Measurements of the electrostatic potential of Rosetta at comet 67P

Odelstad, Elias; Stenberg-Wieser, Gabriella; Wieser, Martin; Eriksson, Anders; Nilsson, Hans; Johansson, Folke

doi:10.1093/mnras/stx2232

Cited by 51 publications

(96 citation statements)

References 43 publications

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“…Being negative during the period selected, it repels electrons. In the tenuous neutral density environment encountered at 3 au, for an electron temperature of 7 eV and an electron density of 400 cm −3 , the Debye length is about 1 m. The charge sheath extends typically to a radius of three times the Debye length, that is, to about 3 m for the plasma conditions encountered by Rosetta during the period under study (Odelstad et al 2016). The spacecraft potential field decays therefore beyond the location of the sensors.…”

Section: Rpc-lap Electron Densitymentioning

confidence: 94%

Ionospheric plasma of comet 67P probed byRosettaat 3 au from the Sun

Galand

Héritier

Odelstad

et al. 2016

Mon. Not. R. Astron. Soc.

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170

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We propose to identify the main sources of ionization of the plasma in the coma of comet 67P/Churyumov-Gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (<20 km) and large heliocentric distances (>3 au). The ionospheric model proposed is used as an organizing element of a multi-instrument data set from the Rosetta Plasma Consortium (RPC) plasma and particle sensors, from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis and from the Microwave Instrument on the Rosetta Orbiter, all on board the ESA/Rosetta spacecraft. The calculated ionospheric density driven by Rosetta observations is compared to the RPC-Langmuir Probe and RPC-Mutual Impedance Probe electron density. The main cometary plasma sources identified are photoionization of solar extreme ultraviolet (EUV) radiation and energetic electron-impact ionization. Over the northern, summer hemisphere, the solar EUV radiation is found to drive the electron density -with occasional periods when energetic electrons are also significant. Over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. The bulk of the electron population is warm with temperature of the order of 7-10 eV. For increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population.

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Section: Rpc-lap Electron Densitymentioning

confidence: 94%

Ionospheric plasma of comet 67P probed byRosettaat 3 au from the Sun

Galand

Héritier

Odelstad

et al. 2016

Mon. Not. R. Astron. Soc.

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170

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“…At U B >− α V S/C , the photoelectron current falls off exponentially with increasing U B , with a characteristic e ‐folding typically on the order of 1–2 V. This regime change at U B =− α V S/C is typically identifiable as a sharp knee in the sweeps, hereafter denoted V ph , from which an estimate of the spacecraft potential can be obtained as V S/C =− V ph / α . α has been shown to generally be in the range 0.7–1 by Odelstad et al (). Such spacecraft potential measurements have previously been used by Odelstad et al (, ) to demonstrate the overall pervasiveness of warm (∼5 eV) electrons in the coma of 67P; in this paper we more carefully examine this during the times when the spacecraft is inside the diamagnetic cavity.…”

Section: Instrumentation and Measurementsmentioning

confidence: 98%

“…α has been shown to generally be in the range 0.7–1 by Odelstad et al (). Such spacecraft potential measurements have previously been used by Odelstad et al (, ) to demonstrate the overall pervasiveness of warm (∼5 eV) electrons in the coma of 67P; in this paper we more carefully examine this during the times when the spacecraft is inside the diamagnetic cavity.…”

Section: Instrumentation and Measurementsmentioning

confidence: 98%

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

et al. 2018

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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 ( ≲0.11emeV) 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.

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“…RPC-LAP data also indicate the presence of a colder electron population with a temperature below 0.1 eV, which is not considered in this study, since the warm population is dominating the electron flux to the spacecraft (Odelstad et al, 2015(Odelstad et al, , 2017(Odelstad et al, , 2018. The electron temperature has to be chosen carefully, since the spacecraft potential is highly dependent on this value.…”

Section: Plasma Model and Simulation Environmentmentioning

confidence: 99%

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

Bergman

Wieser

et al. 2020

JGR Space Physics

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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.Plain Language Summary The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko for 2 years, providing data giving new insights into the nature of comets. The Ion Composition Analyzer (RPC-ICA) on board the spacecraft measures positive ions in the vicinity of the comet. The instrument can measure low-energy ions, which play an important part in the processes taking place in this environment. To fully understand the environment around the comet, we have to understand these low-energy ions. Unfortunately, this part of the RPC-ICA data is distorted by the spacecraft potential. A spacecraft in space interacts with the surrounding environment, which charges the spacecraft surface to a positive or negative potential. Rosetta was commonly charged to a negative potential throughout the mission, which means that the positive ions measured by RPC-ICA were attracted to the spacecraft. Consequently, both the energy and the travel direction of the ions changed before detection. We investigate how the low-energy ions measured by RPC-ICA have been affected by the spacecraft potential. We use the Spacecraft Plasma Interaction Software to model these effects. The results give us a lower energy limit above which we can trust the measurements and show that some parts of the instrument are more heavily affected than others.

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Measurements of the electrostatic potential of Rosetta at comet 67P

Cited by 51 publications

References 43 publications

Ionospheric plasma of comet 67P probed byRosettaat 3 au from the Sun

Ionospheric plasma of comet 67P probed byRosettaat 3 au from the Sun

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

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

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