120 results found
Vigren E, André M, Edberg NJT, et al., 2017, Effective ion speeds at ∼200–250 km from comet 67P/Churyumov–Gerasimenko near perihelion, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S142-S148, ISSN: 0035-8711
In 2015 August, comet 67P/Churyumov–Gerasimenko, the target comet of the ESA Rosetta mission, reached its perihelion at ∼1.24 au. Here, we estimate for a three-day period near perihelion, effective ion speeds at distances ∼200–250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2–8 km s−1, are markedly higher than the expected neutral outflow velocity of ∼1 km s−1. This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions’ kinetic energy associated with radial drift motion is decreasing.
Heritier KL, Henri P, Vallières X, et al., 2017, Vertical structure of the near-surface expanding ionosphere of comet 67P probed by Rosetta, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S118-S129, ISSN: 0035-8711
The plasma environment has been measured for the first time near the surface of a comet. This unique data set has been acquired at 67P/Churyumov–Gerasimenko during ESA/Rosetta spacecraft's final descent on 2016 September 30. The heliocentric distance was 3.8 au and the comet was weakly outgassing. Electron density was continuously measured with Rosetta Plasma Consortium (RPC)–Mutual Impedance Probe (MIP) and RPC–LAngmuir Probe (LAP) during the descent from a cometocentric distance of 20 km down to the surface. Data set from both instruments have been cross-calibrated for redundancy and accuracy. To analyse this data set, we have developed a model driven by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis–COmetary Pressure Sensor total neutral density. The two ionization sources considered are solar extreme ultraviolet radiation and energetic electrons. The latter are estimated from the RPC–Ion and Electron Sensor (IES) and corrected for the spacecraft potential probed by RPC–LAP. We have compared the results of the model to the electron densities measured by RPC–MIP and RPC–LAP at the location of the spacecraft. We find good agreement between observed and modelled electron densities. The energetic electrons have access to the surface of the nucleus and contribute as the main ionization source. As predicted, the measurements exhibit a peak in the ionospheric density close to the surface. The location and magnitude of the peak are estimated analytically. The measured ionospheric densities cannot be explained with a constant outflow velocity model. The use of a neutral model with an expanding outflow is critical to explain the plasma observations.
Beth A, Altwegg K, Balsiger H, et al., 2017, First in situ detection of the cometary ammonium ion NH4+ (protonated ammonia NH3) in the coma of 67P/C-G near perihelion, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S562-S572, ISSN: 0035-8711
In this paper, we report the first in situ detection of the ammonium ion NH+44+ at 67P/Churyumov–Gerasimenko (67P/C-G) in a cometary coma, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). Unlike neutral and ion spectrometers onboard previous cometary missions, the ROSINA/DFMS spectrometer, when operated in ion mode, offers the capability to distinguish NH+44+ from H2O+ in a cometary coma. We present here the ion data analysis of mass-to-charge ratios 18 and 19 at high spectral resolution and compare the results with an ionospheric model to put these results into context. The model confirms that the ammonium ion NH+44+ is one of the most abundant ion species, as predicted, in the coma near perihelion.
Vigren E, Altwegg K, Edberg NJT, et al., 2017, Erratum: “Model–observation comparisons of electron number densities in the coma of 67P/Churyumov-Gerasimenko during 2015 January” (2016, AJ, 152, 59), Astronomical Journal, Vol: 153, Pages: 50-50, ISSN: 0004-6256
Galand M, Héritier KL, Odelstad E, et al., 2016, Ionospheric plasma of comet 67P probed by Rosetta at 3 AU from the Sun, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S331-S351, ISSN: 1365-2966
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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