T. E. Cravens

Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure

The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

The plasma environment of Mars

When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.

Comet Hyakutake x‐ray source: Charge transfer of solar wind heavy ions

Recently, Lisse et al. (1996) reported on exciting observations by the Rontgen X-ray satellite (ROSAT) of x-ray and extreme ultraviolet emissions from comet C/Hyakutake 1996 B2. The spatial distribution of the emissions was displaced sunward of the nucleus and the spatial extent was about 105 km. Lisse et al. (1996) suggested that the emission could be explained by thermal bremsstrahlung associated with hot electrons, possibly due to solar wind interaction effects. In the present paper, an alternate emission mechanism is proposed. The solar wind contains a large number of minor/heavy ion species with a range of charge states, such as O6+, C5+, N5+, and Si10+. These ions will readily charge transfer with cometary neutrals, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and x-ray part of the spectrum. The photon emission rate is proportional to the solar wind heavy ion flux and hence to the solar wind flux and, with some assumptions concerning the solar wind velocity, to the solar wind number density. The emission rate should be greatest downstream of the bow shock along the sun-comet axis in agreement with the observed spatial distribution. The x-ray images are really images of the line of sight integration of the solar wind density convoluted with the cometary neutral density. A total EUV/x-ray luminosity for comet Hyakutake from this charge transfer mechanism agrees with the observed luminosity of 4 × 1015 ergs s−1 within a factor of two.

Model of Titans ionosphere with detailed hydrocarbon ion chemistry

Abstract We have modified the previous model of Keller, et al., 1992 of the ionosphere of Titan. The model is a one-dimensional photochemical model and takes into account newly measured gas phase kinetic rates, particularly those of the higher mass hydrocarbons and nitriles. The model neutral atmospheres developed by Yung et al., 1984 ( Astrophys. J. Suppl. 55, 465), Yung 1987 Icarus 72, 468) and Toublanc et al., 1995 (Icarus 113, 2) were used in our model. The current model includes more neutral and ion species and discriminates among the various higher mass hydrocarbon and nitrile ion species. Ion neutral chemistry produces HCNH+ as the single major ion species at the ionospheric peak (an altitude of 1055 km), however, the total density of all the higher mass hydrocarbons is more than that of HCNH+. The higher mass hydrocarbons include such species as : c-C3H+3, C5H+5, and C3H+5. Another important ion species is H2C3N+. Based on this model we expect the higher mass channels (e.g. 39 amu, 41 amu, 53 amu, 65 amu, 67 amu, and 69 amu) of the Cassini Ion-Neutral Mass Spectrometer to measure higher densities than previous models have predicted.

A pulsating auroral X-ray hot spot on Jupiter

Jupiters X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planets polar regions. Here we report high-spatial-resolution observations that demonstrate that most of Jupiters northern auroral X-rays come from a ‘hot spot’ located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared and ultraviolet emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.

Models of Venus neutral upper atmosphere - Structure and composition

Abstract Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO 2 , O, CO, He, N, N 2 , and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides. Estimates are made of other species, including O 2 and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.

A model of the ionosphere of Titan

We have developed a one-dimensional photochemical model to study both the composition and density structure of Titans ionosphere. The model neutral atmosphere developed by Yung et al. (1984) and Yung (1987) was used as a basis to model the chemistry of Titans ionosphere. Ionization rates due both to photoionization by solar EUV flux and to electron impact ionization by photoelectrons and Saturnian magnetospheric electrons were included. The major neutral species (nitrogen and methane) are ionized to produce N2+, N+, CH4+, CH3+, CH2+, and CH+ ions. Ion-neutral chemistry converts these ions to the major ion species H2CN+ (in agreement with Atreya (1986), Strobel (1985), and Ip (1990a)), as well as C2H5+, CH5+, HCN+, and more complex hydrocarbon ions (CnHm+). Inclusion of both forms of ionization gave a peak electron density of ≈ 3030 cm−3 at an altitude of ≈ 1175 km along the terminator. The radio occultation experiment on board Voyager 1 (Lindal et al., 1983) put limits on the maximum ionospheric electron density of 5 × 10³ cm−3 and 3 × 10³ cm−3 along the morning and evening terminators, respectively. The total external pressure (i.e., dynamic, thermal, and magnetic) upstream of Titan at the time of the Voyager encounter is of the order of the maximum ionospheric thermal pressure. As a consequence one can expect that the solar wind interaction with Venus during periods of high solar wind dynamic pressure might provide a good analogy for the interaction of Titan with the Saturnian magnetospheric plasma.

Solar cycle variability of hot oxygen atoms at Mars

The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O2+ ions. These calculations take into account the calculated vibrational distribution of O2+ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O2+ ions, and the spread of the ion and electron velocities. These production rates were next used in a two-stream model to obtain the energy dependent flux of the hot oxygen atoms as a function of altitude. Finally, the calculated flux at the exobase was input into an exosphere model, based on Liouvilles theorem, to calculate the hot oxygen densities as a function of altitude in the exosphere and the resulting escape flux. It was found that hot oxygen densities vary significantly over the solar cycle; the calculated densities vary from about 2×103 to 6×103 cm−3 at an altitude of 1000 km. The escape flux also varies from about 3×106 to 9×106 cm−2s−1.

Solar wind stagnation near comets

The nature of the solar wind flow near comets is examined analytically in this paper. In particular, typical values for the stagnation pressure and magnetic barrier strength are estimated, taking into account magnetic field line tension and change-exchange cooling of the mass-loaded solar wind. A knowledge of the strength of the magnetic barrier is required in order to determine the location of the ionopause surface which separates the contaminated solar wind plasma from the outflowing plasma of the cometary ionosphere.

Electrons in the ionosphere of Titan

A theoretical model of the spatial and energy distribution of electrons in the ionosphere of Titan has been constructed using the two-stream electron transport method and the electron energy equation. The calculated electron spectra show abrupt decreases that can contribute to the “bite-out” signature observed by the Voyager 1 plasma science instrument. Energy deposition rates in the exosphere of Titan by photoelectrons and magnetospheric electrons are calculated. Calculated N2 EUV airglow emission rates lead to the conclusion that airglow emission due to photoelectron impact is much more important than airglow emission due to magnetospheric electron interactions. Thermal electron temperatures at Titan are calculated for the first time. The electron gas remains well thermalized with the neutral atmosphere for radial distances from the center of Titan less than 3500 km. For radial distances beyond about 4000 km, energy transport dominates the energetics, and the electrons are almost isothermal along magnetic field lines.