The European Space Agency's three Swarm satellites were launched on 22 November 2013 into nearly polar, circular orbits, eventually reaching altitudes of 460 km (Swarm A and C) and 510 km (Swarm B). ...Swarm's multiyear mission is to make precision, multipoint measurements of low‐frequency magnetic and electric fields in Earth's ionosphere for the purpose of characterizing magnetic fields generated both inside and external to the Earth, along with the electric fields and other plasma parameters associated with electric current systems in the ionosphere and magnetosphere. Electric fields perpendicular to the magnetic field
B→ are determined through ion drift velocity
v→i and magnetic field measurements via the relation
E→⊥=−v→i×B→. Ion drift is derived from two‐dimensional images of low‐energy ion distribution functions provided by two Thermal Ion Imager (TII) sensors viewing in the horizontal and vertical planes;
v→i is corrected for spacecraft potential as determined by two Langmuir probes (LPs) which also measure plasma density ne and electron temperature Te. The TII sensors use a microchannel‐plate‐intensified phosphor screen imaged by a charge‐coupled device to generate high‐resolution distribution images (66 × 40 pixels) at a rate of 16 s−1. Images are partially processed on board and further on the ground to generate calibrated data products at a rate of 2 s−1; these include
v→i,
E→⊥, and ion temperature Ti in addition to electron temperature Te and plasma density ne from the LPs.
Key Points
Swarm TII sensors provide ion velocity and temperature from core ion distribution function images
CCD‐based imaging detectors in the TIIs support event rates that are much higher than possible with charge amplifier‐based systems
Swarm LPs use a harmonic‐mode technique to obtain electron density, temperature, and spacecraft potential at high rates
We present Mars' electron temperature (Te) and density (ne) altitude profiles derived from the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission deep dip orbits in April 2015, as measured by the ...Langmuir probe instrument. These orbits had periapsides below 130 km in altitude at low solar zenith angles. The periapsides were above the peak in ne during this period. Using a Chapman function fit, we find that scale height and projected altitude of the ne peak are consistent with models and previous measurements. The peak electron density is slightly higher than earlier works. For the first time, we present in situ measurements of Te altitude profiles in Mars' dayside in the altitude range from ~130 km to 500 km and provide a functional fit. Importantly, Te rises rapidly with altitude from ~180 km to ~300 km. These results and functional fit are important for modeling Mars' ionosphere and understanding atmospheric escape.
Key Points
First in situ measurements of the electron temperature at Mars
Electron density and temperature profiles at Mars
We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low‐energy plasma in the near‐nucleus coma of comet ...67P/Churyumov‐Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10−6, at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low‐energy plasma around the comet.
Key Points
The spatial distribution of plasma around comet 67P is highly structured
Local ionization of neutral gas dominates the plasma environment
Plasma falls off with cometocentric distance as 1/r
Context.
A variety of kinetic electrostatic and electromagnetic waves develop in the solar wind and the relationship between these waves and larger scale structures, such as current sheets and ...ongoing turbulence, remain a topic of investigation. Similarly, the instabilities producing ion-acoustic waves in the solar wind are still an open question.
Aims.
The goals of this paper are to investigate electrostatic Langmuir and ion-acoustic waves in the solar wind at 0.5 AU and determine whether current sheets and associated streaming instabilities can produce the observed waves. The relationship between these waves and currents observed in the solar wind is investigated statistically.
Methods.
Solar Orbiter’s Radio and Plasma Waves instrument suite provides high-resolution snapshots of the fluctuating electric field. The Low Frequency Receiver resolves the waveforms of ion-acoustic waves and the Time Domain Sampler resolves the waveforms of both ion-acoustic and Langmuir waves. Using these waveform data, we determine when these waves are observed in relation to current structures in the solar wind, estimated from the background magnetic field.
Results.
Langmuir and ion-acoustic waves are frequently observed in the solar wind. Ion-acoustic waves are observed about 1% of the time at 0.5 AU. The waves are more likely to be observed in regions of enhanced currents. However, the waves typically do not occur at current structures themselves. The observed currents in the solar wind are too small to drive instability by the relative drift between single ion and electron populations. When multi-component ion or electron distributions are present, the observed currents may be sufficient for instabilities to occur. Ion beams are the most plausible source of ion-acoustic waves in the solar wind. The spacecraft potential is confirmed to be a reliable probe of the background electron density when comparing the peak frequencies of Langmuir waves with the plasma frequency calculated from the spacecraft potential.
Magnetospheric ions with energies less than tens of eV originate from the ionosphere. Positive low‐energy ions are complicated to detect onboard sunlit spacecraft at higher altitudes, which often ...become positively charged to several tens of volts. We use two Cluster spacecraft and study low‐energy ions with a technique based on the detection of the wake behind a charged spacecraft in a supersonic ion flow. We find that low‐energy ions usually dominate the density and the outward flux in the geomagnetic tail lobes during all parts of the solar cycle. The global outflow is of the order of 1026 ions/s and often dominates over the outflow at higher energies. The outflow increases by a factor of 2 with increasing solar EUV flux during a solar cycle. This increase is mainly due to the increased density of the outflowing population, while the outflow velocity does not vary much. Thus, the outflow is limited by the available density in the ionospheric source rather than by the energy available in the magnetosphere to increase the velocity.
Key Points
Low‐energy (eV) ions dominate the magnetotail lobes 60–70% of the time
Much of the atmospheric escape is due to low‐energy (eV) ions
The outflow of low‐energy (eV) ions increases with solar EUV radiation
ABSTRACT
Moment analysis of ion spectrograms measured by the Ion Composition Analyser (ICA) in the coma of comet 67P typically produces an ion number density estimate markedly lower than the number ...density of free electrons as measured by the Mutual Impedance Probe and the dual Langmuir Probe. While there are good reasons to distrust the ion density moment estimate in these circumstances, the issue cannot yet be considered fully understood and it is of interest to see whether any natural non-instrumental cause is possible. An obvious such cause would be whether a significant fraction of the positive charge density resides in positively charged dust grains that are not measured by the ICA. Here, we show that this scenario is highly unlikely, even near perihelion where photoemission is the strongest. In our semi-analytical grain charging model, we balance the current contributions to grains of photoelectron emission and electron attachment so as to find the expected charge state for a grain of a given radius. The charge state is affected by the ambient electron number density, the electron temperature, and the heliocentric distance. While at times the bulk of the dust population around comet 67P could be charged positive, dust charging, including photoelectron emission, should have a negligible influence on the overall ionization balance in the cometary coma simply because the dust particles are not ubiquitous enough.
Because ion-neutral reaction cross sections are energy dependent, the distance from a cometary nucleus within which ions remain collisionally coupled to the neutrals is dictated not only by the ...comet's activity level but also by the electromagnetic fields in the coma. Here we present a 1D model simulating the outward radial motion of water group ions with radial acceleration by an ambipolar electric field interrupted primarily by charge transfer processes with H2O. We also discuss the impact of plasma waves. For a given electric field profile, the model calculates key parameters, including the total ion density, nI, the H3O+/H2O+ number density and flux ratios, Rdens and Rflux, and the mean ion drift speed, , as a function of cometocentric distance. We focus primarily on a coma roughly resembling that of the ESA Rosetta mission target comet 67P/Churyumov-Gerasimenko near its perihelion in 2015 August. In the presence of a weak ambipolar electric field in the radial direction the model results suggest that the neutral coma is not sufficiently dense to keep the mean ion flow speed close to that of the neutrals by the spacecraft location (∼200 km from the nucleus). In addition, for electric field profiles giving nI and within limits constrained by measurements, the Rdens values are significantly higher than values typically observed. However, when including the ion motion in large-amplitude plasma waves in the model, results more compatible with observations are obtained. We suggest that the variable and often low H3O+/H2O+ number density ratios observed may reflect nonradial ion trajectories strongly influenced by electromagnetic forces and/or plasma instabilities, with energization of the ion population by plasma waves.
Context. The electron temperature of the plasma is one important aspect of the environment. Electrons created by photoionization or impact ionization of atmospheric gas have energies ~10 eV. In an ...active comet coma, the gas density is high enough for rapid cooling of the electron gas to the neutral gas temperature (a few hundred kelvin). How cooling evolves in less active comets has not been studied before. Aims. We aim to investigate how electron cooling varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders of magnitude during the Rosetta mission. Methods. We used in situ data from the Rosetta plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps and mutual impedance probe measurements, we determined at which time cold electrons formed at least 25% of the total electron density. We compared the results to what is expected from simple models of electron cooling, using the observed neutral gas density as input. Results. We demonstrate that the slope of the Langmuir probe sweep can be used as a proxy for the presence of cold electrons. We show statistics of cold electron observations over the two-year mission period. We find cold electrons at lower activity than expected by a simple model based on free radial expansion and continuous loss of electron energy. Cold electrons are seen mainly when the gas density indicates that an exobase may have formed. Conclusions. Collisional cooling of electrons following a radial outward path is not sufficient to explain the observations. We suggest that the ambipolar electric field keeps electrons in the inner coma for a much longer time, giving them time to dissipate energy by collisions with the neutrals. We conclude that better models are required to describe the plasma environment of comets. They need to include at least two populations of electrons and the ambipolar field.
A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. Here we combine Langmuir and Mutual Impedance Probe ...measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2–4 km/s, significantly above the expected neutral velocity
≲1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion‐neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials
≲−5 V throughout the cavity, showing that a population of warm (∼5 eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold (
≲0.1eV) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.
Key Points
The ion velocity exceeded the neutral velocity, showing that the ions were not strongly collisionally coupled to the neutral gas
A population of warm electrons was present throughout the parts of the cavity reached by Rosetta, driving the spacecraft potential negative
A population of cold electrons was consistently observed inside the cavity and intermittently also in the surrounding region
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