169 research outputs found
Oxygen Ion Energization Observed At High Altitudes
We present a case study of significant heating (up to 8 keV) perpendicular to
the geomagnetic field of outflowing oxygen ions at high altitude (12 RE)
above the polar cap. The shape of the distribution functions indicates that
most of the heating occurs locally (within 0.2â0.4 RE in altitude). This
is a clear example of local ion energization at much higher altitude than
usually reported. In contrast to many events at lower altitudes, it is not
likely that the locally observed wave fields can cause the observed ion
energization. Also, it is not likely that the ions have drifted from some
nearby energization region to the point of observation. This suggests that
additional fundamentally different ion energization mechanisms are present at
high altitudes. One possibility is that the magnetic moment of the ions is
not conserved, resulting in slower outflow velocities and longer time for ion
energization
The Hydrogen Exospheric Density Profile Measured with ASPERA-3/NPD
We have evaluated the Lyman-α limb emission from the exospheric hydrogen of Mars measured by the neutral particle detector of the ASPERA-3 instrument on Mars Express in 2004 at low solar activity (solar activity index = 42, F10.7=100). We derive estimates for the hydrogen exobase density, n H = 1010 mâ3, and for the apparent temperature, T > 600 K. We conclude that the limb emission measurement is dominated by a hydrogen component that is considerably hotter than the bulk temperature at the exobase. The derived values for the exosphere density and temperature are compared with similar measurements done by the Mariner space probes in the 1969. The values found with Mars Express and Mariner data are brought in a broader context of exosphere models including the possibility of having two hydrogen components in the Martian exosphere. The present observation of the Martian hydrogen exosphere is the first one at high altitudes during low solar activity, and shows that for low solar activity exospheric densities are not higher than for high solar activit
Energetic Hydrogen and Oxygen Atoms Observed on the Nightside of Mars
We present measurements of energetic hydrogen and oxygen atoms (ENAs) on the nightside of Mars detected by the neutral particle detector (NPD) of ASPERA-3 on Mars Express. We focus on the observations for which the field-of-view of NPD was directed at the nightside of Mars or at the region around the limb, thus monitoring the flow of ENAs towards the nightside of the planet. We derive energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 105 cmâ2 srâ1 sâ1, but no oxygen ENA signals above the detection threshold of 104 cmâ2 srâ1 sâ1 are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements (Galli et al., this issue) point to a hydrogen exobase density of 1010 mâ3 and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 1012 mâ3 and a temperature of 200 K predicted by Krasnopolsky and Gladstone (1996). Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENA
Faire lâUnion. La refondation des parties de droite aprĂšs les Ă©lections de 2002
Un nouveau parti de droite, pour le moment dénommé UMP, a été mis en place entre des deux tours de
l'élection présidentielle. Cet article se donne pour objectif d'analyser les enjeux de cette transformation.
La genĂšse de l'UMP permet de saisir dans quelle mesure un parti poli-tique peut ĂȘtre conçu comme la
connexion d'un ensemble de systÚmes de coopération organisés autour de postes à conquérir. En effet,
l'impératif d'unification de la droite française est lié au décalage persistant existant entre les systÚmes
de coopération législatif et présidentiel. Mais les questions soulevées par cette fusion partisane (l'UMP
inclut le RPR, DL et une majeure partie de l'UDF) montrent que cette vision ne peut suffire et que les
partis ne sont pas seulement des systÚmes de coopération mais aussi des systÚmes de production.
DĂšs lors, leurs logiques d'organisation internes pĂšsent sur ce travail de transformation du social en
politique, de la plu-ralité en homogénéité. Dans le cas de l'UMP, il s'agit d'abord de mettre en commun
des modes de fonctionnement, en particulier de définir la place et la légitimité accordées
respectivement aux adhérents et aux élus, mais aussi d'organiser l'expression de la pluralité
idéologique par la mise en place de courants
First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation
Context. Amongst the different features and boundaries encountered around comets, one remains of particular interest to the plasma community: The diamagnetic cavity. Crossed for the first time at 1P/Halley during the Giotto flyby in 1986 and later met more than 700 times by the ESA Rosetta spacecraft around Comet 67P/Churyumov-Gerasimenko, this region, almost free of any magnetic field, surrounds nuclei of active comets. However, previous observations and modelling of this part of the coma have not yet provided a definitive answer as to the origin of such a cavity and on its border, the diamagnetic cavity boundary layer. Aims. We investigate which forces and equilibrium might be at play and balance the magnetic pressure at this boundary down to the spatial and temporal scales of the electrons in the 1D collisionless case. In addition, we scrutinise assumptions made in magneto-hydrodynamic and hybrid simulations of this environment and check for their validity. Methods. We simulated this region at the electron scale by means of 1D3V particle-in-cell simulations and SMILEI code. Results. Across this layer, depending on the magnetic field strength, the electric field is governed by different equilibria, with a thin double-layer forming ahead. In addition, we show that the electron distribution function departs from Maxwellian and/or gyrotropic distributions and that electrons do not behave adiabatically. We demonstrate the need to investigate this region at the electron scale in depth with fully kinetic simulations
Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA
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
Investigating short-time-scale variations in cometary ions around comet 67P
The highly varying plasma environment around comet 67P/ChuryumovâGerasimenko 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 0.3â82 eV/q and the other featuring a 1 s time resolution in the energy range 13â50 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 1462âh of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84âperâcent of these sequences into one of five categories, depending on their appearance in an energyâtime 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
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