144 research outputs found
Positive ion composition in the polar D and E regions measured during moderate ionospheric absorption
During the MAC/Epsilon campaign a mass spectrometer probe was flown on a rocket launched from Andoya (Norway) on 12 November 1987 at 0021 UT providing partial ion density profiles in the altitude range between less than 50 to 125 km. Due to the short sampling period of 0.17 seconds structural features could be observed at approx. 150 m height resolution in the regimes where metal ions occur and where cluster ions are dominant. The observations were made during stable ionospheric absorption of 1 to 1.5 dB. Preliminary results are presented and discussed
On the Solar EUV Deposition in the Inner Comae of Comets with Large Gas Production Rates
In this letter we have made a comparative study of degradation of solar EUV
radiation and EUV-generated photoelectrons in the inner comae of comets having
different gas production rates, Q, with values 1x10^28, 7x10^29, 1x10^31, and
1x10^32 s^-1. We found that in higher-Q comets the radial profile of H2O+
photo-production rate depicts a double-peak structure and that the differences
in sunward and anti-sunward photoionization rates are pronounced. We show that
photoelectron impact ionization is an order of magnitude larger than
photoionization rate near the lower photoionization peak in comets with Q >~
1x10^31 s^-1. The present study reveals the importance of photoelectrons
relative to solar EUV as the ionization source in the inner coma of high-Q
comets
Radial Distribution of Production Rates, Loss Rates and Densities Corresponding to Ion Masses <=40 amu in the Inner Coma of Comet Halley: Composition and Chemistry
In this paper we have studied the chemistry of C, H, N, O, and S compounds
corresponding to ions of masses <=40 amu in the inner coma of the comet
1P/Halley. The production rates, loss rates, and ion mass densities are
calculated using the Analytical Yield Spectrum approach and solving coupled
continuity equation controlled by the steady state photochemical equilibrium
condition. The primary ionization sources in the model are solar EUV photons,
photoelectrons, and auroral electrons of the solar wind origin. The chemical
model couples ion-neutral, electron-neutral, photon-neutral and electron-ion
reactions among ions, neutrals, electrons, and photons through over 600
chemical reactions. Of the 46 ions considered in the model the chemistry of 24
important ions are discussed in this paper. The calculated ion mass densities
are compared with the Giotto Ion Mass Spectrometer (IMS) and Neutral Mass
Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km.Comment: pages 42, figures 9 (total 12), tables
Model for the Production of CO Cameron band emission in Comet 1P/Halley
The abundance of CO2 in comets has been derived using CO Cameron band (a3pi
--> X1Sigma+) emission assuming that photodissociative excitation of CO2 is the
main production process of CO(a3pi). On comet 1P/Halley the Cameron (1-0) band
has been observed by International Ultraviolet Explorer (IUE) on several days
in March 1986. A coupled chemistry-emission model is developed for comet
1P/Halley to assess the importance of various production and loss mechanisms of
CO(a3pi) and to calculate the intensity of Cameron band emission on different
days of IUE observation. Two different solar EUV flux models, EUVAC of Richards
et al. (1994) and SOLAR2000 of Tobiska (2004), and different relative
abundances of CO and CO2, are used to evaluate the role of photon and
photoelectron in producing CO molecule in a3pi state in the cometary coma. It
is found that in comet 1P/Halley 60--70% of the total intensity of the Cameron
band emission is contributed by electron impact excitation of CO and CO2, while
the contribution from photodissociative excitation of CO2 is small (20--30%).
Thus, in the comets where CO and CO2 relative abundances are comparable, the
Cameron band emission is largely governed by electron impact excitation of CO,
and not by the photodissociative excitation of CO2 as assumed earlier. Model
calculated Cameron band 1-0 emission intensity (40 R) is consistent with the
observed IUE slit-averaged brightness (37 +/- 6 R) using EUVAC model solar flux
on 13 March 1986, and also on other days of observations. Since electron impact
excitation is the major production mechanism, the Cameron emission can be used
to derive photoelectron density in the inner coma rather than the CO2
abundance.Comment: 40 pages 8 figure
Evidence for methane and ammonia in the coma of comet P/Halley
Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH_3 , and uniquely NH_3 , in coma gases in order to explain the data. A ratio of production rates Q(NH_3)/Q(H20) = 0.01-Q.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15.
The observations are fit best with Q(CH_4)/Q(H_20) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material
Distant ionospheric photoelectron energy peak observations at Venus
The dayside of the Venus ionosphere at the top of the planet's thick atmosphere is sustained by photoionization. The consequent photoelectrons may be identified by specific peaks in the energy spectrum at 20â30 eV which are mainly due to atomic oxygen photoionization. The ASPERA-4 electron spectrometer has an energy resolution designed to identify the photoelectron production features. Photoelectrons are seen not only in their production region, the sunlit ionosphere, but also at more distant locations on the nightside of the Venus environment. Here, we present a summary of the work to date on observations of photoelectrons at Venus, and their comparison with similar processes at Titan and Mars. We expand further by presenting new examples of the distant photoelectrons measured at Venus in the dark tail and further away from Venus than seen before. The photoelectron and simultaneous ion data are then used to determine the ion escape rate from Venus for one of these intervals. We compare the observed escape rates with other rates measured at Venus, and at other planets, moons and comets. We find that the escape rates are grouped by object type when plotted against body radius
On the origin and evolution of the material in 67P/Churyumov-Gerasimenko
International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects
Thymus vulgaris, un cas de polymorphisme chimique pour comprendre l'écologie évolutive des composés secondaires
MONTPELLIER-SupAgro La Gaillarde (341722306) / SudocSudocFranceF
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