Structural peculiarities of circumstellar envelopes of the young A0E herbig stars

Circumstellar peculiarities of the A0-type Herbig stars are analyzed on the basis of high-resolution CCD spectroscopic data, obtained in 1991-1994 at the ESO and the Crimean Astrophysical Observatory (about 250 spectrograms). The results of investigation of the rapid line profile variability in H , H , HeI 5876 and DNa lines are presented for AB Aur, HD 163296, HD 36112 and HD 100546. The conclusion is made that the behaviour of these lines can be explained in the framework of the model containing an equatorially concentrated and azimuthally inhomogeneous stellar wind and an external cool shell occasionally losing matter in the form of infall on to the star.     

Periods of active permafrost layer formation during the geological history of Mars: Implications for circum-polar and mid-latitude surface processes

Permafrost is ground remaining frozen (temperatures are below the freezing point of water) for more than two consecutive years. An active layer in permafrost regions is defined as a near-surface layer that undergoes freeze-thaw cycles due to day-average surface and soil temperatures oscillating about the freezing point of water. A “dry” active layer may occur in parched soils without free water or ice but significant geomorphic change through cryoturbation is not produced in these environments. A wet active layer is currently absent on Mars. We use recent calculations on the astronomical forcing of climate change to assess the conditions under which an extensive active layer could form on Mars during past climate history. Our examination of insolation patterns and surface topography predicts that an active layer should form on Mars in the geological past at high latitudes as well as on pole-facing slopes at mid-latitudes during repetitive periods of high obliquity. We examine global high-resolution MOLA topography and geological features on Mars and find that a distinctive latitudinal zonality of the occurrence of steep slopes and an asymmetry of steep slopes at mid-latitudes can be attributed to the effect of active layer processes. We conclude that the formation of an active layer during periods of enhanced obliquity throughout the most recent period of the history of Mars (the Amazonian) has led to significant degradation of impact craters, rapidly decreasing the steep slopes characterizing pristine landforms. Our analysis suggests that an active layer has not been present on Mars in the last ∼5 Ma, and that conditions favoring the formation of an active layer were reached in only about 20% of the obliquity excursions between 5 and 10 Ma ago. Conditions favoring an active layer are not predicted to be common in the next 10 Ma. The much higher obliquity excursions predicted for the earlier Amazonian appear to be responsible for the significant reduction in magnitude of crater interior slopes observed at higher latitudes on Mars. The observed slope asymmetry at mid-latitudes suggests direct insolation control, and hence low atmospheric pressure, during the high obliquity periods throughout the Amazonian. We formulate predictions on the nature and distribution of candidate active layer features that could be revealed by higher resolution imaging data.     

The 27-Day and 22-Year Cycles in Solar and Geomagnetic Activity

We study the evolution of the longitudinal asymmetry in solar activity through the wave packet technique applied to the period domain of 25 – 31 days (centered at the 27-day solar rotation period) for the sunspot number and geomagnetic aa index. We observe the occurrence of alternating smaller and larger amplitudes of the 11-year cycle, resulting in a 22-year periodicity in the 27-day signal. The evolution of the 22-year cycle shows a change of regime around the year 1912 when the 22-year period disappears from the sunspot number series and appears in the aa index. Other changes, such as a change in the correlation between solar and geomagnetic activity, took place at the same time. Splitting the 27-day frequency domain of aa index shows an 11-year cycle for higher frequencies and a pure22-year cycle for lower frequencies, which we attribute to higher latitude coronal holes. This evidence is particularly clear after 1940, which is another benchmark in the evolution of the aa index. We discuss briefly the mechanisms that could account for the observed features of the 22-year cycle evolution.     

Modeling the dynamics of the land-sea breeze circulation for air quality modeling

Conventional atmospheric dispersion and air quality models require that the wind field be known with a higher resolution than is currently available from field monitoring stations in most coastal areas. In this paper, a numerical model is developed to predict the wind flow field during the land-sea breeze. The form and assumptions and method of solution of the model are described. The model output is compared to atmospheric data taken from a field study conducted in the Santa Barbara Channel and Ventura-Oxnard plain in southern California.     

Heinrich-type glacial surges in a low-order dynamical climate model

Recent studies suggest the occurrence of sporadic episodes during which the ice streams that discharge ice sheets become enormously active, producing large numbers of icebergs (reflected in North Atlantic sea cores as Heinrich events) and possibly causing the partial collapse of the ice sheets. To simulate the mechanism of internal thermo-hydrodynamical instability implied by such behavior in the context of a more general paleoclimate dynamics model (PDM), we introduce a new sliding-catastrophe function that can account for ice-sheet surges. In particular, using simple scaling estimates derived from the equations of motion and thermo-conductivity for ice flow, we express this function in terms of the thickness, density, viscosity, heat-capacity, and heat-conductivity of ice. Analysis of the properties of this function suggests that these Heinrich-type instability events might be of three possible kinds: the first type of event occurs in periods of glacial maximum when temperature conditions on the ice surface are extremely cold, but internal friction within bottom boundary layer is also at its maximum and is strong enough to melt ice and cause its surge. The second type of event may happen during an interglacial, when the ice thickness is small but relatively warm climatic conditions on the upper surface of ice can be easily advected with the flow of ice to the bottom where even a small additional heating due to friction may cause melting. The third and, perhaps, most interesting type of event is one that may occur during ice sheet growth; in this period particles of ice reaching the bottom still remember the warm temperature conditions of the previous interglacial and additional heating due to increasing friction associated with the growing ice sheet may again cause melting. To the extent that the upper glacier surface temperature depends on atmospheric carbon dioxide concentration, this third case introduces the interesting possibility that earlier CO₂ concentrations may be as important for the present-day climate as its current value. We present results of numerical experiments demonstrating how these three kinds of instability can originate and interact with other components of the global climate system to produce variations of the Heinrich-event type. In particular, according to our model the climate system seems more vulnerable to surges during the penultimate interglacial period than in the present one, which may contribute to an explanation of the recent results of the Greenland Ice Core Project.     

The Mechanism of Reactive NO3 Uptake on Dry NaX (X=Cl, Br)

A simple kinetic mechanism of nitrate radicals uptake on dry sea-salt NaCl, NaBr surfaces is proposed. The mechanism includes adsorption/desorption equilibrium and unimolecular decomposition of the adsorbed complex: NO₃(g) + NaX(s) (NO₃-NaX)(s); (NO₃-NaX)(s) NaNO₃ + X(s) Two techniques were used: the matrix isolation ESR and mass spectrometry. The uptake coefficient () is found to be dependent on exposure time of salt to NO₃ for raw coating. The initial (t0) is higher than the observable steady-state _obs. At room temperature _obs is independent of [NO₃] at low [NO₃] = 3 × 10⁹ - 10¹¹ cm^-3, but it is inversely proportional to [NO₃] at concentrations higher than 10¹² cm^-3. At temperatures above 100 °C, _obs becomes independent of [NO₃] in a wider range of [NO₃]. An increased number of dislocations is supposed to exist in the case of raw coating. Due to a wide spread of the surface sites binding energy with the ionic lattice near dislocations, the part of surface complexes has lower binding energy and "burns" more rapidly. That burning determines the transition from (t0) down to _obs.The kinetic parameters and elementary rate coefficients are obtained. The recommended for low atmospheric NO₃ concentration are in the range of 0.002 ± 0.04 for NaCl and 0.1-0.3 for NaBr depending on a mechanism of the (t) relaxation.     

Global geological map of Venus

The surface area of Venus (∼460×10⁶ km²) is ∼90% of that of the Earth. Using Magellan radar image and altimetry data, supplemented by Venera-15/16 radar images, we compiled a global geologic map of Venus at a scale of 1:10 M. We outline the history of geological mapping of the Earth and planets to illustrate the importance of utilizing the dual stratigraphic classification approach to geological mapping. Using this established approach, we identify 13 distinctive units on the surface of Venus and a series of structures and related features. We present the history and evolution of the definition and characterization of these units, explore and assess alternate methods and approaches that have been suggested, and trace the sequence of mapping from small areas to regional and global scales. We outline the specific defining nature and characteristics of these units, map their distribution, and assess their stratigraphic relationships. On the basis of these data, we then compare local and regional stratigraphic columns and compile a global stratigraphic column, defining rock-stratigraphic units, time-stratigraphic units, and geological time units. We use superposed craters, stratigraphic relationships and impact crater parabola degradation to assess the geologic time represented by the global stratigraphic column. Using the characteristics of these units, we interpret the geological processes that were responsible for their formation. On the basis of unit superposition and stratigraphic relationships, we interpret the sequence of events and processes recorded in the global stratigraphic column. The earliest part of the history of Venus (Pre-Fortunian) predates the observed surface geological features and units, although remnants may exist in the form of deformed rocks and minerals. We find that the observable geological history of Venus can be subdivided into three distinctive phases. The earlier phase (Fortunian Period, its lower stratigraphic boundary cannot be determined with the available data sets) involved intense deformation and building of regions of thicker crust (tessera). This was followed by the Guineverian Period. Distributed deformed plains, mountain belts, and regional interconnected groove belts characterize the first part and the vast majority of coronae began to form during this time. The second part of the Guineverian Period involved global emplacement of vast and mildly deformed plains of volcanic origin. A period of global wrinkle ridge formation largely followed the emplacement of these plains. The third phase (Atlian Period) involved the formation of prominent rift zones and fields of lava flows unmodified by wrinkle ridges that are often associated with large shield volcanoes and, in places, with earlier-formed coronae. Atlian volcanism may continue to the present. About 70% of the exposed surface of Venus was resurfaced during the Guineverian Period and only about 16% during the Atlian Period. Estimates of model absolute ages suggest that the Atlian Period was about twice as long as the Guineverian and, thus, characterized by significantly reduced rates of volcanism and tectonism. The three major phases of activity documented in the global stratigraphy and geological map, and their interpreted temporal relations, provide a basis for assessing the geodynamical processes operating earlier in Venus history that led to the preserved record.     

Comparing seismicity declustering techniques by means of the joint use of Allan Factor and Morisita index

In this paper, we propose to compare different declustering methods on the basis of the time-correlation and the space-clustering of the residual earthquake catalog after the declustering techniques have been applied. To this aim, we applied two point process clustering measures, the Allan Factor and the Morisita Index, for the identification and quantification of temporal correlation and spatial clustering in point processes, respectively. We used our joint space–time approach to study the earthquake space–time point processes of southern California and Switzerland with surrounding area, declustered by using the method of Gardner and Knopoff (with Grünthal and Uhmhammer window) and that of Reasenberg (with different setting parameters). Our results show that the residual declustered catalog is still characterized by time-correlated structures at long timescales; however, the cutoff timescale that is the lowest timescale above which the time-correlation is visible is higher with the Reasenberg method while is smaller with the Gardner and Knopoff method with Grünthal window. The space-clustering analysis performed by means of the Morisita Index suggests that the declustering technique effectively reduces the spatial clustering of the seismicity of Switzerland, but does not change the spatial properties of the residual seismic catalogue of the southern California.