摘要:

The Voyager Neptune/Interstellar Mission is the continuation of the NASA program of exploration of the outer solar system. The prior phases of the Voyager program included encounters with Jupiter, Saturn, and Uranus as summarized in theJournal of Geophysical Research(volume 86, pages 8121–8841, 1981; volume 88, pages 8625–9018, 1983; and volume 92, pages 14,873–15,375, 1987). Following the successes of these prior phases of the Voyager mission, a new phase was undertaken with the objectives of exploring the Neptune system and continuing the investigation of the interplanetary and interstellar media. An additional objective was to preserve the capability to extend the investigations to include a search for the heliopause. Summaries of results from the Neptune and Triton encounters are reported in the following p

ISSN:0148-0227    

DOI:10.1029/91JA02174

年代:1991

数据来源: WILEY

摘要:

Voyager radio occultation and infrared spectrometer measurements are used to obtain an estimate of the helium abundance in the atmosphere of Neptune. It is found that the shape of the measured spectrum cannot be well matched by spectra calculated from atmospheric models that include only gaseous opacity. The most plausible explanation of the observed spectral shape appears to require the existence of an additional opacity source associated with clouds or hazes. The data can be fit with either a tropospheric or a stratospheric cloud model. If a tropospheric cloud is invoked, an optical thickness at 200 cm−1between 1 and 8 is required, depending on the particle size assumed. For a stratospheric cloud an optical thickness between 0.2 and 0.8 is required. Optical constants for methane ice are used, but ethane has similar optical properties in this spectral region. Assuming no modification to the temperature profile is required other than a molecular weight adjustment, a combination of gas and cloud opacities produces a good fit to the spectrum for a helium mole fractionqHe= 0.190 ± 0.032, corresponding to a mass fractionY= 0.32 ± 0.05. Within the measurement uncertainties the Neptune helium abundance may be compatible with that of Uranus and, marginally, with the protosolar value. However, models in which the outer part of the primitive solar nebular was enriched in CO and N2, leading to an enhanced atmospheric He/H2ratio, cannot be ruled out at this t

ISSN:0148-0227    

DOI:10.1029/91JA01703

年代:1991

数据来源: WILEY

摘要:

Data from the Voyager infrared spectrometer and radiometer (IRIS) investigation are used in determining the albedo, effective temperature, and energy balance of Neptune. From broadband radiometric observations made at phase angles of 14° and 134°, together with measurements at intermediate phase angles from the literature, an orbital mean value for the bolometric Bond albedo,Ā= 0.290 ± 0.067, is obtained; this yields an equilibrium temperatureTeq= 46.6 ± 1.1 K. From thermal spectra obtained over latitudes from pole to pole an effective temperature = 59.3 ± 0.8 K is derived. This represents a substantial improvement over previously determined values. The energy balance of Neptune is thereforeE= 2.61 ± 0.28, which is in agreement with previous results. The reduced uncertainty in this value is due to the improved determination of the effective tempe

ISSN:0148-0227    

DOI:10.1029/91JA01087

年代:1991

数据来源: WILEY

摘要:

Spatially resolved Voyager infrared spectra from a global mapping sequence are used to characterize the thermal structure of the atmosphere of Neptune between approximately 50 and 100 mbar. A zonal mean meridional temperature cross section is obtained, which shows a minimum in the upper tropospheric and lower stratospheric temperatures at southern mid latitudes with maxima at the equator and high southern latitudes. This is qualitatively similar to the structure observed on Uranus, even though the obliquities and internal heat fluxes of the two planets are quite different. Thermal wind calculations show zonal wind speeds decreasing with altitude at all latitudes. Within the framework of a linear radiative‐dynamical model, the thermal structure and cloud top winds together imply a frictional damping time comparable in magnitude to the radiative relaxation time; similar results have previously been obtained for the other three giant planets. Periodogram techniques for used to search the observed longitude variations of temperature for possible planetary‐scale wave features. Data from two latitude bins are analyzed; one of the bins includes the Great Dark Spot, and the other is centered on the southern mid‐latitude minimum in the zonal mean temperature. A weakly significant feature near planetary wavenumber 4 is identified at the latitude of the spot. The rest of the variance can be attributed to instrument noise and possibly a broadband meteorological spectrum indistinguishable from white

ISSN:0148-0227    

DOI:10.1029/91JA01859

年代:1991

数据来源: WILEY

摘要:

High temporal and spatial resolution images acquired from Voyager cameras have been used to measure cloud motions to improve the meridional profile of the zonal mean circulation on Neptune. A wide range of atmospheric periods between 12 and 21 hours is revealed by the average cloud motions, consistent with the previous observations. New observations have expanded latitudinal coverage, improved the determination of streak motions, especially near 30°N, and added statistical weight to latitudes already covered by previous measurements. Both new and earlier observations have been subjected to quality control procedures to reduce dispersion caused by erroneous observations. The resulting data set emphasizes short time intervals to maximize target recognition. Most reliable cloud targets last less than one Neptune rotation, many only a fraction of it. A broad equatorial retrograde jet extends from approximately 50°S to at least 45°N (the northernmost latitude at which discrete cloud features have been seen). A relatively narrow prograde jet of at least 300 m s−1is found near 70°S. The wind observations have a high degree of variability, some of which is due to variability of motions; but at many latitudes it reflects observation errors arising from rapid evolution of Neptune's clouds. A bias observed in the measured meridional component is most likely due to a small discrepancy between the true rotation pole of Neptune and the pole position used in the data reduction. Zonal motions and morphology suggest the global circulation to be symmetric about the eq

ISSN:0148-0227    

DOI:10.1029/91JA01701

年代:1991

数据来源: WILEY

摘要:

Emission from the acetylene and ethane bands at 729 and 822 cm‐1detected in the Voyager infrared spectra of Neptune has been analyzed. A large selection of low‐spatial resolution spectra was used to derive the disk‐averaged abundances of C2H2and C2H6. Under the assumption of uniform vertical distributions above the saturation region, a C2H2mixing ratio of 6−4+14x 10−8and a C2H6mixing ratio of 1.5−0.5+2.5x 10−6were inferred. The accuracy of the retrievals is limited by the large uncertainty in the stratospheric temperature structure. The maximum contribution to the observed C2H2and C2H6emission comes from the 0.2‐ and 0.7‐mbar regions, respectively. Mixing ratio profiles derived from photochemical modeling, which are not constant with height above the saturation region, indicate that the hydrocarbon emission is most sensitive to the assumed eddy diffusion coefficient in the millibar region. Either the C2H2or the C2H6emission can be reproduced by the photochemical model to within the accuracy of the retrievals, but not both simultaneously. Best fits to both emission features simultaneously occur with C2H2mixing ratios a factor of 2 too high and C2H6mixing ratios a factor of 2 too low. We consider this agreement satisfactory considering the unknowns in the chemical and photolytic processes. A set of Voyager spectra at higher spatial resolution was used to study the latitudinal variation of the C2H2 emission between 30°N and 80°S. Zonal mean radiances at the C2H2peak show a minimum near 50°–60°S and maxima near the south pole and equator. This behavior is similar to that observed at 350 and 250 cm−1, where the lower stratosphere and troposphere are sounded. The mid‐latitude minimum can be explained by a fivefold depletion of acetylene or a temperature decrease of about 15 K (or any combination of the two effects) in the 0.03‐ to 2‐mbar region. The latitude variation in the C2H2emission could result from a circulation pattern forced from deep levels, with upwelling at mid‐latitudes and sub

ISSN:0148-0227    

DOI:10.1029/91JA01930

年代:1991

数据来源: WILEY

摘要:

With the completion of the Voyager 2 encounter with Neptune we have now surveyed the plasma wave spectra of five planetary magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune. Here we provide a first general comparison of the various plasma wave modes at each of the planets with the use of a common format for displaying the spectra. The general conclusions are that many of the same types of wave modes are present in each of the magnetospheres, despite great differences in the magnetospheres' sizes, heliocentric distances, energy sources, plasma sources, and magnetic dipole orientations. There are, however, great differences in the relative and absolute intensity of some of the wave modes. Virtually ubiquitous in planetary magnetospheres are electron cyclotron harmonic bands and whistler mode emissions such as hiss and chorus. Ion cyclotron harmonic emissions have been observed where the observed local magnetic field strength was great enough to move these low‐frequency waves into the Voyager plasma wave receiver's frequency range. Broadband electrostatic noise has also been observed in the majority of the magnetospheres. In addition to a general survey of the magnetospheric wave modes, an initial assessment of the role of plasma waves in the precipitation of charged particles is presented. Waves seem to have obvious contributions in this aspect for Earth, Jupiter, and Uranus. Weaker wave amplitudes observed at Saturn and Neptune may possibly be due to the specific geometry of the flybys or to quiescent states of the magnetospheres during the encounter

ISSN:0148-0227    

DOI:10.1029/91JA01819

年代:1991

数据来源: WILEY

摘要:

This paper provides a synopsis of the ion observations near Neptune, from the upstream solar wind through the magnetosheath, magnetosphere, and back out through the magnetosheath to the solar wind. The main emphasis is on the magnetosphere; complete density and temperature profiles are shown for this region. Observations are consistent with the presence of two ion species, H+and N+, which increase in density and temperature with decreasing distance from Neptune. Calculations of the flux tube content and energy invariant imply that N+escapes directly from Triton's ionosphere, whereas H+is created by ionization of a large neutral H cloud emanating from Triton and extending inward to 8RN. InsideL=8, substantial plasma losses occur, precipitation into Neptune's atmosphere is one loss mechanism, and charge exchange with a hypothesized Neptune‐centered H cloud may also be important Given expected source rates of about lxlO25ions/s, transport times must be very fast, of the order of 105s atL=1

ISSN:0148-0227    

DOI:10.1029/91JA01598

年代:1991

数据来源: WILEY

摘要:

One of the strongest plasma wave signals observed during the Voyager 2 encounter with Neptune is a narrowband emission between 3.0 and 4.3 kHz that was detected over a period of roughly 2 hours around closest approach. The emission occurs below the electron cyclotron frequency and the low‐frequency cutoff of the radio continuum radiation. Of the naturally occurring signals in the Earth's auroral zone and in Jupiter's magnetosphere this emission most resembles trapped Z mode waves found near the left‐hand cutoff frequency. Using this identification, we obtain a plasma density profile that is independent of the plasma temperature. These densities greatly exceed those measured by the plasma science instrument on Voyager but are lower than estimates based on other models of Neptunian plasma wave phenomenology. If this wave mode is not a natural emission, it might arise from an unusual interaction of the spacecraft with the cold, dense ambient pla

ISSN:0148-0227    

DOI:10.1029/91JA00085

年代:1991

数据来源: WILEY

摘要:

A spherical harmonic model of the planetary magnetic field of Neptune is obtained from the Voyager 2 encounter observations. The model is obtained by partial solution to the underdetermined inverse problem (eighth‐order and degree spherical harmonic model) using generalized inverse techniques. Dipole and quadrupole Schmidt‐normalized spherical harmonic coefficients are determined, or resolved, independently of higher‐order terms. Much additional information is also obtained regarding octupole and higher‐order components of the field. Model parameter resolution and uniqueness are addressed using new visual representations of the “resolution matrix.” The model yields a dipole of magnitude 0.14 GRN3, tilted by 47° toward 72° west longitude. Neptune's quadrupole is relatively large, equal to or exceeding in magnitude the (surface) dipole field. The octupole is likewise very large but less well constrained. The magnetic fields of Neptune and Uranus are very much alike and equally unlike those of the other known magnetized planets. Characteristics of this complex magnetic field are illustrated, using contour maps of the field on the planet's surface, and discussed in the context of dynamo generation in the relatively poorly conducting

ISSN:0148-0227    

DOI:10.1029/91JA01165

年代:1991

数据来源: WILEY