| 1. |
- Bergman, Sofia, 1991-, et al.
(författare)
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Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 503:2, s. 2733-2745
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Tidskriftsartikel (refereegranskat)abstract
- Y Comets are constantly interacting with the solar wind. When the comet activity is high enough, this leads to the creation of a magnetic field free region around the nucleus known as the diamagnetic cavity. It has been suggested that the ion-neutral drag force is balancing the magnetic pressure at the cavity boundary, but after the visit of Rosetta to comet 67P/Churyumov-Gerasimenko the coupling between ions and neutrals inside the cavity has been debated, at least for moderately active comets. In this study, we use data from the ion composition analyser to determine the bulk speeds and temperatures of the low-energy ions in the diamagnetic cavity of comet 67P. The low-energy ions are affected by the negative spacecraft potential, and we use the Spacecraft Plasma Interaction Software to model the resulting influence on the detected energy spectra. We find bulk speeds of 5-10 km s(-1) with a most probable speed of 7 km s(-1), significantly above the velocity of the neutral particles. This indicates that the collisional coupling between ions and neutrals is not strong enough to keep the ions at the same speed as the neutrals inside the cavity. The temperatures are in the range 0.7-1.6 eV, with a peak probability at 1.0 eV. We attribute the major part of the temperature to the fact that ions are born at different locations in the coma, and hence are accelerated over different distances before reaching the spacecraft.
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| 2. |
- Bergman, Sofia, 1991-, et al.
(författare)
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The Influence of Varying Spacecraft Potentials and Debye Lengths on In Situ Low-Energy Ion Measurements
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:4
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Johansson, Fredrik Leffe, 1988-, et al.
(författare)
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Ionisation and EUV attenuation at comet 67P
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Context. The new cross-calibrated density dataset from the Rosetta Plasma Consortium (RPC) is ideal for investigating the comet 67P/Churyumov-Gerasimenko ionosphere and its long-term evolution as the gas production rate varied over three orders of magnitude. Although event-based studies have, at times, shown the importance of 20-200 eV electrons for the ionisation of the cometary gas, mission-wide statistics have not been made before. Aims. We attempt to build on previous successful modelling efforts (with good accuracy, but poor precision) at selected events to obtain a more generalised understanding, also encompassing the peak activity near perihelion.Methods. Using the neutral gas production as measured by ROSINA/COPS, in conjunction with recent findings on the bulk cometary ion flow, as well as estimates of photoionisation and electron-impact ionisation from RPC instruments, we construct an ionosphere model and compare it to the new cross-calibrated electron density datasetResults. We find that the photoionisation and elevated ion flow speeds as measured by LAP produce self-consistent densities in a simple cometary ionosphere model based on the cross-calibrated density dataset. The ion velocities are also consistent with the radial ICA ion bulk flows, and are a factor of five times larger than the neutral speeds. Also, the consistent photoionisation estimate lends further evidence that the solar EUV is attenuated everywhere in the cometary ionosphere at peak activities. We also find that electron-impact ionisation seems to increase with decreasing cometocentric distance. This points towards an external source of hot electrons that are accelerated by a (generally radial) ambipolar electric field, which also have been hypothesised to be the mechanism behind the elevated ion speeds.Conclusions. The cometary ionospheric densities as measured by Rosetta is consistent with a model where an ambipolar electric field strongly affects the distribution of the plasma, and collisions play only a minor role. The attenuation of the EUV in the cometary ionosphere reported cannot be local, and is only readily explained by a significant population of nanodust, produced beyond 2000 km in the comet-sun direction via erosion or fragmentation of larger grains.
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| 4. |
- Johansson, Fredrik Leffe, 1988-, et al.
(författare)
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Plasma densitites, flow and Solar EUV flux at comet 67P : A cross-calibration approach
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
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Tidskriftsartikel (refereegranskat)abstract
- Context.During its two year mission at comet 67P, Rosetta nearly continuously monitored the inner coma plasma environment forgas production rates varying over three orders of magnitude, at distances to the nucleus from a few to a few hundred km. To achievethe best possible measurements, cross-calibration of the plasma instruments is needed.Aims.To provide a consistent plasma density data set for the full mission, in the process providing a statistical characterisation of theplasma processes in the inner coma and their evolution.Methods.We construct physical models for two different methods to cross-calibrate the spacecraft potential and the ion current asmeasured by the Rosetta Langmuir Probes (LAP) to the electron density as measured by the Mutual Impedance Probe (MIP). We alsodescribe the methods used to estimate spacecraft potential, and validate 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 rangecompared to any plasma instrument alone and, at times, improve the temporal resolution from 0.24-0.74 Hz to 57.8 Hz. The physicalmodel also yields, at 3 hour time resolution, ion flow speeds as well as a proxy for the solar EUV flux from the photoemission fromthe Langmuir Probes.Conclusions.We report on two independent estimates of the ion flow speed which are consistent with the bulk H2O+ion velocitiesas measured by ICA. We find the ion flow to be much faster than the neutral gas, lending further evidence that the ions are mostlycollisionally decoupled from the neutrals in the coma. Also, the measured EUV flux is perfectly consistent with independent measurements previously published in Johansson et al. (2017) and lends support for the conclusions drawn therein regarding an attenuationof solar EUV from a distant nanograin dust population between the comet and the Sun, when the comet activity was high. The newdensity dataset is consistent with the existing MIP density dataset, but facilitates plasma analysis at much shorter timescales, with anincreased temporal resolution of a factor of (up to) 240 and covers also long time periods where densities were too low to be measuredby MIP.
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| 5. |
- Johansson, Fredrik Leffe, 1988-
(författare)
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Rosetta Observations of Plasma and Dust at Comet 67P
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In-situ observations of cometary plasma are not made because they are easy. The historic ESA Rosetta mission was launched in 2004 and traversed space for ten years before arriving at comet 67P/Churyumov-Gerasimenko, which it studied in unprecedented detail for two years. For the Rosetta Dual Langmuir Probe Experiment (LAP), the challenge was increased by the sensors being situated on short booms near a significantly negatively charged spacecraft, which deflects low-energy charged particles away from our instrument. To disentangle the cometary plasma signature in our signal, we create a charging model for the particular design of the Rosetta spacecraft through 3D Particle-in-Cell/hybrid spacecraft-plasma interaction simulations, which also can be applicable to similarly designed spacecraft in cold plasma environments. By virtue of this model, we find a way to cross-calibrate (with the Mutual Impedance probe, MIP) the LAP spacecraft potential to a plasma density estimate with increased temporal resolution and dynamic range than any single plasma instrument alone.To characterise and disentangle the Sun-driven photoelectric current from the positive cometary ion current signal, using three different methods (where we believe one is novel), we find a signature of an attenuation of the Extreme Ultraviolet (EUV) radiation from the Sun that follows the cometary out-gassing activity. We discuss possible reasons for this, where the scattering and absorption of radiation by ~20 nm sized dust grains created by the disintegration of far larger cometary dust grains far from the nucleus appears most likely.By cross-calibrating also our current measurements to MIP, we find a cometary ion speed estimate, which, when applied to a simple comet ionosphere model using the LAP photoemission as a photoionisation proxy, predicts the measured comet plasma densities near perihelion, when comet activity was highest. This demonstrates that the LAP cross-calibration estimates are self-consistent, but also strongly suggests that the EUV attenuation we reported is apparent also in the comet ionosphere, as less plasma is ionised by EUV radiation. The ion speed estimates from LAP are consistent with recent results of cometary water ion velocities from the Ion Composition Analyser (ICA), and much elevated above the comet neutral speed, often by a factor of 5. This verifies that the cometary ions are not collisionally coupled to the neutrals, and instead rapidly accelerated by some electric field, such as an ambipolar electric field or from plasma wave activity.
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