摘要:

The temperature and composition of the upper atmosphere of Titan have been inferred by observing an occultation of the sun by Titan, using the Voyager 1 ultraviolet spectrometer. The temperature is 176±20 K near the evening terminator and 196±20 K near the morning terminator. The major constituent is N2with a density of 2.7±0.2 × 108cm−3at 3840 km. The mixing ratio of CH4is 8±3% at a radial distance of 3700 km near the evening terminator where [CH4] ≃1.2 × 108cm−3. On the morning terminator the [CH4] ≃ 1.2 × 108cm−3level is about 20 km lower in the atmosphere. The acetylene mixing ratio above 3400 km is measured at the 1 to 2% level. Below 3300 km it decreases to between 0.1 and 0.3%. A layer of absorbing molecules, possibly polymers, is present at both morning and evening terminators. Near the evening terminator the layer is located between 3350 and 3600 km. Near the morning terminator it is located about 100 km lower in the atmosphere. A simple photochemical model suggests that the homopause is located at 3500 ± 70 km with an eddy diffusion coefficient of 1−0.7+2× 108cm² s−1, which decreases to ∼10³ cm² s−1in the l

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01351

年代:1982

数据来源: WILEY

摘要:

Analysis of Titan's EUV emission spectra obtained at the Voyager 1 encounter demonstrates that electron impact on N2above 3600 km accounts for the bulk of the observed emission short of Lyman α. In conjunction with the UVS solar occultation data it is concluded that N2is the major component of Titan's upper atmosphere, with upper limit mixing ratios at 3900 km on NeI, ArI, CO, H2, and HI of 0.01, 0.06, 0.05, 0.06, and 0.1, respectively. Magnetospheric electrons interact with Titan's sunlit hemisphere to produce a power dissipation rate of ≃2 × 109W in the exosphere and ≃3 × 109W below the exobase, with optical signatures from numerous N2bands, NI, and NII multiplets. The N2C4′ (0‐0) Rydberg band at 958 Å acts as an optical probe of Titan's exosphere because of transmission losses caused by fluorescence and predissociation. Magnetospheric electron precipitation produces an average dayside electron density of ≃3 × 10³ cm−3between 3600 and 4000 km, the region of bright limb emission. When Titan is within Saturn's magnetosphere, magnetospheric electron impact dissociation of N2generates an N atom escape rate of ≃3 × 1026s−1from Titan's exosphere. A nonthermal H atom escape rate of ≃2 × 1026s−1is estimated from magnetospheric electron impact ionization of N2followed by reactions with CH4and H2and recombina

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01361

年代:1982

数据来源: WILEY

摘要:

The Voyager 1 spacecraft had a close encounter (miss distance = 6970 km) with Titan (diameter = 5140 km) on November 12, 1980, while this large satellite was located within the Saturnian magnetosphere at a local solar time of 1330. No clear evidence was found for any intrinsic magnetic field nor for the development of a bow shock wave as the corotating Saturnian magnetoplasma convected past Titan. However, a strong electrodynamic interaction was evidenced with the observation of a very well developed, induced bipolar magnetic tail. Three thin current carrying regions were crossed, corresponding to the inbound and outbound tail magnetopause and an imbedded tail neutral sheet. An asymmetry in the tail axis orientation of 20° with respect to the corotation direction was observed. Also seen was an asymmetry in the structure of the two tail lobes. This interaction is unique among those in the solar system observed to date, being intermediate in the characteristic Mach numbers, both sonic and Alfvenic, when compared with Titan (or Venus) in the solar wind or Io in the Jovian magnetosphere. This paper presents the results of the analysis of the magnetic field data, which suggest an interpretation qualitatively described by the draping of the Saturnian magnetic field around the ionosphere of Titan

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01369

年代:1982

数据来源: WILEY

摘要:

Electron and ion measurements made by the Voyager 1 plasma science instrument revealed a plasma wake surrounding Titan in Saturn's rotating magnetosphere. This wake is characterized by a plasma that is more dense and cooler than the surrounding subsonic magnetospheric plasma. The density enhancement is produced by the deflection of magnetospheric plasma around Titan and the addition of exospheric ions picked up by the rotating magnetosphere. By using simple models for ion pickup in the ion exosphere outside Titan's magnetic tail and ion flow within the boundaries of the tail, the interaction between Saturn's rotating magnetosphere and Titan is shown to resemble the interaction between the solar wind and Venus. Outside the magnetic tail of Titan, pickup of H+formed by ionization of the H exosphere is indicated when synthetic and observed ion spectra are matched. Close to the boundary of the tail, a reduction in plasma flow speed is found, providing evidence for mass loading by the addition of N2+/H2CN+and N+to the flowing plasma. The boundary of the tail is indicated by a sharp reduction in the flux of high‐energy electrons, which are removed by inelastic scattering with the atmosphere and centrifugal drift produced when the electrons traverse the magnetic field draped around Titan. Within the tail the plasma is structured as the result of spatial and/or temporal variations. The ion mass cannot be determined uniquely in the tail; however, one measurement suggests the presence of a heavy ion with a mass of order 28 amu: One candidate is H2CN+, suggested as the dominant topside ion of the ionosphere, which may flow from the ionosphere into the tai

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01383

年代:1982

数据来源: WILEY

摘要:

During the Voyager 1 flyby of Saturn's moon Titan, the plasma wave instrument detected several types of plasma wave emissions. On the inbound leg a broad region of intense low‐frequency noise was detected on the side of Titan facing away from Saturn. This noise has characteristics similar to the electric field turbulence observed in the magnetosheath at the earth and the ionosheath at Venus and is believed to be generated by newly created ions that are being accelerated in the vicinity of Titan by the corotational electric field. During the pass through the induced magnetic tail of Titan, a series of upper hybrid resonance emissions were observed. The electron density profile inferred from these emissions shows three distinct peaks with densities of ∼40 cm−3, the first peak corresponding to the entry into the magnetic tail, the second peak corresponding to the neutral sheet crossing from the northern to the southern tail lobe, and the third, somewhat smaller, peak corresponding to the outbound exit from the tail. Large depressions in the magnetic field strength are observed coincident with each of the density peaks. These effects indicate that a dense plume of plasma is being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn. By equating the magnetic field pressure in the tail lobe to the plasma pressure in the neutral sheet, the temperature of the plasma is estimated to be about 8600°K. This low temperature suggests that the plasma originates from the ionosphere of Titan, probably forming a plume of plasma with a θ or H cross section extending downstream from Titan. Within the tail lobes, a second type of low‐frequency electric field noise was observed, with characteristics very similar to a type of noise called broadband electrostatic noise, which is found in the earth's magnetic tail. As in the case of the earth, this noise is most intense near the outer boundary of the plasma sheet and is almost completely absent in the high‐density region near the ne

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01395

年代:1982

数据来源: WILEY

摘要:

During the Voyager 1 Titan flyby, unusual radio emissions were observed by the planetary radio astronomy experiment in the 20‐ to 97‐kHz frequency range. In this paper we show that Titan itself is not the source of the observed radio emission. Rather, we attribute the emission features to modification of the normal Saturn kilometric radiation by propagation effects in enhanced density structures within the Titan wake. Furthermore, spiky emissions observed in the magnetic wake of Titan are interpreted in terms of local electrostatic instabilities at the electron plasma frequency. From these measurements we derive a range of electron densities in the wake region, and we discuss the consistency of the resu

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01405

年代:1982

数据来源: WILEY

摘要:

The close fly‐by of the Voyager 1 spacecraft to the Saturnian satellite Titan provided an opportunity for the low energy charged particle (LECP) experiment to investigate the influences of Titan on the magnetosphere energetic particle distributions. Magnetic field data from the magnetometer (MAG) experiment are used with angular distribution data from LECP to study the changes in the pitch angle distributions of ions (40

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01411

年代:1982

数据来源: WILEY

摘要:

A fortuitous set of interplanetary and solar events which occured on October 12, 1977, allowed the observations by several interplanetary spacecraft, IMP 7 and 8, of features in charged particle angular distributions which suggest the presence of a closed magnetic loop. This loop must have extended beyond the orbit of earth and have been connected to particle sources and/or mirrors at both ends. This magnetic topology is similar to a suggestion made by Gold (1960). The salient feature of the anisotropies which calls attention to the possible presence of a loop is the occurrence of a trapping‐type electron distribution peaked at 90° to the magnetic field, while the ions are strongly field aligned and bidirection

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01419

年代:1982

数据来源: WILEY

摘要:

The profile of the helium resonance line at λ 584 Å from the full solar disk has been investigated through analysis of data obtained by sounding rockets flown in 1977 and in 1980. The instrument, a helium‐filled spectrometer, made use of a curve‐of‐growth technique. In the analysis, the solar He I line profile was represented by a Gaussian distribution. Between 6.5 and 13% of the Gaussian core area was found to be missing due to self‐reversal. Data from our 1980 experiment gave a line width of 101 ± 10 mÅ (full width at half maximum). The 1977 line width, obtained with less experimental reliability, was 128 ± 20 mÅ (full width at half maximum). Results obtained in a similar experiment in 1974, corrected for background radiation and for solar line self‐reversal, gave a revised 1974 line width of 103 mÅ (full width at half maximum). While the measured line width did vary, a comparison of the 1974, 1977, and 1980 data showed no consistent relationship between the measured widths and solar activity. An observation of the solar He I 537‐Å emissions during the 1980 sounding rocket flight provided a measurement of that line width, giving a value of 148 ± 27 mÅ (full w

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01433

年代:1982

数据来源: WILEY

摘要:

An expression for the total powerPTtransferred from the solar wind to an open magnetosphere is derived. The power is transmitted through an ‘open’ magnetopause with a nonzero normal component of the magnetic field, which is identified as a rotational discontinuity. The total powerPTconsists of (1) the powerPEMrepresenting the electromagnetic energy transfer and (2) the powerPKErepresenting the rate of kinetic energy carried by particles penetrating into the magnetosphere. It is found thatPEM≃VSWBSWψ,PKE≃ (½MA‐ 1)PEMandPT≃ ½MAPEM, whereVSW,BSW, andMAare the velocity, magnetic field, and the Alfvén‐Mach number in the solar wind, respectively, and Ψ is the open magnetic flux in the magnetosphere. The Alfvén‐Mach number of the flow at the magnetopause determines the nature of the local energy transfer; the power per unit area transferred from the solar wind to the magnetosphere consists mainly of kinetic energy. The electromagnetic energy ratePEMcontrols the near‐earth magnetospheric activity, whereas the kinetic energy ratePKE(≃ 3‐4PEM) should dominate the dynamics

ISSN:0148-0227    

DOI:10.1029/JA087iA03p01439

年代:1982

数据来源: WILEY