30 km from the centre
of comet 67P/Churyumov–Gerasimenko by the Rosetta spacecraft during
its close flyby on 28 March 2015. There are two electron ...populations:
one cold at kBTe≈0.2 eV and one
warm at kBTe≈2 eV. The ions are
dominated by a cold (a few hundredths of electronvolt) distribution of
water group ions with a bulk speed of (3–3.7) km s−1. A
warm kBTe≈6 eV ion population,
which also is present, has no influence on the ion acoustic waves due
to its low density of only 0.25 % of the plasma density.
Near closest approach the propagation direction was within
50∘ from the direction of the bulk velocity. The waves, which
in the plasma frame appear below the ion plasma frequency
fpi≈2 kHz, are Doppler-shifted to the
spacecraft frame where they cover a frequency range up to
approximately 4 kHz.
The waves are detected in a region of space where the
magnetic field is piled up and draped around the inner part of the
ionised coma. Estimates of the current associated with the magnetic
field gradient as observed by Rosetta are used as input to
calculations of dispersion relations for current-driven ion acoustic
waves, using kinetic theory. Agreement between theory and observations
is obtained for electron and ion distributions with the properties
described above. The wave power decreases over cometocentric
distances from 24 to 30 km. The main difference between the
plasma at closest approach and in the region where the waves are
decaying is the absence of a significant current in the latter. Wave
observations and theory combined supplement the particle measurements
that are difficult at low energies and complicated by spacecraft
charging.
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.
Context. The first long-term in-situ observation of the plasma environment in the vicinity of a comet, as provided by the European Rosetta spacecraft. Aims. Here we offer characterisation of the ...solar wind flow near 67P/Churyumov-Gerasimenko (67P) and its long term evolution during low nucleus activity. We also aim to quantify and interpret the deflection and deceleration of the flow expected from ionization of neutral cometary particles within the undisturbed solar wind. Methods. We have analysed in situ ion and magnetic field data and combined this with hybrid modeling of the interaction between the solar wind and the comet atmosphere. Results. The solar wind deflection is increasing with decreasing heliocentric distances, and exhibits very little deceleration. This is seen both in observations and in modeled solar wind protons. According to our model, energy and momentum are transferred from the solar wind to the coma in a single region, centered on the nucleus, with a size in the order of 1000 km. This interaction affects, over larger scales, the downstream modeled solar wind flow. The energy gained by the cometary ions is a small fraction of the energy available in the solar wind. Conclusions. The deflection of the solar wind is the strongest and clearest signature of the mass-loading for a small, low-activity comet, whereas there is little deceleration of the solar wind.
Upstream solar wind speed at comet 67P Nilsson, H; Moeslinger, A; Williamson, H N ...
Astronomy and astrophysics (Berlin),
03/2022, Volume:
659
Journal Article
Peer reviewed
Open access
Context. Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. The solar wind was observed for much of this time, but was significantly deflected and to some extent slowed down by ...the interaction with the coma. Aims. We use the different changes in the speed of H+ and He2+ when they interact with the coma to estimate the upstream speed of the solar wind. The different changes in the speed are due to the different mass per charge of the particles, while the electric force per charge due to the interaction is the same. A major assumption is that the speeds of H+ and He2+ were the same in the upstream region. This is investigated. Methods. We derived a method for reconstructing the upstream solar wind from H+ and He2+ observations. The method is based on the assumption that the interaction of the comet with the solar wind can be described by an electric potential that is the same for both H+ and He2+. This is compared to estimates from the Tao model and to OMNI and Mars Express data that we propagated to the observation point. Results. The reconstruction agrees well with the Tao model for most of the observations, in particular for the statistical distribution of the solar wind speed. The electrostatic potential relative to the upstream solar wind is derived and shows values from a few dozen volts at large heliocentric distances to about 1 kV during solar events and close to perihelion. The reconstructed values of the solar wind for periods of high electrostatic potential also agree well with propagated observations and model results. Conclusions. The reconstructed upstream solar wind speed during the Rosetta mission agrees well with the Tao model. The Tao model captures some slowing down of high-speed streams as compared to observations at Earth or Mars. At low solar wind speeds, below 400 km s−1, the agreement is better between our reconstruction and Mars observations than with the Tao model. The magnitude of the reconstructed electrostatic potential is a good measure of the slowing-down of the solar wind at the observation point.
In 2015 August, comet 67P/Churyumov-Gerasimenko, the target comet of the ESA Rosetta mission, reached its perihelion at similar to 1.24 au. Here, we estimate for a three-day period near perihelion, ...effective ion speeds at distances similar to 200-250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2-8 km s(-1), are markedly higher than the expected neutral outflow velocity of similar to 1 km s(-1). This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions' kinetic energy associated with radial drift motion is decreasing.
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