128 results found
Beth A, Galand MIF, 2018, Effects of the convective field on weakly outgassing comets., Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S824-S841, ISSN: 0035-8711
By applying a kinetic approach, we have developed two models in order to assess the influence of one main driver of plasma acceleration, the convective electric field, on the cometary ion distribution at 67P/Churyumov-Gerasimenko (67P/C-G). This electric field is carried by the solar wind and corresponds to the acceleration undergone by cometary ions ultimately picked up. We have quantified its contribution on ion number density and mean velocity profiles, supported by an intercomparison with the recent literature. We found that the ion number density should reflect a departure from the observed ∼1/r law. We discuss reasons for this discrepancy.
Chadney JM, Koskinen TT, Galand M, et al., 2017, Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b, Astronomy and Astrophysics, Vol: 608, ISSN: 0004-6361
Stellar flares are a frequent occurrence on young low-mass stars around whichmany detected exoplanets orbit. Flares are energetic, impulsive events, andtheir impact on exoplanetary atmospheres needs to be taken into account wheninterpreting transit observations. We have developed a model to describe theupper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. Themodel simulates thermal escape from the upper atmospheres of close-in EGPs.Ionisation by solar radiation and electron impact is included and photochemicaland diffusive transport processes are simulated. This model is used to studythe effect of stellar flares from the solar-like G star HD209458 and the youngK star HD189733 on their respective planets. A hypothetical HD209458b-likeplanet orbiting the active M star AU Mic is also simulated. We find that theneutral upper atmosphere of EGPs is not significantly affected by typicalflares. Therefore, stellar flares alone would not cause large enough changes inplanetary mass loss to explain the variations in HD189733b transit depth seenin previous studies, although we show that it may be possible that an extremestellar proton event could result in the required mass loss. Our simulations dohowever reveal an enhancement in electron number density in the ionosphere ofthese planets, the peak of which is located in the layer where stellar X-raysare absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flarelevels and enhanced electron densities last from about 3 to 10 hours after theonset of the flare. The strength of the flare and the width of its spectralenergy distribution affect the range of altitudes that see enhancements inionisation. A large broadband continuum component in the XUV portion of theflaring spectrum in very young flare stars, such as AU Mic, results in a broadrange of altitudes affected in planets orbiting this star.
Hajra R, Henri P, Vallières X, et al., 2017, Impact of a cometary outburst on its ionosphere: Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016, Astronomy and Astrophysics, Vol: 607, Pages: 1-10, ISSN: 0004-6361
We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of ∼0.2 m s-1. The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at ∼0950 UT. In response to the outburst, the neutral density increased by a factor of ∼1.8 and the local plasma density increased by a factor of ∼3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ∼2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (~5 nT) and an abrupt rotation (∼36.4°) in response to the outburst. A weakening of 10-100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.
During the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission's deep-dip #2 campaign of 17–22 April 2015, spacecraft instruments observed all of the physical parameters needed to assess the photo-chemical-equilibrium (PCE) explanation for ionospheric variability at a fixed altitude (135 km) near the peak of the Martian ionosphere. MAVEN measurements of electron density, electron temperature, neutral CO2 density, and solar irradiance were collected during 28 orbits. When inserted into the PCE equation, the measurements of varying PCE drivers correlated with the observed electron density variations to within instrumental uncertainty levels. The dominant source of this positive correlation was the variability of CO2 densities associated with the longitudinal wave-2 component of nonmigrating tides in the Martian thermosphere.
Eriksson AI, Engelhardt IAD, Andre M, et al., 2017, Cold and warm electrons at comet 67P/Churyumov-Gerasimenko, Astronomy and Astrophysics, Vol: 605, ISSN: 0004-6361
Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce.Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P/Churyumov-Gerasimenko and show filamentation of this plasma.Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas.Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5–10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order − 10 V.Conclusions. The warm (5–10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming fil
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