| 1. |
- Nilsson, Hans, et al.
(författare)
-
Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA
- 2017
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 469:Suppl_2, s. S252-S261
-
Tidskriftsartikel (refereegranskat)abstract
- Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.
|
|
| 2. |
- Odelstad, Elias, et al.
(författare)
-
Measurements of the electrostatic potential of Rosetta at comet 67P
- 2017
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469:Suppl. 2, s. S568-S581
-
Tidskriftsartikel (refereegranskat)abstract
- We present and compare measurements of the spacecraft potential (Vs/c) of the Rosetta spacecraft throughout its stay in the inner coma of comet 67P/Churyumov-Gerasimenko, by the Rosetta Plasma Consortium-LAngmuir Probe (RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. Vs/c has mainly been negative, driven by the high temperature (~5-10 eV) of the coma photoelectrons. The magnitude of the negative Vs/c traces heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft being the dominant source of the observed plasma. LAP only picks up a portion of the full Vs/c since the two probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally inside the potential field of the spacecraft. Comparing with the minimum energy of positive ions collected by ICA, we find numerous cases with strong correlation between the two instruments, from which the fraction of Vs/c picked up by LAP is found to vary between about 0.7 and 1. We also find an ICA energy offset of 13.7 eV (95 per cent CI: [12.5, 15.0]). Many cases of poor correlation between the instruments are also observed, predominantly when local ion production is weak and accelerated ions dominate the flux, or during quiet periods with low dynamic range in Vs/c and consequently low signal-to-noise ratios.
|
|
| 3. |
- Wieser, Gabriella Stenberg, et al.
(författare)
-
Investigating short-time-scale variations in cometary ions around comet 67P
- 2017
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 469, s. S522-S534
-
Tidskriftsartikel (refereegranskat)abstract
- The highly varying plasma environment around comet 67P/Churyumov-Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3-82 eV/q and the other featuring a 1 s time resolution in the energy range 13-50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy-time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.
|
|
| 4. |
- Simon Wedlund, Cyril, et al.
(författare)
-
Hybrid modelling of cometary plasma environments : I. Impact of photoionisation, charge-exchange and electron ionisation on bow shock and cometopause at 67P/Churyumov-Gerasimenko
- 2017
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 604
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The ESA/Rosetta mission made it possible to monitor the plasma environment of a comet, from near aphelion to perihelion conditions. To understand the complex dynamics and plasma structures found at the comet, a modelling effort must be carried out in parallel. Aims. Firstly, we present a 3D hybrid model of the cometary plasma environment including photoionisation, solar wind charge exchange, and electron ionisation reactions; this model is used in stationary and dynamic conditions (mimicking the solar wind variations), and is thus especially adapted to a weakly outgassing comet such as 67P/Churyumov-Gerasimenko, the target of the ESA/Rosetta mission. Secondly, we use the model to study the respective effects of ionisation processes on the formation of the dayside macroscopic magnetic and density boundaries upstream of comet 67P in perihelion conditions at 1.3 AU. Thirdly, we explore and discuss the effects of these processes on the magnetic field line draping, ionisation rates, and composition in the context of the Rosetta mission. Methods. We used a new quasi-neutral hybrid model, originally designed for weakly magnetised planetary bodies, such as Venus, Mars, and Titan, and adapted here to comets. Ionisation processes were monitored individually and together following a probabilistic interaction scheme. Three-dimensional paraboloid fits of the bow shock surface, identified for a magnetosonic Mach number equal to 2, and of the cometopause surface, were performed for a more quantitative analysis. Results. We show that charge exchange and electron ionisation play a major role in the formation of a bow shock-like structure far upstream, while photoionisation is the main driver at and below the cometopause boundary, within 1000 km cometocentric distance. Charge exchange contributes to 42% of the total production rate in the simulation box, whereas production rates from electron ionisation and photoionisation reach 33% and 25%, respectively. We also discuss implications for Rosetta's observations, regarding the detection of the bow shock and the cometopause.
|
|
