The composition and vertical structure of the lower cloud deck on Venus

The opportunity to determine the planetwide temperature and cloud structure of Venus using radio occultation techniques arose with Pioneer Venus. Amplitude and Doppler data provided by the radio occultation experiment offered a unique and powerful means of examining the atmospheric properties in the lower cloud region.Absorption due to gaseous components of the atmosphere was subtracted from the measured absorption coefficient profiles before they were used to compute cloud mass contents. This absorption was found to represent a small part of the total absorption, depending on the latitude. In the main cloud deck, gaseous absorption contributes 10 to 20%, however, at the bottom of the detected absorption layer the sulfuric acid vapor contributes up to 100% due to increased vapor pressures. The clouds are the primary contributing absorbers in the 1- to 3-bar level of the Venus atmosphere. Below about 3 bars, depending on the latitude, absorption due to sulfuric acid vapor dominates.If a cloud particle model consisting of a solid nonabsorbing dielectric sphere with a concentric liquid sulfuric acid coating is invoked, the absorptivity of the particles increases from that of a pure sulfuric acid liquid sphere, and the mass content derived from the absorption coefficient profiles decreases. As the ratio of the core radius to the total radius (q) increases, absorption increases by more than a factor of 10 for high values of q. In the case of pure sulfuric acid droplets, the conductivity is sufficiently high that some of the field is excluded from the interior of the droplet thereby reducing the absorption. When a dielectric core of nonabsorbing material is introduced, the surface charge density is reduced and the absorption increases.The mass contents for all orbits in the equatorial region of Venus were calculated using values of q from 0 to 1. The resulting profiles match the probe mass content profiles at similar locations when a q of 0.97 is chosen.The wavelength dependence of the absorption for the spherical shell model varies with q from 1/λ² for pure liquid to λ^0.2 for a large core. A q of from 0.96 to 0.98 results in a wavelength dependence of 1/λ^1.0 to 1/λ^1.4 which matches the radio occultation absorption wavelength dependence and the microwave opacity wavelength dependence.Mass content profiles using a q of 0.97 were determined for occultations in the polar, collar, midlatitudinal, and equatorial regions assuming q remains constant over the planet. The results show considerable variability in both the level and the magnitude of the lower cloud deck. The cloud layer is lowest in altitude in the polar region. This might be expected as the temperature profile is cooler in the polar region than over the rest of the planet. The mass content is greatest in the polar and collar regions; however, many of the collar profiles were cut off due to fluctuations resulting from increased turbulence in the collar region. The mass contents are least dense in the midlatitude regions. There is a sharp lower boundary at about 1.5 bars in the equatorial and midlatitude regions and at about 2.5 bars in the polar region. Measurements made by the Particle Size Spectrometer and nephelometers also showed sharp lower cloud boundaries at this level.     

An instrument to measure solar magnetic fields by an atomic-beam method

Using the atomic-beam technique in absorption, a new kind of very narrow-passband filter is obtained, which may also be used as a high-resolution spectrograph and as a Babcock window for detecting solar magnetic fields without a spectrograph.     

Some developments of the magnetic beam absorption filter

In the present paper some experimental arrangements are shown which utilize the magnetic filter described by Cimino et al. (1968). For a single cell we have elaborated an elementary theory in the following cases: (i) absorption by an atomic beam in a uniform magnetic field (i.e. pure damping profile); (ii) atomic beam in a non-uniform magnetic field; (iii) vapours in a uniform magnetic field (i.e. gaussian distribution); (iv) vapours in a non-uniform magnetic field.     

Liquid content of the lower clouds of venus as determined from mariner 10 radio occultation

S-band (13.06-cm) and X-band (3.56-cm) radio occulation data obtained during the flyby of Venus by Mariner 10 on February 5, 1974 were analyzed to obtain the effects of dispersive microwave absorption by the clouds of Venus. The received power profiles were first corrected for the effects of refraction in the atmosphere of Venus, programmed changes in the pointing direction of the high-gain antenna, and limit-cycle motion of the spacecraft attitude control system. The resulting excess attenuation profiles presumbaly due to cloud absorption have been inverted discretely to obtain profiles of absorption coefficient at the two wavelenghts. The ratios of the absorptivities are consistent with a sulfuric acid-water mixture as the constituent of the absorbing clouds, having a sulfuric acid concentration of 75 ± 25%. Three absorption peaks are evident in the profiles at altitudes of 68, 60, and 48 km. With a sulfuric acid concentration of 75%, the upper cloud has a peak liquid content of 0.08 g/m³, and an integrated content of 0.024 g/cm², which corresponds roughly to terrestrial stratus or altostratus clouds. The major absorption layer has a peak of 1.1 g/m³ at an altitude of 48 km, with an integrated content of 0.5 g/cm², similar to that of terrestrial cumulus and cumulonimbus clouds. The absorption ratios for the middle cloud at 60 km are not consistent with a sulfuric acid-water mixture.     

The regional park of the Nebrodi Mts. (Sicily): a contribution to an integrated groundwater management

 The Nebrodi Park, located in the north-eastern sector of Sicily, is of particular environmental and scientific interest. This is due to the morphological heterogeneity of the protected region that presents a wide variety of landscapes and ecosystems. In spite of the recent efforts of the Regional Government and Municipalities, environmental resources, especially groundwater, risk progressive depauperation and pollution. In this paper, the authors consider the hydrogeological karst unit of Monte Traura-Rocche del Crasto, the most productive in the Nebrodi belt (zone B of the regional park). This unit is underexploited and needs a careful hydrogeological study in order to supply coastal aquifers. Geochemical and geophysical prospecting has been carried out in this area. Analyses of water springs, sampled in different seasons, have suggested patterns of groundwater circulation. Apparent resistivity measurements have yielded complementary information about the recharge processes of deep aquifers. Goals of the project to which this research belongs are the rational exploitation and protection of reservoirs such as Monte Traura and the recovery of the degraded aquifers, proposing new strategies of regional management for the water resources in this comer of Sicily. Received: 27 November 1996 · Accepted: 5 January 1998     

A geophysical and geochemical approach for seawater intrusion assessment in the Acquedolci coastal aquifer (Northern Sicily)

Vertical electrical sounding (VES’) surveys and chemical analyses of groundwater have been executed in the coastal plain of Acquedolci (Northern Sicily), with the aim to circumscribe seawater intrusion phenomena. This urbanized area is representative of a more general problem interesting most of Mediterranean littoral areas, where intensive overdraft favors a heavy seawater intrusion through the coastline. Aquifer resistivity seems decisively to be conditioned by the chlorine and magnesium content in the main aquifer of the region. Schlumberger VES’, together with piezometric and chemical–physical information of groundwater, allowed us to perform hydrogeological and geophysical elaborations, to propose the occurrence of a relatively narrow belt marked by fresh–salt water mixing. In the considered plain, pollution risk studies have been already realized by authors with the proposal to identify—by parametric and synthetic zoning of significant hydrogeological elements—the most vulnerable sectors. In detail, an intrinsic vulnerability mapping has been already performed, applying the well-known SINTACS system. This paper intends to give—in this sector of Sicily—an example of integration of different methodologies, showing the role of geophysics to describe the degradation of aquifers on the whole as well as to assess their pollution risk better.     

Calibration of base flow separation methods with streamflow conductivity

The conductivity mass-balance (CMB) method can be used to calibrate analytical base flow separation methods. The principal CMB assumptions are base flow conductivity is equal to streamflow conductivity at lowest flows, runoff conductivity is equal to streamflow conductivity at highest flows, and base flow and runoff conductivities are assumed to be constants over the period of record. To test the CMB assumptions, fluid conductivities of ground water, surface runoff, and streamflow were measured during wet and dry conditions in a 12-km(2) stream basin. Ground water conductivities at wells varied an average of 6% from dry to wet conditions, while stream conductivities varied 58%. Shallow ground water conductivity varied significantly with distance from the stream, with lowest conductivities of 87 microS/cm near the divide, a maximum of 520 microS/cm 59 m from the stream, and 215 microS/cm 22 m from the stream. Runoff conductivities measured in three rain events remained nearly constant, with lower conductivities of 35 microS/cm near the divide and 50 microS/cm near the stream. The CMB method was applied to the records from 10 USGS stream-gauging stations in Texas, Kentucky, Georgia, and Florida to calibrate the USGS base flow separation technique, HYSEP, by varying the time parameter 2N*. There is a statistically significant relationship between basin areas and calibrated values of 2N*, expressed as N = 0.46A(0.44), with N in days and A in km(2). The widely accepted relationship N = 0.83A(0.2) is not valid for these basins. Other analytic methods can also be calibrated with the CMB method.