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- 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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| 2. |
- 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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| 3. |
- 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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