摘要:

During the week of June 4–8, 1990, a Chapman Conference on Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions was held at Balatonfüred, Hungary. The meeting was coorganized by J. G. Luhmann and M. Tatrallyay, under the auspices of the AGU and the Hungarian Academy of Sciences. Aproximately 85 participants attended some 72 presentations in oral and poster form. The topics on which the sessions focused included atmosphere evolution, present atmospheres, exospheres and ionospheres, solar wind interactions, and past and future missions. The international and multidisciplinary makeup of the audience provided the basis for a wide range of discussions and brought out a number of remaining controversies and questions to be answered by future resear

ISSN:0148-0227    

DOI:10.1029/91JA01217

年代:1991

数据来源: WILEY

摘要:

Probe data showing the presence of waves in Venus's middle and upper atmosphere [Seiff et al., 1980; Seiff and Kirk,1982] are critically reevaluated and extended to 138 km, near the level of in‐situ data taken by the Pioneer Venus orbiter. Uncertainties in temperature are determined. They are typically about 0.1 times amplitude, thus supporting the reality of large amplitude oscillations approaching 40 K at 120 km. Growth rates above 100 km follow approximately the inverse square root of density until “saturation” occurs (in the sense that lapse rates become adiabatic in the expanding segment of the wave). The waves then break at the 120 km level, providing a source for the “friction” required in models to match the observed day‐night temperature contrast in Venus's lower thermosphere [Seiff, 1982;Bougher,1984]. The data correlate to an unexpected degree with temperatures from the Pioneer Venus orbiter atmospheric drag (OAD) experiment taken at altitudes of 140 to 165 km, which, especially for the night probe, extend not only the mean temperature structure, but also the oscillation structure of the probe data at the same local Venus time. OAD temperatures depend on local Venus time and altitude, but, in the limited number of observations, appear independent of observing date over periods of up to 11 days, and correlate as described with probe data taken 65 to 137 days earlier. These observations lead to the suggestion that the thermospheric waves are solar‐fixed, induced either by the major subsidence across the terminators or as continuations upward of waves in the middle atmosphere. The wave structure in the large probe sounding below 100 km is similar to, but does not quantitatively support the solar‐tidal model of Pechmann and Ingersoll, which gives much larger amplitudes and differ

ISSN:0148-0227    

DOI:10.1029/91JA01101

年代:1991

数据来源: WILEY

摘要:

During 1988 and 1990, the star sensor aboard the Pioneer Venus orbiter (PVO) was used to search for optical pulses from lightning on the nightside of Venus. In our previous effort to use the star sensor for this purpose [Borucki et al.,1981], we reported an upper limit to the lightning activity, but we were unable to show that lightning was being detected because the signal rate was indistinguishable from the false alarm rate. Because the periapsis altitude has increased by nearly a factor of ten since 1979, the star scanner views a much larger area of Venus than it did previously. This increased viewing area should provide an increased signal rate because the amplitude of optical pulses should still be above the detection threshold of the sensor if the flashes are as bright as terrestrial flashes. Because the false alarm rate did not increase, the increased viewing area allows a more sensitive search for lightning activity. Useful data were obtained for 53 orbits in 1988 and 55 orbits in 1990. During this period, approximately 83 s of search time plus 7749 s of control data were obtained. Our results again find no optical evidence for lightning activity. Within the region that was observed during 1988, the results imply that the upper bound to short‐duration flashes is 4×10−7flashes/km²/s for flashes that are at least 50% as bright as typical terrestrial lightning. During 1990, when the 2‐Hz filter was used, the results imply an upper bound of 1×10−7flashes/km²/s for long‐duration flashes at least 1.6% as bright as typical terrestrial lightning flashes or 33% as bright as the pulses observed by the Venera 9. The upper bounds to the flash rates for the 1988 and 1990 searches are twice and one half the global terrestrial rate, respectively. These two searches covered the region from 60°N latitude to 30°S latitude, 250° to 350° longitude, and the region from 45°N latitude to 55°S latitude, 155° to 300° longitude. Both searches sampled much of the nightside region from the dawn terminator to within 4 hours of the dusk terminator. These searches covered a much larger latitude range than any previous search. Our results show that the Beta and Phoebe Regio areas previously identified byRussell et al.[1988] as areas with high rates of lightning activity were not active during the two seasons of our observations. When we assume that our upper bounds to the nightside flash rate are representative of the entire planet, the results imply that the global flash rate and energy dissipation rate derived byKrasnopol'sky[1983] from his observation of a single storm are too high. Experimental measurements carried out to determine the amount of scattering occurring in the star scanner optics indicate that our previously determined upper bound to lightning activity is too low. The present results supplant the past results. The apparent conflict between the radio data that indicate the presence of a large amount of lightning activity and the optical data that indicate no lightning on the nightside, can be resolved by noting that theoretical considerations indicate lightning activity is to be expecte

ISSN:0148-0227    

DOI:10.1029/91JA01097

年代:1991

数据来源: WILEY

摘要:

Global average models for the thermospheres of Venus, Earth, and Mars are used to calculate the solar cycle variations of the global mean temperatures using the compositional profiles for each planet. All models use the same absolute solar EUV and UV fluxes but scaled for planetary distances. Photochemistry appropriate to each planet is used (or assumed) with similar rate coefficients for common physical and chemical processes. In particular, the CO215‐μm cooling rates for all three planets are determined using a common rate coefficient for collisional excitation of the CO2bending mode by atomic oxygen (relaxation rate 10−12cm³ s−1). Solar EUV and UV heating efficiencies are calculated self‐consistently for Earth, but prescribed for Venus (15/22%) and Mars (18/22%) according to independent calculations. Eddy diffusion profiles are prescribed for each planet in accord with previous studies that compared model predictions with available satellite observations. The global mean models are run to steady state for both solar minimum (F10.7 = 70) and solar maximum (F10.7 = 240) conditions. The results show that the solar cycle global mean exospheric temperature variation is about 76 K for Venus (172 to 248 K), 518 K for Earth (737 to 1255 K), and 110 K for Mars (180 to 290 K). A thermal balance analysis shows that the small exospheric temperature variation on Venus occurs because of the strong radiative damping by CO215‐μm cooling. The peak CO2cooling occurs at the altitude of maximum solar heating, and efficiently radiates it to space. On Earth, the increased solar heating occurs at a higher altitude than the peak in the infrared cooling. Consequently, it must be thermally conducted down to the altitude of the peak infrared cooling before it is radiated to space. An increase in the thermally conducted heat flux requires an increase in the vertical temperature gradient which results in a larger exospheric temperature variation. On Mars, the increased solar heating also occurs at a higher altitude than the peak cooling and likewise must be conducted downward before radiating to space. Furthermore, CO2cooling is not as effective on Mars as it is on Venus because of lower O/CO2ratios. These two factors yield a Mars solar cycle variation of global mean temperatures that is larger th

ISSN:0148-0227    

DOI:10.1029/91JA01162

年代:1991

数据来源: WILEY

摘要:

A remarkable feature of the ionosphere of Venus is the presence of nightward supersonic flows at high altitude near the terminator. In general the steady flow of an ideal gas admits a subsonic‐supersonic transition only in the presence of special conditions, such as a convergence of the flow followed by divergence, or external forces. In this paper we show that the relatively high pressure dayside plasma wells up slowly, and at high altitude it is accelerated horizontally through a relatively constricted region near the terminator toward the low‐density nightside. In effect, the plasma flows through a “nozzle” that is first converging, then diverging, permitting the transition to supersonic flow. Analysis of results from previously published models of the plasma flow in the upper ionosphere of Venus shows how such a “nozzle” is formed. The model plasma does indeed accelerate to supersonic speeds, reaching sonic speed just behind the terminator. The computed speeds prove to be close to those observed by the Pioneer Venus orbiter, and the ion transport rates are sufficient to produce and maintain the nightside

ISSN:0148-0227    

DOI:10.1029/91JA00318

年代:1991

数据来源: WILEY

摘要:

Pioneer Venus radio occultation measurements of the nightside ionosphere of Venus collected from 1979 to 1986 have made it possible to study its behavior at times of both solar maximum and solar minimum. Although some solar maximum measurements are similar in nature to those observed at solar minimum, which have an average peak density of about 7×10³ cm−3, others show much higher peak densities, reaching values of about 4×104cm−3. These elevated peak densities also occur at higher altitudes. The integrated electron column densities for these measurements are also much higher, indicating the presence of substantial ionization above the main peak. The magnitudes of both the peak density and the integrated content above the peak are anticorrelated with the solar wind dynamic pressure, leading to the interpretation that these enhancements during solar maximum are due to transterminator transport of O+ions from the dayside when the solar wind dynamic pressure is low enough to permit a sufficiently high dayside ionopause. The resulting ionization peak can be many times the concentration produced by energetic electron fluxes impacting the neutral atmosphere on the nightside, which apparently form the remaining source of the nightside peak at such times during solar maximum, when transterminator flow is cut off by high solar wind pressure depressing the dayside ionopause, and during solar minimum, when the ionopause is always dep

ISSN:0148-0227    

DOI:10.1029/91JA00672

年代:1991

数据来源: WILEY

摘要:

In situ measurements by the Pioneer Venus orbiting spacecraft, conducted during solar maximum only, have shown that magnetization (permeation of large‐scale magnetic fields) of the ionosphere of Venus occurs under high solar wind dynamic pressure and that this takes place most frequently near the subsolar region. Magnetization is also revealed in electron density profiles by the presence of disturbed topside plasma and/or a ledge near the altitude of 200 km. In this paper we use remote sensing radio occultation measurements to study magnetization of the ionospheres of Venus and Mars based on these characteristics. For Venus we take advantage of the unique data set consisting of 148 electron density profiles deduced from Pioneer Venus radio occultation measurements. We demonstrate that radio occultation measurements yield results on frequency of occurrence of magnetization during solar maximum that are similar to those obtained from the Pioneer Venus in situ magnetic field measurements. During solar minimum, for which direct ionospheric measurements have never been made, we find that magnetization of the Venus ionosphere is more pervasive than at solar maximum. Magnetization extends to higher solar zenith angles (SZA) and appears stronger than at solar maximum. These results confirm that during solar minimum, the high solar wind dynamic pressure state is more prevalent at Venus because the ionospheric plasma pressure is weaker than at solar maximum. Comparison of a large number of electron density profiles of Mars (deduced from radio occultation measurements by the Viking 1 and 2 and Mariner 9 spacecraft for SZA>46°) with those of Venus shows an absence of the ledge and disturbed topside plasma observed in the Venus profiles. These results, however, do not constitute evidence against magnetization of the ionosphere of Mars, as Shinagawa and Cravens (1989) have shown in their one‐dimensional MHD models that, even when the ionosphere of Mars is highly magnetized, the magnetic structure differs from that at Venus, and a ledge does not form in its electron density prof

ISSN:0148-0227    

DOI:10.1029/91JA00313

年代:1991

数据来源: WILEY

摘要:

The MHD Venus ionospheric model developed by Shinagawa and Cravens (1988) has been improved by including the energy equations for ions and electrons in a self‐consistent manner. This new model reproduces observed electron density and magnetic field profiles very well, and the basic MHD processes of the Venus ionosphere, as described by Shinagawa and Cravens (1988), remain virtually unchanged. The results indicate that including energetics does not significantly alter the density and magnetic field profiles. Under unmagnetized conditions, it is necessary to impose heat fluxes for both ions and electrons in order to reproduce the observed plasma temperature profiles, which are consistent with the studies by Cravens et al. (1979, 1980) and Kim et al. (1990). In the magnetized ionosphere, it is likely that a heat source for the ions is present at higher altitudes. On the other hand, the observed very high electron temperatures can be reproduced with a reduced conductivity or with a heat source at high altitudes. It is also found that heating processes do not play a significant role in the dynamics at low altitudes. Thus a nearly supersonic downward velocity layer in the lower ionosphere of Venus, proposed by Cloutier et al. (1987), is unlikely, suggesting that their flow/field model is not applicable to the solar wind‐Venus interaction and other unmagnetized bodies in magnetized plasma fl

ISSN:0148-0227    

DOI:10.1029/90JA02505

年代:1991

数据来源: WILEY

摘要:

The Viking retarding potential analyzer (RPA) flown to Mars in 1976 had negative retarding potential sweeps in addition to the positive ion sweeps. An analysis of the electron mode sweeps made in Viking 1 above the ionosphere is presented here. A spectrum covering the range 0 to −78 V was recorded in 1 s, and electron observations were made at intervals of 4 or 8 s. The derived electron concentrations and temperatures in the solar wind are lower than the generally accepted values, but the maximum electron pressures derived above the ionosphere are about as high as can be expected for generally accepted solar wind values. Except in the highly variable or turbulent region from about 2500 km down to 500 km, the scans show a high degree of consistency from scan to scan. The initial concentrations near 15,000 km were slightly less than 1 cm−3characterized by a temperature of about 40,000 K, plus what is interpreted as a backstreaming component from the planetary shock of about 0.1 cm−3at about 250,000 K. The shock appeared to be quasi‐parallel, as there was no evidence of a sharp transition anywhere near the expected shock position; it appears that the shock transition region extended from about 3500 down to 850 km. The hottest region was near 850 km, where there is clear evidence of an electron component too hot for the RPA to measure; it produced abundant secondary emission and is assumed to be characterized by energies of about 200 eV so as to be efficient in the production of secondaries. The electron concentrations there averaged about

ISSN:0148-0227    

DOI:10.1029/91JA00317

年代:1991

数据来源: WILEY

摘要:

Magnetic fields are induced by the solar wind in the ionospheres of planets such as Venus and Mars, which possess only weak, or nonexistent, intrinsic magnetic fields. The magnetometer on the Pioneer Venus Orbiter (PVO) observed large‐scale magnetic fields in the ionosphere when the solar wind dynamic pressure was high. During high solar wind dynamic pressure conditions the ionopause was observed to be broad and at low altitudes, whereas during low solar wind dynamic pressure conditions the ionopause was located at high altitudes (z>300 km) and was narrow (Δz≈ 20 km). We review the results of recent theoretical models of the magnetic field in the ionosphere of Venus, and, in particular, the one‐dimensional MHD model of Shinagawa and Cravens (1988) and the two‐dimensional kinematic dynamo model of Cravens et al. (1990). The results of these models are used as a starting point for an explanation of the behavior of the ionopause. It is well known that during low solar wind dynamic pressure conditions the location of the ionopause is determined by vertical pressure balance between the ionospheric thermal pressure and the magnetic pressure of the overlying magnetic barrier. The structure of the ionopause is not well understood; we suggest that the sensitivity of horizontal plasma transport to the presence of a magnetic field and the convergence of the vertical flow provide the explanation for the narrowness of the Venus ionopause. However, during high solar wind dynamic pressure conditions, ion‐neutral friction also plays an important role in determining the thickness of the

ISSN:0148-0227    

DOI:10.1029/91JA00674

年代:1991

数据来源: WILEY