摘要:

Observations of energetic charged particles obtained by Pioneers 10 and 11 near the orbits of the inner Jovian satellites are reviewed with particular emphasis on the implications of these observations with regard to possible models of the access of charged particles to the satellite surfaces. The observed effects on particle pitch angle distributions and the observed energy dependence of the intensity depletions seen at the satellite orbits are compared with predictions of satellite sweepup based on several different access models. The two major uncertainties which hamper the comparisons are those associated with the satellite conductivities and the ionospheric dynamo electric field power spectrum. The satellite conductivity is important because it governs the access of the particles to the satellite surface and therefore the lifetime τ; the dynamo power spectrum is important because it controls the magnitude and energy dependence of the radial diffusion coefficient. In spite of these uncertainties we can nevertheless make the following conclusions. (1) The electron pitch angle distributions at Io's orbit are compatible with expectations based on sweeping. (2) The energy dependences of the observed electron depletions at all three inner satellites (Amalthea, Io, and Europa) are incompatible with expectations based on a perfect conductor model of a satellite and its flux tube but are compatible with the energy dependence expected for perfectly insulating or partially conducting satellites; the energy dependences of the proton losses at all three satellites also appear to be compatible with absorption by an insulating or partially conducting satellite but only if certain forms of the dynamo power spectrum are assumed. (3) However, the proton losses at Io are observed to be much stronger than the electron losses, in contradiction to expectations based on sweeping. (4) The most attractive explanation for the proton‐electron discrepancy at Io is that the large proton losses at Io's orbit are principally due to enhanced pitch angle scattering in the region of higher plasma density and probably greater turbulence associated with the satellite rather than to satellite sweeping alo

ISSN:8755-1209    

DOI:10.1029/RG017i003p00369

年代:1979

数据来源: WILEY

摘要:

Recent developments in photochemical models of the upper atmosphere of Jupiter and Titan are reviewed. Measurements of the Jovian Lyman alpha airglow favor a homopause value of approximately 107cm² s−1for the eddy diffusion coefficient. A relatively hot exosphere as implied by the electron density measurements results in an extended atmosphere of hydrogen. Larger than solar C/H and N/H ratios are obtained on reinterpretation of the sunlight reflected from Jupiter; and substantially lower ultraviolet photoabsorption cross sections measured for methane and ethane influence the distribution of CH4, NH3, and their photochemical products. Possible coupling between PH3and NH3may inhibit ultraviolet destruction of NH3on Jupiter. The red color of the Great Red Spot and other minor spots may be due to the presence of red phosphorus crystals formed after photolysis of PH3. The detection of such gases as PH3, GeH4, and CO in the stratosphere of Jupiter gives an indication of the presence of thermochemical nonequilibrium processes, and a strong vertical mixing may be required to explain their large observed upper atmospheric abundances. This argument is further strengthened by a laboratory finding that C2H2can efficiently scavenge photolysis products of PH3. Other alternate mechanisms to account for the presence of CO in the stratosphere are the oxygen atom transfer from the Galilean satellites to Jupiter either by radial diffusion, or along magnetic flux tubes, or on ablation of meteoroidal material. Subsequent reactions of the thermal oxygen atoms with photolysis products of methane lead to the production of CO. Finally, the discussion of reducing atmospheres is extended to include Titan. It is found that if NH3has outgassed from Titan and if the surface temperature during the initial phases of its evolution was 150°K or greater, due perhaps to a CH4‐H2greenhouse, it is inevitable that Titan would have accumulated a thick atmosphere of nitrogen over geologic time. The pressure‐induced thermal opacity of 20 bars of nitrogen can also explain the recently measured 200°K surface temperature of Titan. On the other hand, the composition of Titan's atmosphere would be far less certain if the surface temperature were as low

ISSN:8755-1209    

DOI:10.1029/RG017i003p00388

年代:1979

数据来源: WILEY

摘要:

Quantitative modeling of the near‐earth plasma environment in the 0‐ to 100‐keV energy range has made significant strides in the last decade in response to the needs of spacecraft users and to the substantial improvements in the modeling of the earth's electric and magnetic fields. Concurrent with this has been an extensive growth in the observational data base necessary to evaluate these models. In this paper, models of the plasmasphere and near‐geosynchronous environment are reviewed and compared with the data; also discussed are potential users, their needs, and the ability of the models to meet those needs. The review indicates that although several significant problems are not resolved, quantitative low‐energy plasma models are at the stage that magnetic field models were at a decade ago and substantial gains are expected in the next

ISSN:8755-1209    

DOI:10.1029/RG017i003p00397

年代:1979

数据来源: WILEY

摘要:

In recent years, much evidence has been accumulated which supports the existence of spatially limited regions of strong electric fields (>100 mV/m) parallel to the magnetic field. Evidence which will be reviewed comes from rocket measurements, satellite observations, and plasma cloud releases above the polar ionosphere. The most plausible explanation of such structures is provided by the concept of double layers or electrostatic shocks, which are double layers moving in relation to the plasma. We also review some recent laboratory investigations of double layers which show that double layers can exist for long periods of time. The production of these structures requires large currents. The theory of double layers has so far concentrated on the nature of stationary double layers, and theoretical proof is now available that the Vlasov equation allows for stationary double layer solutions in one or two dimensions. A full three‐dimensional treatment is not available. Necessary conditions for the existence of double layers (generalized Bohm criterion), relations between thickness and strength of the layer, and the field signature are studied. Comparison with observations reveals many similarities, but major problems (e.g., lack of magnetic field alignment of accelerated particles and particle heating in contrast to free acceleration) remain. All double layers observed so far are parts of a large‐scale electric circuit carrying a current driven by a generator and dissipated in a load. Thus the whole circuit must be analyzed in order to understand properly the generation and nature of double lay

ISSN:8755-1209    

DOI:10.1029/RG017i003p00418

年代:1979

数据来源: WILEY

摘要:

The Astrophysical Observatory of the Smithsonian Institution (APO) made measurements of the solar constant at many locations on the earth's surface from 1902 to 1962. These measurements have been interpreted by various authors to show that the sun has both a long‐term secular change in brightness and cyclic variations. The same data set has been used by other authors to show that the sun is constant in output. The APO solar constant data are reexamined in this review in order to clarify what the APO staff did and what their results say about the behavior of the sun. There are serious problems with the internal consistency of the APO solar constant measurements indicated by the general lack of a common signal between stations or between different measurement methods. If the overall data set is considered, there is no evidence for cyclic variations or any long‐term trend in the solar constant greater than a few tenths of a percent. Overall the solar constant appears to be constant to within about 0.1% over the period 1923–1954. Most of the observed variations are explained by errors in the data reduction scheme which failed to remove all the effects of atmospheric extinction or by improper changes in the radiation scale. The solar constant values are independent of solar activity. Although this null conclusion based on the examination of this data set is not encouraging as an easy explanation for climatic change during the twentieth century, the conclusion is valuable in that it sets constraints on the required long‐term accuracy and reproducibility needed in the upcoming satellite observations of the solar c

ISSN:8755-1209    

DOI:10.1029/RG017i003p00427

年代:1979

数据来源: WILEY

摘要:

Particulate matter in the atmosphere is a major source of contamination, having an impact upon visibility, health, and even climate. These aerosols are predominantly of natural origin. A minor component, the anthropogenic organic aerosol, has been studied extensively because of its relation to smog. Much less is known about the far more abundant natural organic aerosol. Estimates of the annual global production rate of the latter range between 75 and 1000 million metric tons. In contrast, the corresponding rate for man‐made organic aerosol is only between 0.2% and 17% of the natural rate. Thus the natural organic aerosol production rate is exceeded greatly only by that of natural sea‐salt aerosol and possibly by that of natural nitrate. By far the largest proportion of the natural organic aerosol is derived from vegetation by means of a photochemical reaction of its vapors, a process which is observed as haze formation. This reaction of olefins has been studied extensively and is believed to be identical with that involved in smog production by anthropogenic olefins. In fact, the resultant products in the two cases are virtually indistinguishable, although minor differences do exist. An additional possible source for natural organic aerosols is the release of waxy particles from the tips of pine needles by means of electrical brush discharge from clouds. Yet other small amounts come from biological cellular debris and possibly even from volcanos. The role of natural organic aerosols in condensation and precipitation has been studied. Claims for the effectiveness of organic aerosols as ice‐forming nuclei have been made. Natural organic aerosols have been suggested as possible sources of petroleum and of humus in the ground. Biodegradation mechanisms for this result have been considered. If it is verified, the impact upon pollution control strategies would be conside

ISSN:8755-1209    

DOI:10.1029/RG017i003p00459

年代:1979

数据来源: WILEY